In the years after the Second World War, the city of Los Angeles began to sit under a pale haze that stung the eyes and cracked the leaves of crops in the fields around it. Everyone assumed the cause was obvious: smoke. The city closed down open burning and tightened controls on chimneys. The haze stayed.
The puzzle was handed to Arie Haagen-Smit, a biochemist at the California Institute of Technology, who had spent his career working out what gives pineapples and wine their aroma. He was used to identifying substances present in tiny quantities. In the early 1950s he took samples of the city’s air, concentrated them, and ran them through his laboratory glassware. What he found was ozone — a gas nobody was emitting. He then showed how it appeared: unburnt fuel vapours and oxides of nitrogen from vehicle exhaust, sitting in strong sunlight, reacted together to make it.
That result changed how pollution was understood. The damaging substance was not the one coming out of the pipe; it was one manufactured in the air afterwards. And nobody could have guessed it by looking. It had to be sampled, separated and measured.
This chapter will answer each of these questions.
4.1 Pollution Monitoring: What Is Measured and Why
Every environment carries substances that do not belong to it. A city street carries soot from diesel engines; a field carries the residue of a spray applied last month; a canal carries whatever the town upstream has finished with. None of this is visible in any useful way. The question that opens this chapter is a practical one: if you cannot see a problem, how do you establish that it exists, how large it is, and whether it is getting better or worse?
The answer is measurement, carried out to a fixed method, at fixed places, over long periods. That activity is called pollution monitoring. It is not the same as controlling pollution. Monitoring produces numbers; control is what is done with them. But without the numbers, control has nothing to aim at and no way of knowing whether it worked.
Pollution and the substances that cause it
Pollution is the addition to air, water or soil of a substance or form of energy at a rate faster than the environment can disperse, break down or absorb it, producing a change that harms living things or damages property. The definition matters because of what it leaves out. A substance is not a pollutant merely because it is unnatural, nor is it harmless merely because it is natural. Volcanic ash and forest-fire smoke are natural; both damage lungs. What counts is the quantity present, over what period, and what that quantity does.
(Pollution is the addition of a substance or of energy — such as heat, noise or light — to the environment in amounts that exceed its capacity to disperse or break them down, causing harm to living organisms or to built and natural assets.)
The agent doing the damage is the pollutant. In air, the term is narrowed slightly.
(An air pollutant is any substance present in the air — as a solid particle, a liquid droplet or a gas — in a concentration high enough to have an adverse effect on human health, on other organisms, on crops, or on materials and structures.)
4.1.1 Pollutants, and the Idea of a Pollutant Load
A pollutant does harm through its concentration, which is the amount of the substance present in a given volume of air or water. Gaseous pollutants in air are reported either as a mass in a volume, in micrograms per cubic metre (µg/m³), or as a volume ratio, in parts per million (ppm) or parts per billion (ppb). Particulate pollutants are reported only as mass per volume, in µg/m³, because a mixture of dust of every size and composition has no meaningful volume ratio.
µg/m³ and ppm are not the same kind of number One microgram is a millionth of a gram, and 1 ppm means one molecule of the gas in a million molecules of air. Converting between them depends on the molecular mass of the gas and on temperature and pressure, so the two units can never be swapped without that calculation.
It is also useful to know where the material entered from. A point source discharges from a single identifiable outlet — a factory stack, an effluent pipe, a chimney. A non-point source is spread over an area with no single outlet — run-off carrying fertiliser from a hundred fields, dust lifted off unpaved roads, exhaust from tens of thousands of vehicles moving through a city. Point sources are far easier to regulate, because there is something to inspect and someone responsible for it. Much of the difficulty of pollution control lies in the fact that the largest loads are often non-point.
4.1.2 Primary and Secondary Pollutants
Not every pollutant is released as such. Some enter the atmosphere already in the harmful form; others are made in the air after release, out of substances that were comparatively less harmful when they left the source. This distinction organises the whole of air quality work, because the two kinds behave differently and are controlled differently.
A primary pollutant is emitted directly into the atmosphere from an identifiable source, in the same chemical form in which it does harm. Ash thrown up by a volcanic eruption, sulphur dioxide (SO₂) leaving the stack of a coal-fired plant, carbon monoxide (CO) leaving a vehicle’s tailpipe, and lead released by certain industrial processes are all primary pollutants.
(A primary pollutant is one that is emitted directly into the atmosphere from a source, in the form in which it exerts its harmful effect.)
A secondary pollutant is not emitted at all. It forms in the atmosphere when primary pollutants react with one another, with ordinary constituents of air, or under the influence of sunlight. Ground-level ozone (O₃) is the standard example, and the story that opened this chapter is how it was identified: unburnt hydrocarbons and oxides of nitrogen, both emitted by vehicles, react in strong sunlight to produce it. Sulphates and nitrates formed in the air from SO₂ and oxides of nitrogen, and then existing as fine particles, are secondary pollutants too, as is peroxyacetyl nitrate (PAN), formed from oxides of nitrogen and organic vapours.
(A secondary pollutant is one that is not emitted directly but is formed in the atmosphere by chemical or photochemical reaction between primary pollutants, or between primary pollutants and normal atmospheric constituents.)
Some substances belong to both categories at once. Nitrogen dioxide (NO₂) is emitted directly in small amounts by high-temperature combustion, but most of what is measured in city air has formed in the atmosphere from nitric oxide (NO). The categories describe pathways, not fixed lists.
The consequences of the distinction are practical, and they show up in Fig. 4.1-1. A secondary pollutant peaks away from its source, not at it — downwind, and at the time of day when sunlight is strongest. A monitoring station placed at the factory gate will therefore record the primary pollutant faithfully and miss the secondary one almost entirely. And it cannot be controlled by fitting a device to the chimney, because there is nothing in the chimney to catch. It can only be controlled by cutting the emission of the substances that make it — its precursors.
Table 4.1 – Primary vs secondary pollutants | Feature | Primary pollutant | Secondary pollutant | |—|—|—| | Origin | Emitted directly from a source | Formed in the atmosphere from other pollutants | | Chemical change before it acts | None | Chemical or photochemical reaction required | | Where concentration peaks | At or very near the source | Downwind of the source, often several kilometres away | | Time of peak | Follows the pattern of emission, e.g. traffic peaks | Often mid-afternoon, when sunlight is strongest | | Method of control | Treat or trap the emission at the source | Reduce emission of the precursor substances | | Examples | SO₂, CO, oxides of nitrogen as emitted, ash, lead | Ground-level ozone, PAN, sulphate and nitrate particles |
Q1. Ground-level ozone comes out of no chimney and no exhaust pipe. So where does it come from, and what is such a pollutant called?
A. It is manufactured in the air itself. Unburnt hydrocarbons and oxides of nitrogen, both released by vehicle engines, react together under strong sunlight to produce ozone. A pollutant formed in the atmosphere in this way, rather than emitted directly, is called a secondary pollutant.
4.1.3 Why Air Pollution Is Monitored
It is worth being explicit about what monitoring is for, because the design of a network follows directly from its purpose.
The first purpose is to establish the present status — what concentrations people are actually breathing in a given place, expressed as numbers that can be compared with a standard. Until this exists, a claim that a city’s air is bad or good is only an impression.
The second is to detect trends. A single reading is nearly meaningless, because concentrations swing with the hour, the traffic and the weather. A run of readings taken by the same method at the same site for several years shows whether the level is rising, falling or steady. This is the only honest way to judge whether a control measure worked. When a fuel is changed or a class of vehicle removed, the question “did it help?” can be answered by comparing the trend before and after.
The third is to identify sources and understand behaviour. Because concentrations vary with wind direction, time of day and season, a set of readings analysed against meteorological data indicates which sources are contributing, how far pollutants travel, and which weather conditions trap them near the ground. Winter mornings, when cold air sits under a layer of warmer air and stops vertical mixing, produce far higher readings from the same emissions than a windy afternoon does.
The fourth is to support decisions: setting priorities between sources, deciding where new industry may be sited, planning a town, and warning the public on days when levels are high. The fifth is to generate background data — records of what the air was like before a project began, against which its later effects can be assessed.
Monitoring also raises a question of who is measured. A network of a few stations reports an average for a city, but exposure is not evenly shared. A household beside a truck route or a construction site, or one that burns solid fuel indoors, breathes air quite unlike the city average — and is usually the household least able to move away from it.
4.1.4 Ambient Air Quality Monitoring: Gaseous and Particulate
A distinction runs through all air monitoring. Source monitoring measures what a chimney or exhaust is emitting, at the point of emission, and answers the question “is this plant complying with its emission limit?” Ambient air quality monitoring measures the general outdoor air that people and crops are actually exposed to, away from any single source, and answers the question “what is the state of the air here?”
(Ambient air is the general outdoor air of a locality, at a place and height where people are normally exposed to it, and not the air inside a stack, duct or enclosed space.)
Ambient air carries pollutants in two physical forms, and the two require entirely different sampling arrangements — which is why the syllabus treats them separately.
Gaseous pollutants are individual molecules mixed among the molecules of air: sulphur dioxide, oxides of nitrogen, carbon monoxide, ozone, ammonia, and volatile organic compounds such as benzene. They cannot be filtered out, because they pass through any filter as freely as the air does. To measure them, a known volume of air must be drawn through a liquid or solid medium that absorbs or reacts with the particular gas, or passed through an instrument that detects the gas by a physical property such as its absorption of infrared light.
Particulate pollutants are solid particles or liquid droplets suspended in air: soot, road dust, fly ash, sea salt, pollen, and the fine sulphate and nitrate particles formed in the atmosphere. These are trapped physically. A known volume of air is drawn through a filter paper, and the increase in the filter’s mass gives the mass of particulate matter collected. Dividing by the volume of air sampled gives the concentration in µg/m³.
\[C = \frac{m}{V}\]
where \(C\) is the concentration of particulate matter in µg/m³, \(m\) is the mass of material collected on the filter in micrograms (µg), and \(V\) is the volume of air drawn through the filter in cubic metres (m³).
Particulate matter is classified by size, because size decides where a particle goes when it is inhaled. Larger particles are caught in the nose and upper airways. Particulate matter of aerodynamic diameter 10 micrometres or less (PM₁₀), also called respirable suspended particulate matter, passes into the lower airways. Particulate matter of aerodynamic diameter 2.5 micrometres or less (PM₂.₅) penetrates deep into the lungs. This is why total suspended dust is a poor measure of risk on its own, and why standards specify size fractions.
how small is PM₂.₅? A human hair is roughly 50–70 µm across. A PM₁₀ particle is about a seventh of that width, and a PM₂.₅ particle about a twenty-fifth. Neither is visible individually; a haze is what millions of them look like together.
Because the two forms are collected differently, they are also sampled for different lengths of time. A gaseous sample is typically collected over a shorter period and repeated through the day; a particulate sample must be drawn for many hours to deposit enough mass on the filter to be weighed reliably. The detail of both is taken up in section 4.5.
On three different days, note down the published air quality reading for your town or the nearest city that reports one, together with the date, the pollutant named, its unit, and the averaging period given. Then write one sentence on what changed between the three days — the weather, the day of the week, or something else you observed.
4.2 Carbon Credits and Carbon Trading
Sulphur dioxide and particulate matter do their damage close to where they are released, which is why they are regulated by fixing a limit on each chimney and each exhaust pipe. Carbon dioxide behaves differently. A tonne of it released in Jaipur, Johannesburg or Jakarta mixes into the same atmosphere and has the same effect on the global heat balance. It causes no local illness and no local corrosion.
That difference changes the logic of regulation. If the harm does not depend on where the tonne is emitted, then it also does not matter where it is prevented. What matters is only the total. This is the idea on which carbon credits and carbon trading rest: fix the total, and then let the reductions happen wherever they are cheapest to achieve.
4.2.1 What a Carbon Credit Is
A carbon credit is a tradable certificate or permit that represents the right to emit one tonne of carbon dioxide, or an amount of another greenhouse gas having the same warming effect.
(A carbon credit is a tradable permit or certificate representing the right to emit one tonne of carbon dioxide equivalent. It can be held, surrendered against actual emissions, or sold to another party.)
The phrase carbon dioxide equivalent (CO₂e) is doing real work in that definition. Greenhouse gases differ greatly in how strongly and how long they warm the atmosphere; a tonne of methane traps far more heat over a given period than a tonne of carbon dioxide. To keep a single accounting unit, each gas is converted into the mass of carbon dioxide that would have the same warming effect. All credits are then denominated in tonnes of CO₂e, and gases as different as methane, nitrous oxide and the industrial fluorinated gases can be handled on one ledger.
how much is one tonne of carbon dioxide? Roughly the amount released by burning about 430 litres of petrol — a little over four hundred litres of fuel, represented by a single certificate. It is a unit of accounting, not a small quantity.
Credits reach the market in two ways, and the distinction matters because their reliability differs.
Table 4.2 – Allowance-based credits vs offset credits | Feature | Allowance-based credits | Offset credits | |—|—|—| | Where they come from | Issued by a regulator up to the total permitted emission | Earned by a project that reduces or avoids emissions | | What they represent | Permission to emit, already counted within the cap | A reduction claimed to have occurred somewhere else | | Basis of the quantity | A number fixed by the regulator | A comparison with an estimate of what would otherwise have happened | | Ease of verification | Straightforward — the total issued is known | Difficult — the comparison is with an outcome that did not occur | | Main risk | The cap may be set too generously | The claimed reduction may not be additional, or may not last |
4.2.2 How Trading Regulates Emissions
Carbon trading is the buying and selling of these credits within a system in which total emissions have been capped. The mechanism, usually called cap and trade, works in four steps, shown in Fig. 4.2-1.
First, a regulating authority fixes a cap: the total quantity of greenhouse gas that all the regulated sources together may emit in a period. Second, credits equal to that total are issued — allocated free, or sold by auction. Third, each source must, at the end of the period, surrender credits matching its actual emissions. Fourth, sources may trade credits among themselves.
(Carbon trading is a market-based approach to emission control in which a total emission limit is fixed and divided into tradable credits, allowing regulated sources to buy or sell those credits so that the required reduction is achieved at the lowest total cost.)
Why the trading step matters is best seen with numbers. Suppose two plants are together required to cut 100 tonnes of CO₂e. Reducing emissions costs Plant A ₹800 per tonne, because it can retune an old boiler; it costs Plant B ₹2,000 per tonne, because it would have to replace equipment.
If each is simply ordered to cut 50 tonnes, the total cost of the reduction is
\[(50 \times 800) + (50 \times 2000) = 40{,}000 + 1{,}00{,}000 = ₹1{,}40{,}000\]
Now allow trading. Plant A cuts all 100 tonnes at a cost of
\[100 \times 800 = ₹80{,}000\]
and sells its 50 surplus credits to Plant B at, say, ₹1,200 each. Plant B pays ₹60,000 for the credits — less than the ₹1,00,000 it would have spent cutting the emissions itself — while Plant A receives ₹60,000 for work that cost it ₹40,000.
The atmosphere sees exactly the same 100-tonne reduction in both cases. But the real resources used up to achieve it fall from ₹1,40,000 to ₹80,000, and both plants are better off than under the fixed order. That is the whole argument for trading: because the cap is fixed, the environmental outcome is fixed, and the market only decides who does the cutting.
Alongside these compliance markets there are voluntary markets, in which firms and individuals buy credits they are under no legal obligation to hold, in order to offset emissions they have chosen to account for. Here there is no cap at all, and the environmental value of the purchase rests entirely on whether the underlying project is genuine.
4.2.3 What Carbon Markets Can and Cannot Do
The case for carbon markets is that they place a price on something that was previously free. Where emitting costs nothing, there is no financial reason to avoid it; where each tonne carries a price, every investment decision — which boiler to buy, which fuel to burn, whether to insulate a building — has that price built into it. Supporters add that the search for cheap reductions channels finance and technology towards places where emissions can be cut most cheaply, which are often lower-income countries.
The criticisms are equally substantial, and a student should be able to state them precisely rather than as general approval or disapproval.
The first concerns the cap. The environmental outcome depends entirely on how tight it is. A generous cap produces a low credit price, a busy market and almost no reduction. Several early trading systems issued more credits than the emissions that actually occurred, and prices fell close to zero.
The second concerns additionality, and applies to offset credits. A credit is supposed to represent a reduction that would not have happened anyway. But that requires comparing the project with a hypothetical alternative — what the emissions would have been without it — which cannot be observed and must be estimated. If the plant would have been built efficiently in any case, the credit is real on paper and empty in the atmosphere.
The third is permanence and verification. A reduction claimed from a tree-planting project depends on the trees remaining standing for decades. Checking such a claim, year after year, requires an inspection system whose cost and independence are themselves in question.
The fourth is about local pollutants. Trading is defensible for carbon dioxide precisely because location does not matter. It is not defensible for sulphur dioxide, particulates or heavy metals, where an unchanged emission concentrated in one locality harms the people living there, whatever is happening elsewhere. A plant that buys carbon credits instead of replacing an old boiler goes on emitting its local pollutants too.
There is also a distributional question that is easy to miss inside the arithmetic. The party that pays for a credit and the party that lives beside the emission are rarely the same. A market can deliver the cheapest total reduction and still leave the burden of what continues to be emitted concentrated in a few localities — which is why carbon trading is treated everywhere as a complement to direct regulation of local air quality, not a substitute for it.
4.3 Vehicular Pollution and Its Regulation
Stand at a busy crossing in any Indian city in the late afternoon and you are standing inside a chemical reactor. Every stopped engine is releasing carbon monoxide and unburnt fuel vapour; every acceleration from the lights releases oxides of nitrogen; the wheels are lifting dust that settled on the road overnight. The mixture is at its densest exactly where people are — at breathing height, on a footpath, in the middle of a settlement.
This is what makes vehicular pollution a distinct regulatory problem. An industrial estate can be placed away from housing and its chimneys made tall enough to disperse what they emit. Vehicles cannot be moved away from people, because carrying people is what they are for. Their emissions are released at ground level, in the places of greatest exposure, from millions of separate outlets that no inspector can visit.
4.3.1 Causes of Excessive Vehicular Pollution
Vehicles pollute for several distinct reasons, and it is worth separating them, because each has a different remedy.
The number of vehicles. The most straightforward cause is growth in the fleet. Where the number of vehicles doubles, the improvement achieved by making each vehicle cleaner can be wiped out entirely. This is why tightening standards on new vehicles reduces emissions per kilometre while total emissions in a city may still rise.
Incomplete combustion. An engine is designed to convert fuel into carbon dioxide and water. It never does so completely. Where the air supply is insufficient or the mixture burns unevenly, carbon monoxide and unburnt hydrocarbons leave the tailpipe. Where combustion is hot and efficient, less of these appear but more oxides of nitrogen do, because at high temperature the nitrogen and oxygen of the air itself combine. The two families of pollutant therefore pull in opposite directions, and engine design is a compromise between them.
Fuel quality. The fuel decides what the exhaust can contain. Sulphur in diesel or petrol emerges as sulphur dioxide and as fine sulphate particles; it also poisons the catalyst in an exhaust after-treatment device, so a vehicle designed for clean fuel will not stay clean on dirty fuel. Lead, once added to petrol to improve its burning quality, was emitted directly into city air until it was removed. Adulteration — mixing cheaper solvents or kerosene into petrol or diesel — undoes fuel standards entirely and is difficult to detect at the pump.
Engine type and age. Older engine designs pollute far more per kilometre. The two-stroke engines once common in three-wheelers and scooters burn a petrol–oil mixture and emit heavy smoke and unburnt hydrocarbons. Beyond design, every engine deteriorates: worn rings, a clogged air filter, injectors out of adjustment and a failed catalytic converter all raise emissions steeply while the vehicle still runs perfectly well as far as the driver can tell.
Traffic conditions. Emissions per kilometre depend strongly on how a vehicle is driven. A vehicle crawling in congestion, repeatedly stopping and starting, burns far more fuel for each kilometre covered than the same vehicle at a steady moderate speed. Congestion is therefore not only a consequence of vehicle numbers but an independent multiplier of their emissions.
Sources other than the tailpipe. Fuel evaporates from tanks and during refuelling, releasing hydrocarbons without any combustion at all. Tyres and brakes shed particles. Traffic lifts road dust back into the air, which in dry Indian cities can rival the tailpipe as a source of coarse particulate matter. Fig. 4.3-1 sets these pathways out together.
The pollutants involved were introduced in section 4.1. What is worth noticing here is that vehicles supply both the primary pollutants of the street — carbon monoxide, particulate matter, oxides of nitrogen — and, through their hydrocarbons and oxides of nitrogen, the precursors of the secondary pollutants that form over the city later in the day.
4.3.2 Pollution-Emission Standards for New Vehicles
The first line of regulation is applied before a vehicle is ever sold. A mass emission standard fixes the maximum quantity of each pollutant that a new vehicle model may emit, expressed as a mass per distance driven — grams per kilometre (g/km) — measured over a prescribed test cycle on a laboratory rig that simulates driving. A model that fails is not approved for sale. Because the requirement falls on the manufacturer at the design stage, it is enforceable at a few hundred points instead of a few million.
Bharat Stage emission standards India regulates new vehicles through the Bharat Stage norms, introduced in the 1990s and modelled on the European emission standards, each stage tightening the permitted grams per kilometre of carbon monoxide, hydrocarbons, oxides of nitrogen and particulate matter. Bharat Stage IV applied across the country from 2017, and the country then moved directly to Bharat Stage VI from April 2020, skipping the intervening stage. The move required a matching change in fuel: Bharat Stage VI petrol and diesel carry a maximum sulphur content of 10 parts per million, against 50 parts per million earlier, because the after-treatment devices the standard demands cannot survive higher sulphur.
Two points about such standards deserve emphasis. First, a standard on new vehicles takes effect only as the fleet turns over; vehicles already on the road continue under the rules they were built to, so the full benefit arrives a decade or more after the rule. Second, the standard governs a test cycle, and a vehicle’s behaviour in real traffic — cold starts, congestion, overloading, a badly maintained engine — can differ substantially from its behaviour on the rig.
4.3.3 The CNG Programme
Changing the fuel changes the exhaust more fundamentally than any adjustment to the engine. Compressed Natural Gas (CNG) is natural gas, largely methane, stored in cylinders at high pressure. Because it is a light gaseous hydrocarbon containing almost no sulphur and no heavy compounds, its combustion produces very little particulate matter and almost no sulphur dioxide, and less carbon monoxide and fewer unburnt heavy hydrocarbons than petrol or diesel.
Table 4.3 – Petrol, diesel and CNG as vehicle fuels | Feature | Petrol | Diesel | CNG | |—|—|—|—| | Physical form | Liquid | Liquid | Gas, stored compressed in heavy cylinders | | Particulate emission | Low to moderate | High, and largely fine particles | Very low | | Sulphur dioxide emission | Depends on fuel sulphur content | Depends on fuel sulphur content | Negligible | | Oxides of nitrogen | Moderate | High | Moderate — not eliminated | | Carbon monoxide and unburnt hydrocarbons | Moderate | Lower | Lower, though some unburnt methane escapes | | Main practical constraint | — | — | Requires pipeline supply, filling stations and cylinder space in the vehicle |
the conversion of Delhi’s public transport fleet to CNG, from the late 1990s Context: By the 1990s Delhi’s air carried very high levels of particulate matter and sulphur dioxide. A large share came from diesel buses and from two-stroke three-wheelers and taxis, all operating in dense traffic at ground level, in a city where winter conditions regularly trap pollutants near the surface. What happened: Public interest litigation before the Supreme Court of India led to directions requiring the city’s buses, three-wheelers and taxis to run on compressed natural gas rather than diesel or petrol. Response: The conversion was carried out over several years around the turn of the century. It required more than a change of vehicle: gas had to be brought to the city by pipeline, filling stations built, cylinders fitted and certified, and drivers and mechanics trained. Old two-stroke three-wheelers were withdrawn in the same period. Supply was the binding constraint throughout, and queues at the early filling stations were long. Outcome: Emissions of particulate matter and sulphur dioxide from the converted fleet fell sharply, and the programme is widely cited as evidence that a fuel switch can be carried out at city scale. Its effect on the city’s overall air quality was smaller and shorter-lived than expected, because the number of private cars and two-wheelers continued to grow rapidly, and because CNG combustion does little to reduce oxides of nitrogen. How much of the later deterioration reflects those two factors, and how much reflects sources outside the transport sector altogether, remains contested.
4.3.4 Inspection and Maintenance of In-Use Vehicles
A standard applied to a new vehicle says nothing about that vehicle five years later. An inspection and maintenance programme addresses the vehicles already on the road, by testing them periodically and requiring repair where they fail.
In India this takes the form of the Pollution Under Control certificate. A petrol vehicle is tested at idle for the concentration of carbon monoxide in its exhaust, expressed as a percentage by volume, and for unburnt hydrocarbons in parts per million; a diesel vehicle is tested for smoke density, measured by how much light the exhaust plume blocks. A vehicle within the prescribed limits is issued a certificate valid for a fixed period.
The reasoning is sound. A small proportion of badly maintained vehicles typically accounts for a disproportionate share of total emissions, and returning them to reasonable condition is far cheaper than replacing them. A driver has no other way of learning that the engine has drifted out of tune, because fuel consumption changes slowly and the exhaust is invisible.
The limitations are equally real and should be stated plainly. The test is a snapshot taken at idle, and idle behaviour is a poor guide to emissions under load. It measures carbon monoxide, hydrocarbons and smoke, but not oxides of nitrogen, which are the pollutants of most concern from modern diesel engines. A vehicle can be tuned to pass and adjusted back afterwards. And where testing centres are numerous and lightly supervised, the certificate can become a document obtained rather than a test passed. An inspection programme is only as good as the integrity of the testing network behind it.
4.3.5 Phasing Out Old Commercial Vehicles
Commercial vehicles — buses, trucks, taxis, three-wheelers — matter out of proportion to their numbers, for two reasons. They cover very high distances each year, so each vehicle’s annual emission is large. And they are kept in service far longer than private vehicles, so the oldest and dirtiest technology survives longest in exactly the class that runs the most kilometres.
Phasing out therefore means setting an age beyond which a commercial vehicle may not operate, or may not be re-registered, in a given area, usually accompanied by a scrappage arrangement so that the vehicle is dismantled rather than sold on to a smaller town. The effect is quick, because it does not wait for a fleet to turn over naturally.
An age limit falls on a specific group of people. A truck or an auto-rickshaw is usually the single largest asset an owner-driver possesses, and its residual value is the family’s security. A phase-out that reaches the environmental goal by transferring the loss to the smallest operators solves one problem by creating another, which is why such schemes are generally paired with compensation, credit for replacement, or a transition period — and why the design of that support is as much part of the policy as the age limit itself.
4.3.6 Promotion of Public Transport
The measures so far reduce the emission of each vehicle-kilometre travelled. The last measure reduces the number of vehicle-kilometres that need to be travelled at all, by moving the same people in fewer vehicles.
The arithmetic is decisive. Take a bus that emits about 900 g of carbon dioxide per kilometre and carries 45 passengers, and a car that emits about 150 g per kilometre and carries, as is typical, 1.5 people. Per passenger-kilometre:
\[\text{bus} = \frac{900}{45} = 20 \ \text{g per passenger-km}\]
\[\text{car} = \frac{150}{1.5} = 100 \ \text{g per passenger-km}\]
The bus emits five times more than the car per kilometre of road, and one-fifth as much per person moved. The figures are illustrative rather than exact, but the ratio is robust, and it holds for particulates and oxides of nitrogen as well as for carbon dioxide. It also explains why the same bus, running nearly empty at midday, offers no advantage at all — the whole benefit of public transport depends on occupancy.
Promotion of public transport therefore has two halves. Supply: more buses, rail-based systems where passenger volumes justify the capital cost, dedicated lanes so that buses are not delayed by the congestion they are meant to relieve, and attention to the walk at each end of the journey, since a route that begins with a kilometre on an unlit road without a footpath will not attract passengers. Demand: measures that make the alternative less attractive, such as parking charges or restricted access to central areas.
There is an honest complication, and it belongs in the account. Where road capacity is freed by people shifting to buses and nothing else changes, the free space attracts new car trips that were previously not worth making. Congestion returns towards its earlier level. This is why transport specialists treat improved public transport and restraint on private vehicle use as two halves of one policy, each of which underperforms without the other.
For one journey you make regularly, work out the distance and estimate how many people travel in the vehicle you use. Then find out whether a shared or public option covers the same route, how long it takes, and where it starts and ends relative to your door. Write four lines on what would have to change for the shared option to be the reasonable choice — cost, time, frequency, or the walk at either end.
4.4 Monitoring the Atmosphere: Designing a Monitoring Network
Suppose you were given a budget for four monitoring stations and asked to describe the air of a city of two million people. Where would you put them? Beside the busiest road, where the readings will be highest? In a quiet colony, where most people actually live? Next to the industrial estate, or upwind of it? How often would each station take a reading, and for how long each time?
None of these questions has a single correct answer, but none of them can be avoided either, because the answers decide what the data will mean. A network is not a collection of instruments; it is a design, and every number it produces carries the design’s assumptions inside it.
4.4.1 Monitoring at the Emission Source and of Ambient Air
The first decision is what is being measured. Source monitoring, also called stack monitoring, measures the gas inside a chimney or duct, at the point where it leaves the plant. Ambient air quality monitoring, introduced in section 4.1, measures the general outdoor air that people breathe.
The two are answering different questions, and neither substitutes for the other. Source monitoring establishes whether a particular plant is complying with the limit set for it, and it can attribute what it finds to a known party. Ambient monitoring establishes what the air of a place is actually like, but it cannot by itself say who is responsible for it, because what arrives at the station is a mixture from every source in the district plus whatever the wind has brought from outside.
Table 4.4 – Source monitoring vs ambient air quality monitoring | Feature | Source monitoring | Ambient air quality monitoring | |—|—|—| | Where the sample is taken | Inside the stack or duct, at the point of emission | In the open air, at a height at which people are exposed | | Condition of the gas sampled | Hot, moving fast, undiluted, high concentration | At air temperature, dilute, low concentration | | Usual reporting unit | Mass per volume of flue gas, mg/Nm³, or mass per unit output | Micrograms per cubic metre, µg/m³ | | Question answered | Is this plant within its emission limit? | What is the quality of the air in this locality? | | Attribution to a source | Direct — the source is known | Indirect — requires wind data and modelling |
Sampling inside a stack has a technical requirement worth naming, because it explains why the work is specialised. The gas is moving quickly, and if the sampling probe draws air in faster or slower than the surrounding stream is flowing, particles of different sizes are captured in the wrong proportions — heavier particles carry on past a slow probe, and are over-collected by a fast one. Sampling must therefore be isokinetic, meaning the velocity into the probe is matched to the velocity of the gas stream. Ambient sampling faces no such problem, because the air is barely moving relative to the sampler.
4.4.2 Criteria for Siting Monitoring Stations
The governing idea is representativeness. A site is representative if the concentrations recorded there reflect the air of the area the station is meant to describe, and not some local peculiarity of the spot itself.
Several practical requirements follow from this, and they are laid down in national sampling standards.
Free flow of air around the intake. The sampler must not sit in a confined space, a corner, a courtyard, or under or above a balcony, where air stagnates or eddies. Air must reach it from every direction.
Distance from local sources. The intake must be far enough from any single chimney, generator or busy junction that it is not simply measuring that source — unless measuring that source is the station’s declared purpose. There must also be clearance from trees and from absorbing building surfaces, which take up gases and shed dust and so alter what the instrument sees.
Height of the intake. The probe is placed at a height representing the breathing zone rather than the ground surface. For monitoring beside a road, an intake around three metres above street level is used: high enough to avoid re-collecting dust thrown up by wheels immediately below and to be out of reach of interference, low enough to represent what a person on the footpath breathes.
Distance from unpaved surfaces. Sampling close to unpaved roads produces readings dominated by dust raised by passing vehicles rather than by the pollution being studied, so a substantial separation is prescribed.
Practicability and continuity. The site needs a reliable electricity supply, physical security for costly instruments, shelter from extreme weather, and access for the staff who change filters and reagents. Above all, it must be available for many years. Trend detection is the main purpose of monitoring, and a trend is destroyed if the station has to move, or if the land use around it changes so that the same instrument is describing a different place.
4.4.3 Types of Stations
Stations are classified by what they are meant to represent, because the same instrument at two different sites answers two different questions.
A kerbside or traffic station sits close to a busy road and records the highest exposures experienced by pedestrians, traffic police and street vendors. Its readings are not the city’s average and are not meant to be.
An urban background station sits away from any individual road or plant — in a park, a residential colony, on a school roof — and records what the general population of the city breathes. These stations carry most of the weight in describing a city.
An industrial station is placed in or downwind of an industrial area, to record the contribution of that activity to local air.
A rural or background station sits well outside the urban area, in a place with no traffic and no industry, and records the air arriving at the city from outside. Without it, there is no way to tell how much of an urban reading is generated locally and how much has simply blown in.
A survey or source-oriented station is set up temporarily and close to a specific source, to answer a specific question about that source’s impact, and is dismantled when the question is answered.
4.4.4 Number and Distribution of Stations
How many stations a city needs depends on how much the pollutant varies across it. A pollutant emitted by traffic varies sharply over a few hundred metres, so describing it well takes many stations. Fine particles that have travelled a long way vary gently across a whole city, so a few will do.
In practice the number is decided from the area to be covered, the size and distribution of the population, the land-use pattern, the known distribution of sources, and — inescapably — the resources available, since each station costs money to build and much more to run for a decade.
Their placement is guided by what is already known about the air. Where a previous survey or a dispersion model has produced isopleths — lines on a map joining points of equal concentration, in the way that contour lines join points of equal height — the map shows directly where concentrations are high, where they change steeply, and where a station would add most information. Where no such map exists, emission densities, land use and the prevailing wind direction are used to identify the areas of expected high concentration.
Two principles hold whatever the number. Stations must cover the range of conditions, not only the worst spots, or the network will describe a city that does not exist. And the network must include at least one site upwind of the built-up area, since without it the local contribution cannot be separated from what arrives from elsewhere.
4.4.5 Frequency and Duration of Data Collection
Two separate quantities are being fixed here. Duration is how long each individual sample is collected for; frequency is how often such a sample is taken.
Duration is decided by the standard the data will be compared against. Standards are written for specified averaging periods — commonly a 24-hour average and an annual average — and a measurement is meaningful only against a standard with the same period. A one-hour reading cannot be compared with a 24-hour standard.
Frequency is decided by the need to represent every kind of condition. Concentrations differ between weekdays and Sundays, between winter and monsoon, between calm nights and windy afternoons. A sampling schedule must spread its samples across days of the week and across the seasons, or the annual average it produces will be biased by whatever conditions happened to be over-sampled.
The manual network operated in India under the National Air Monitoring Programme, described in section 4.6, samples on this principle: 24-hour samples are collected twice a week at each station, with gaseous pollutants sampled in four-hourly steps and particulate matter in eight-hourly steps within that period. Twice a week across a full year gives
\[2 \times 52 = 104 \ \text{observations per station per year}\]
spread evenly across all seasons — enough to compute a defensible annual average from a manual network, though far less detail than a continuous analyser provides.
Continuous instruments change this arithmetic completely, reporting an average every few minutes and so capturing short peaks that a twice-weekly sample would miss entirely. They also cost far more to buy, calibrate and maintain, which is the reason both kinds of network exist side by side.
Q3. Who decides where a monitoring station is placed, how many a city needs, and how often it should take readings?
A. The regulating authority does, working to national sampling standards. Placement follows the criterion of representativeness — free flow of air around the intake, correct height, prescribed clearances from roads, trees and buildings, and long-term availability of the site. Numbers follow from the area, the population, the land-use pattern, how sharply the pollutant varies across the city, and the resources available. Frequency and duration follow from the averaging periods of the standards the data will be compared against, and from the need to sample all days of the week and all seasons.
4.4.6 Characteristics of Ambient Air Sampling
Whatever the technique, an ambient air sample has to satisfy a few basic conditions, and they are worth stating briefly because every method in the next section is built to meet them.
The sample must be representative of the air at that place and time — drawn from freely moving outdoor air, at the correct height, with nothing between the intake and the open sky.
The volume of air sampled must be known accurately. Every concentration is a mass divided by a volume, so an error in the measured volume passes directly into the result. This is why samplers carry a flow-measuring device and why the flow rate is checked at the start and end of each run.
The flow rate must be steady and appropriate. Too fast, and a bubbler’s absorbing solution cannot capture the gas passing through it; too slow, and too little material is collected to measure.
The sampling period must be long enough to collect a measurable quantity, and must match the averaging period of the standard.
The pollutant must not be lost or altered between the intake and the collecting medium. Reactive gases can be adsorbed on the walls of a long sampling line, and particles can settle in it, so lines are kept short and made of inert material.
Blanks and calibration must be run. A blank — a filter or reagent carried to the site and handled identically but with no air drawn through it — reveals contamination from handling and transport, which is then subtracted. Instruments are calibrated against gases of known concentration, since an uncalibrated reading is a number without a scale.
Meteorological data must be recorded alongside. Wind speed and direction, temperature, humidity and rainfall are what make a concentration interpretable. Without them, a low reading cannot be distinguished from a windy day, and a high one from a still one.
4.6 Air Quality Standards and the Institutions Behind Them
Sections 4.4 and 4.5 produced numbers. A number on its own decides nothing. To say that yesterday’s particulate concentration was 96 µg/m³ is to say very little until there is something to compare it against — a figure agreed in advance, on stated grounds, for that pollutant over that averaging period. That agreed figure is an air quality standard, and the institutions that set it, monitor against it and act on it are the subject of this section.
4.6.1 What an Air Quality Standard Is
An air quality standard is a legally notified limit on the concentration of a specified pollutant in ambient air, averaged over a stated period, set at a level intended to protect human health, vegetation and property with a margin of safety.
(An air quality standard is the maximum permissible concentration of a specified pollutant in ambient air, averaged over a stated period, notified by the competent authority as the level below which the air is regarded as acceptable for the protection of human health, vegetation and property.)
Three elements of that definition do the work, and dropping any of them makes the standard meaningless.
The pollutant must be specified, because there is no such thing as a standard for “pollution” in general. Each substance behaves differently in the body and in the atmosphere, and each needs its own limit.
The averaging period must be stated. A limit of 80 µg/m³ means nothing until one knows whether it applies to an hour, a day or a year. Short-period standards protect against acute episodes — the still winter night when concentrations climb; long-period standards protect against the cumulative effect of years of moderate exposure. Most pollutants need both, and the short-period limit is always the higher number, because brief peaks are tolerable in a way that a permanent level is not.
The margin of safety reflects an uncomfortable scientific fact. For most air pollutants no threshold has been identified below which effects are certainly zero; risk declines as concentration declines, without disappearing at any particular value. A standard is therefore not the boundary between poison and safety. It is a judgement about what level of residual risk is accepted, taking into account what is achievable, and it is periodically revised as evidence accumulates and as control becomes more feasible.
Why standards matter
Standards convert monitoring into administration. They give a monitoring network something to test against, so that data becomes a finding of compliance or exceedance rather than a table of figures. They give a uniform yardstick, so that one city’s air can be compared with another’s and one year with the next. They allow priorities to be set: the pollutant and the area furthest from the standard is where control effort earns most. They provide the legal basis on which authorities can require abatement, refuse consent to a polluting activity, or decide where an industry may be sited. And they give the public a plain criterion for judging what is being done — the reason air quality data is now published daily in many countries.
4.6.2 Objectives of Air Quality Standards
The objectives are usually stated in four parts, and they follow from the discussion above.
To indicate the level of air quality necessary, with an adequate margin of safety, to protect public health, vegetation and property. This is the primary purpose: to express in a number what the air ought to be like.
To assist in establishing priorities for abatement and control. Comparing measured concentrations against standards shows which pollutants and which localities are worst placed, and therefore where limited money and enforcement capacity should go first.
To provide a uniform yardstick for assessing air quality at the national level. Without common limits and common averaging periods, results from different states could not be compared or aggregated.
To indicate the need for, and extent of, monitoring programmes. The standards decide what must be measured, at what averaging period, and hence what kind of network the country needs — which is why this section follows the two that describe how monitoring is done.
4.6.3 World Health Organization Guidelines and National Standards
A distinction that students frequently blur is between a guideline and a standard.
The World Health Organization publishes global air quality guidelines. These are recommendations derived from the epidemiological and toxicological evidence on health effects, and they answer only one question: at what concentration does the evidence indicate the risk becomes small? They consider health alone. They are not law anywhere, and the Organization publishes interim targets alongside them precisely because the guideline values are far below what many countries can currently achieve.
A national standard is a different kind of object. It is notified under a country’s own law, is enforceable, and is arrived at by weighing the health evidence against what is technically and economically attainable within a reasonable period. National standards for gases and particulate matter are framed with the World Health Organization guidelines as their scientific reference point, but they are generally set at higher concentrations, and they are tightened as control capacity improves. The gap between the two is not a scandal to be exposed nor a detail to be hidden; it is the visible record of a judgement about feasibility, and the direction of travel over the decades has been steadily downward on both sides.
India’s national ambient air quality standards are notified by the Central Pollution Control Board under the Air (Prevention and Control of Pollution) Act, 1981. They were first adopted in the early 1980s and have been revised since — most substantially in 2009, when the earlier separate limits for industrial areas were removed, the number of pollutants covered was extended to twelve, and PM₂.₅ was included for the first time. Two area categories are now used: one covering industrial, residential, rural and other areas, and a stricter one for ecologically sensitive areas notified as such.
Table 4.6 – Selected Indian national ambient air quality standards, in µg/m³ except where stated, for industrial, residential, rural and other areas | Pollutant | Averaging period | Limit | |—|—|—| | Sulphur dioxide (SO₂) | Annual | 50 µg/m³ | | Sulphur dioxide (SO₂) | 24 hours | 80 µg/m³ | | Nitrogen dioxide (NO₂) | Annual | 40 µg/m³ | | Nitrogen dioxide (NO₂) | 24 hours | 80 µg/m³ | | Particulate matter, PM₁₀ | Annual | 60 µg/m³ | | Particulate matter, PM₁₀ | 24 hours | 100 µg/m³ | | Particulate matter, PM₂.₅ | Annual | 40 µg/m³ | | Particulate matter, PM₂.₅ | 24 hours | 60 µg/m³ | | Ozone (O₃) | 8 hours | 100 µg/m³ | | Carbon monoxide (CO) | 8 hours | 2 mg/m³ | | Lead (Pb) | Annual | 0.5 µg/m³ |
Standards are revised from time to time, and the values above should always be checked against the current notification before being used for any actual assessment.
Read the table by columns rather than by rows. Every pollutant appears with a period attached, and the 24-hour limit is consistently higher than the annual one, for the reason given in section 4.6.1. Notice also that carbon monoxide is expressed in mg/m³, not µg/m³ — it is present at concentrations a thousand times greater than the others, so reporting it in micrograms would produce unwieldy four- and five-figure numbers. Lead sits at the opposite extreme, at 0.5 µg/m³, because a heavy metal that accumulates in the body is dangerous at concentrations at which a common gas is harmless.
Compare these with the World Health Organization guideline values, which were revised downward in 2021 on the strength of evidence that harm continues at concentrations previously thought acceptable: an annual guideline of 5 µg/m³ for PM₂.₅ and 15 µg/m³ for PM₁₀, and 10 µg/m³ for nitrogen dioxide. The Indian annual standard for PM₂.₅ is therefore several times the guideline figure, as Fig. 4.6-1 shows.
4.6.4 National Ambient Air Quality Monitoring and the National Air Monitoring Programme
Standards require a network to test them against. The Central Pollution Control Board began building one in the mid-1980s, under the name National Ambient Air Quality Monitoring (NAAQM). It was later renamed the National Air Monitoring Programme (NAMP), the name under which it now runs, and it was progressively expanded to cover several hundred operating stations spread over cities and towns in most states and Union Territories.
Its design reflects everything in sections 4.4 and 4.5. It is built mainly on manual sampling — High Volume Samplers with gaseous attachments and bubbler systems — because that is what allows a network of that size to be operated at all. Each station is sampled for 24 hours twice a week, giving of the order of a hundred sets of observations in a year, spread across the seasons. The pollutants measured routinely at all stations are sulphur dioxide, oxides of nitrogen and particulate matter, with additional parameters including trace metals and polycyclic aromatic hydrocarbons monitored at selected locations, and meteorological data recorded alongside so that the readings can be interpreted against wind and weather. Operation is shared: the Board itself, the State Pollution Control Boards, Pollution Control Committees in the Union Territories, and a number of research institutes and universities run stations within the programme.
Objectives of the National Air Monitoring Programme
The programme states its purposes as follows, and each maps onto a use of data discussed earlier in this chapter.
To determine the present status of air quality and its trends across the country, so that improvement or deterioration can be established rather than asserted.
To identify cities and towns not meeting the standards, and to establish why the gap exists — which sources, which seasons, which conditions.
To control and regulate pollution from industry and other sources so that the standards are met, using the data as the basis for regulatory action.
To understand the natural cleansing processes of the atmosphere — dilution, dispersion, wind transport, dry deposition and chemical transformation — since how quickly an area clears itself determines how much emission it can absorb.
To provide background air quality data for planning purposes: deciding where industry may be located, and how a town should be laid out.
Q4. What exactly is an “air quality standard”, and which bodies lay one down in India?
A. An air quality standard is a legally notified maximum concentration for a named pollutant, averaged over a stated period, set to protect health, vegetation and property with a margin of safety. In India these national ambient air quality standards are laid down by the Central Pollution Control Board under the Air (Prevention and Control of Pollution) Act, 1981, and compliance with them is monitored through the National Air Monitoring Programme, operated by the Board together with the State Pollution Control Boards and Pollution Control Committees.
4.6.5 The Central Pollution Control Board
Central Pollution Control Board (CPCB) What it is: A statutory body of the Government of India, constituted in 1974 under the Water (Prevention and Control of Pollution) Act, 1974, and functioning as the technical wing of the Ministry of Environment, Forest and Climate Change. It also exercises powers and functions under the Air (Prevention and Control of Pollution) Act, 1981 and the Environment (Protection) Act, 1986, and has its head office in New Delhi with zonal offices and laboratories. Mission: To promote the cleanliness of streams and wells through the prevention, control and abatement of water pollution, and to improve the quality of air and prevent, control or abate air pollution in the country. How it works: It lays down national standards for ambient air quality and for effluents and emissions; operates and coordinates national monitoring programmes; collects, compiles and publishes technical and statistical data on pollution and prepares manuals and codes of practice; advises the Central Government on matters concerning pollution; carries out and sponsors investigation and research; organises training for people engaged in pollution control work; runs public awareness programmes through mass media; provides technical assistance and guidance to the State Boards and resolves disputes among them; and acts as the State Board for the Union Territories, where that function has largely been delegated to local administrations. Note: It is an apex technical and coordinating body rather than a police force. Day-to-day inspection and enforcement against individual polluting units rest largely with the State Boards, and the effectiveness of national standards therefore depends on capacity at the state level.
4.6.6 The State Pollution Control Boards
State Pollution Control Boards (SPCBs) What it is: Statutory bodies constituted by each state government under the same Acts, with the corresponding function in the Union Territories performed by Pollution Control Committees. Mission: To plan and execute programmes for the prevention, control and abatement of air and water pollution within the state, and to enforce the applicable standards there. How it works: It advises the state government on pollution matters and plans comprehensive state programmes; inspects sewage and trade effluent works, industrial plants and air pollution control equipment to assess their performance; lays down, modifies and enforces effluent and emission standards for discharges within the state, consistent with the national standards; grants, refuses and revokes the consent an industry requires to establish and to operate; identifies and recommends areas to be declared air pollution control areas; collects information on air and water pollution and conducts and participates in investigation and research; evolves economical and reliable methods for the treatment, utilisation and disposal of sewage and trade effluents, including their use in agriculture; and conducts awareness programmes through mass media. Note: The Board is the level at which regulation actually meets the polluter, which is also where it is most exposed to shortages of staff, laboratory capacity and legal support, and to the pressure that attaches to any decision affecting employment and investment in the state.
Table 4.7 – Central Pollution Control Board and State Pollution Control Board compared | Feature | Central Pollution Control Board | State Pollution Control Board | |—|—|—| | Constituted by | The Central Government, under the Water Act, 1974 | The respective state government, under the same Acts | | Area of operation | The whole country | One state | | Reports to | The Ministry of Environment, Forest and Climate Change | The state government | | Standard-setting role | Lays down national ambient air quality standards and general standards | Applies, and may make stricter, the standards within the state | | Main working role | Coordination, national monitoring, research, publication, technical guidance | Inspection, consent to establish and operate, enforcement against individual units | | Relation to the other | Provides technical assistance to State Boards and resolves disputes among them | Implements within the state and reports data upward |
The three statutes named in this section fit together in a simple sequence. The Water Act of 1974 created the Boards and gave them their water functions; the Air Act of 1981 extended them to air and introduced the consent system for air-polluting industry; and the Environment (Protection) Act of 1986, framed after the Bhopal gas disaster, provided broad umbrella powers to set standards and issue directions across all media.
4.7 Water Testing: Indicators of Water Quality
Water hides its condition better than air does. A canal carrying industrial effluent may run clear; a river in flood may look filthy and be biologically healthy; groundwater that has been pumped for thirty years may taste normal and carry nitrate at several times the safe level. Appearance, colour and smell are unreliable in both directions, which is why water quality is defined by a set of measured characteristics rather than by inspection.
No single measurement describes water. What is used instead is a set of indicators — individual physical, chemical and biological properties, each cheap to measure and each revealing something specific. Taken together, and read against what is known about the catchment, they build a picture. The skill this section teaches is not how to run each test, but how to interpret its result: what a high value implies, what a low value implies, and what the number cannot tell you.
(Water quality is the set of physical, chemical, biological and radiological characteristics of a water body, assessed in relation to the requirements of the organisms living in it or the human use to which it is to be put.)
That last clause is important and is often forgotten. Quality is judged against a purpose. Water perfectly acceptable for irrigation may be unfit to drink; water fit to drink may be too warm or too clear to support a particular fishery. There is no such thing as good water in the abstract.
4.7.1 Electrical Conductivity
Pure water is a poor conductor of electricity. What makes water conduct is the presence of dissolved ions — the charged particles released when salts such as sodium chloride, calcium bicarbonate or magnesium sulphate dissolve. The more ions present, the more freely current passes.
Electrical conductivity is therefore a measure of the total quantity of dissolved ionic material in water, reported in microsiemens per centimetre (µS/cm). It is measured in seconds with a probe, which is what makes it so useful: it gives an immediate indication of total dissolved solids without any chemical analysis at all.
What it means depends on context. Rainwater and clean mountain streams have low conductivity, often below 100 µS/cm. Ordinary river and groundwater in India commonly falls in the range of a few hundred to well over a thousand µS/cm, depending on the rocks and soils the water has passed through. A sudden rise along a river’s length points to something entering it — an effluent discharge, sewage, or drainage from irrigated land carrying dissolved salts. In coastal areas a steady long-term rise in the conductivity of well water is one of the first signs of saline intrusion, as seawater moves into an aquifer that over-pumping has drawn down.
Its limitation is that it is completely non-specific. Conductivity says how much dissolved ionic material is present; it says nothing about what that material is. Harmless calcium bicarbonate and toxic dissolved metals both raise it. It is a screening indicator, and a sharp change is a signal to investigate rather than a finding in itself.
4.7.2 Turbidity
Turbidity is the cloudiness of water caused by suspended particles — silt, clay, fine organic debris, algae, and microscopic organisms. It is measured not by weighing the particles but optically, by shining a beam of light through the sample and measuring how much is scattered sideways, and is reported in nephelometric turbidity units (NTU).
Turbidity rises in a river during heavy rain, when run-off carries soil from the catchment, and it rises where earth is exposed by construction, mining or the removal of vegetation. It also rises with a discharge of sewage or of any effluent carrying solids.
High turbidity matters for three separate reasons. It reduces the light penetrating the water, which limits photosynthesis by submerged plants and so affects the whole aquatic food chain. The suspended particles carry other pollutants adsorbed on their surfaces — metals, pesticides and nutrients travel through a river attached to sediment far more than dissolved in the water. And it interferes with disinfection: bacteria and viruses shelter within and behind particles, where chlorine cannot easily reach them, so turbid water cannot be reliably disinfected without first being filtered or settled. For this reason drinking water treatment aims to bring turbidity to about 1 NTU or below, and high turbidity is treated as a warning of possible microbial contamination even before any test for organisms is done.
4.7.3 pH
pH measures how acidic or alkaline water is. It is defined as the negative logarithm to base 10 of the hydrogen ion concentration:
\[\mathrm{pH} = -\log_{10}[\mathrm{H^+}]\]
where \([\mathrm{H^+}]\) is the concentration of hydrogen ions in moles per litre (mol/L). The scale runs from 0 to 14; a pH of 7 is neutral, values below 7 are acidic and values above 7 are alkaline.
pH is a logarithmic scale, not a linear one Each whole unit is a tenfold change. Water at pH 5 has ten times the hydrogen ion concentration of water at pH 6, and a hundred times that of water at pH 7. A drop of “only two pH units” is therefore a hundredfold change in acidity.
Most natural fresh waters lie between about pH 6.5 and 8.5, held there by dissolved carbonates and bicarbonates that resist change. Water draining forests and swamps can be naturally acidic, because organic matter releases weak organic acids. Departures from the natural range come from acid deposition, mine drainage, and industrial discharges of acids or alkalis.
pH matters far beyond the discomfort of acidic water. It controls the chemical form, and therefore the toxicity and mobility, of many substances. Most metals become more soluble in acidic water, so a fall in pH releases aluminium, lead and other metals from sediments and pipes into the water where organisms and people can take them up. Ammonia does the opposite: it becomes more toxic to fish as pH rises. Aquatic species themselves tolerate only a limited range, and the eggs and young stages are usually the most sensitive, so a change in pH can eliminate a species from a stretch of river without killing a single adult fish.
4.7.4 Dissolved Oxygen
Dissolved oxygen (DO) is the quantity of gaseous oxygen dissolved in water, reported in milligrams per litre (mg/L). It is the single most informative indicator of the health of a river, because almost every organism above the level of bacteria depends on it and because it responds quickly to organic pollution.
Water gains oxygen from the atmosphere at its surface — helped enormously by turbulence, which is why water is best aerated at rapids and weirs — and from photosynthesis by aquatic plants and algae during daylight. It loses oxygen to the respiration of all aquatic organisms and, above all, to the bacteria that decompose organic matter.
The maximum amount water can hold depends strongly on temperature: cold water holds considerably more dissolved oxygen than warm water. This inverse relationship, shown in Fig. 4.7-1, is why the same organic load is far more damaging to a river in May than in January, and why a discharge of warm water is itself a form of pollution.
Interpreting a dissolved oxygen figure is a matter of comparing it with saturation at that temperature. Water close to saturation indicates a healthy, well-aerated system. Values falling below roughly 5 mg/L put most fish under stress; sustained values close to zero indicate that decomposition has consumed everything available, and only organisms tolerant of very low oxygen survive there.
The classic pattern produced by a discharge of organic waste into a river is the oxygen sag curve of Fig. 4.7-2. Immediately downstream of the outfall, bacteria multiply on the incoming organic matter and consume oxygen faster than the river can absorb it, so dissolved oxygen falls steeply. Further downstream the organic matter has largely been consumed, reaeration at the surface catches up, and oxygen recovers. The depth of the sag and the distance needed for recovery together measure how heavy the load was relative to the river’s capacity.
4.7.5 Faecal Waste
The most immediately dangerous contamination of water is not chemical. Human and animal faecal matter entering a water body brings with it the organisms responsible for cholera, typhoid, hepatitis and the diarrhoeal illnesses that remain a major cause of death among young children worldwide.
Testing for each pathogen individually would be slow, expensive and unreliable, since any one of them may be absent from the sample while others are present. Water testing instead uses indicator organisms. These are bacteria that live in large numbers in the intestines of warm-blooded animals, are harmless in themselves, are easy and cheap to detect, and do not multiply appreciably in water. Their presence establishes that faecal material has entered, and therefore that pathogens may have entered with it. Coliform bacteria, and Escherichia coli in particular, are used this way, and results are reported as a count, or a most probable number, per 100 mL of sample.
The standard for drinking water is absolute rather than graded: E. coli should not be detectable in a 100 mL sample. There is no acceptable low level, because a single infective dose is a small number of organisms. For bathing and for surface waters, graded limits are used instead.
4.7.6 Temperature
Temperature is measured more easily than anything else in this list and is often treated as background information. It is in fact a controlling variable, because it governs the behaviour of nearly every other indicator.
It fixes how much oxygen the water can hold, as section 4.7.4 explained. It sets the rate of chemical reactions and of biological processes: bacterial decomposition, and therefore oxygen consumption, roughly accelerates with rising temperature, so warm water is depleted faster by the same organic load. It determines which species can live in the water at all, since most aquatic organisms tolerate only a narrow range, and it acts as a signal for life-history events — the cue for an insect to emerge or a fish to spawn, which arrives at the wrong time if the water is artificially warmed.
Natural variation comes from season, from time of day, from depth in a lake, and from position along a river. Human causes of change include discharge of cooling water from power stations and industry, discharge of warm effluent, removal of the shading vegetation along a bank, and run-off from hot paved urban surfaces. Where the effect is significant it is called thermal pollution, and it is unusual in that nothing has been added to the water at all.
4.7.7 Hardness
Hardness is the concentration of dissolved calcium and magnesium in water, conventionally expressed as the equivalent concentration of calcium carbonate in mg/L. It is acquired naturally as water percolates through limestone and other calcium- and magnesium-bearing rocks, which is why hardness is largely a matter of local geology rather than of pollution.
(Hardness is the total concentration of dissolved calcium and magnesium ions in water, expressed as the equivalent concentration of calcium carbonate (CaCO₃) in mg/L.)
It is distinguished into two kinds. Temporary hardness is due to dissolved bicarbonates, which decompose on boiling and are deposited as carbonate scale — the white crust in a kettle. Permanent hardness is due to sulphates and chlorides of calcium and magnesium, which boiling does not remove.
Hardness is not generally a health concern, and moderately hard water contributes usefully to dietary calcium and magnesium. Its significance is practical and economic. Hard water wastes soap, because calcium and magnesium react with it to form the insoluble scum that leaves a ring on a bucket. It deposits scale inside pipes, boilers, water heaters and industrial equipment, reducing flow and heat transfer and eventually requiring replacement. Very soft water has its own drawback: being less buffered, it is more corrosive to metal pipes, and can dissolve metals from the plumbing into the supply.
4.7.8 Nitrates and Sulphates
Nitrate in water comes mainly from three sources: fertiliser applied to fields and washed off or leached down with irrigation, sewage and animal waste, and the natural breakdown of organic nitrogen. It is highly soluble and is not held by soil particles, so it moves readily down to groundwater — which is why nitrate contamination of wells is common in intensively farmed areas and why it persists for decades after the practice that caused it.
Its significance is twofold. In surface waters, nitrate together with phosphate is a nutrient that drives excessive growth of algae, which on dying are decomposed by bacteria that strip oxygen from the water; the dissolved oxygen collapse that follows can kill fish across a whole lake. In drinking water, high nitrate is a direct health hazard for infants, in whom it interferes with the blood’s ability to carry oxygen. Indian drinking water specifications set a limit of the order of 45 mg/L for nitrate for this reason.
Sulphate occurs naturally where water passes through gypsum and other sulphate-bearing minerals, and is added by industrial discharges, by mine drainage, and by the deposition of sulphur dioxide from the air — the link back to section 4.1 is direct, since atmospheric sulphur dioxide returns to the ground as sulphate in rain. Moderate concentrations are harmless. High concentrations impart a bitter taste and have a laxative effect on people not accustomed to them, and drinking water specifications accordingly set an acceptable limit of the order of 200 mg/L, relaxable where no better source exists. Sulphate also contributes to permanent hardness and to the corrosion of concrete and metal.
Table 4.8 – Water quality indicators: unit, and what a high value usually indicates | Indicator | Usual unit | What a high value usually indicates | |—|—|—| | Electrical conductivity | µS/cm | High dissolved salt content; effluent entry, saline intrusion or drainage from irrigated land | | Turbidity | NTU | Suspended solids from erosion, run-off or discharge; unreliable disinfection | | pH | dimensionless, 0–14 | Above the natural range: alkaline discharge. Below it: acid deposition, mine drainage, organic acids | | Dissolved oxygen | mg/L | A low value, not a high one, indicates trouble: organic pollution or high temperature | | Faecal indicator organisms | count or MPN per 100 mL | Entry of human or animal faecal matter; possible presence of pathogens | | Temperature | °C | Thermal pollution, loss of bank shading, or simply season and time of day | | Hardness | mg/L as CaCO₃ | Dissolved calcium and magnesium, usually from local geology; scale and soap wastage | | Nitrate | mg/L | Fertiliser leaching, sewage or animal waste; risk of eutrophication and, in drinking water, of infant health effects | | Sulphate | mg/L | Mineral dissolution, industrial or mine discharge, or deposition of atmospheric sulphur |
Read the dissolved oxygen row against all the others. It is the only indicator in the table where a low reading is the bad news, which is a common source of error when interpreting a set of results.
4.7.9 Biochemical Oxygen Demand and Chemical Oxygen Demand
The indicators above describe what is in the water. Two further quantities describe what the water will do — specifically, how much oxygen its contents will take out of it. This is what makes them the central measures of organic pollution, and they are treated here as concepts rather than as laboratory procedures.
When organic matter enters water, aerobic bacteria decompose it, and they consume dissolved oxygen while doing so. The greater the quantity of biodegradable organic matter present, the more oxygen will be consumed, and the further the dissolved oxygen will fall. Biochemical Oxygen Demand (BOD) puts a number on this.
(Biochemical Oxygen Demand is the mass of dissolved oxygen consumed by microorganisms in oxidising the biodegradable organic matter present in a sample of water, under stated conditions of temperature and time, expressed in mg/L.)
The stated conditions matter, and the standard ones are incubation for five days at 20 °C — a figure chosen for consistency rather than for any natural significance, since decomposition in a real river continues far beyond five days. Clean river water has a BOD of a few mg/L; untreated domestic sewage is of the order of hundreds of mg/L. Because the test measures precisely what an organic discharge will do to a river’s oxygen, BOD is the standard measure of the strength of sewage and the standard gauge of whether a treatment plant is working.
Chemical Oxygen Demand (COD) answers a related but wider question. Instead of leaving bacteria to consume what they can, the sample is treated with a strong chemical oxidising agent, which oxidises very nearly everything oxidisable — biodegradable organic matter, organic matter that bacteria break down slowly or not at all, and oxidisable inorganic substances as well.
(Chemical Oxygen Demand is the mass of oxygen equivalent to the amount of a strong chemical oxidising agent consumed in oxidising the organic and oxidisable inorganic matter in a sample of water, expressed in mg/L.)
Two consequences follow, and they are the whole point of learning both terms. First, for any given sample, COD is always greater than or equal to BOD, because everything the bacteria could oxidise the chemical oxidant can oxidise too, and more besides. Second, the relationship between the two is itself informative. A sample whose COD is only slightly above its BOD consists mainly of readily biodegradable matter — typical of domestic sewage, and treatable by biological methods. A sample whose COD greatly exceeds its BOD contains a large proportion of material that bacteria cannot handle — typical of certain industrial effluents, which will pass through a biological treatment plant largely unchanged and need chemical or physical treatment instead.
Table 4.9 – Biochemical Oxygen Demand vs Chemical Oxygen Demand | Feature | Biochemical Oxygen Demand (BOD) | Chemical Oxygen Demand (COD) | |—|—|—| | What does the oxidising | Aerobic microorganisms | A strong chemical oxidising agent | | What gets oxidised | Only biodegradable organic matter | Almost all organic matter, plus oxidisable inorganic substances | | Time to obtain a result | Several days of incubation | A few hours | | Unit | mg/L | mg/L | | Relative magnitude for the same sample | Lower | Higher, or at most equal | | What it tells you | How much oxygen the discharge will actually strip from a river | The total oxidisable load, including matter bacteria cannot use | | Chief limitation | Slow; affected by toxic substances that inhibit the bacteria | Overstates the effect on a river, since it counts matter organisms cannot oxidise |
Collect a sample of tap water and a sample of water from a pond or open tank, in clean bottles, and stand both in a glass in good light for an hour. Record for each: colour, smell, whether particles settle to the bottom, and temperature. Then write down which of the nine indicators in this section your observations could and could not have told you about. Do not taste either sample, and do not collect water from a drain, sewage channel, canal bank or any deep or fast-flowing water; take an adult with you.
4.8 Soil Testing: Indicators of Soil Type and Quality
Air moves and water flows, so both carry their pollution away and dilute it. Soil does neither. What enters it largely stays — which is why soil degrades slowly and invisibly, over decades, and why the loss is difficult to reverse within a human lifetime. A field can lose much of its capacity to hold water, to supply nutrients and to support life while continuing to look exactly as it did.
Soil quality is therefore assessed by measurement, on the same logic as air and water. But the object being measured is different in one important respect. Soil is not merely a medium that pollutants sit in; it is a living system whose condition is what is being judged. A soil indicator is chosen to report on that condition.
4.8.1 What Makes a Good Soil Indicator
A soil indicator is a measurable property of soil that reflects its capacity to function — to hold and release water, to store and supply nutrients, to allow roots to grow, and to support the organisms that drive its chemical cycles.
(A soil indicator is a measurable physical, chemical or biological property of soil whose value reflects the soil’s condition and its capacity to perform its functions of supporting plant growth, storing and transmitting water, and cycling nutrients.)
Not every measurable property makes a useful indicator. A good one satisfies several requirements at once, and these are worth stating because they explain why the small set of properties in this section is used rather than the hundreds that could be measured.
It must be sensitive to management and to degradation, changing detectably when the soil is compacted, eroded, over-cultivated or restored. A property fixed by the parent rock and unchanged by anything a farmer does may describe the soil but cannot monitor it.
It must be stable enough to be meaningful — responsive to real change, but not swinging so wildly with the weather of the past week that no trend can be read.
It must be measurable simply, cheaply and repeatably, ideally in the field, because monitoring means measuring many sites many times. A property that requires an expensive laboratory is of little use for routine assessment.
It must be interpretable: there must be an understood relationship between the value and the soil’s actual functioning, so that a number can be read as good or poor rather than merely recorded.
And it should be relevant across a range of soils and land uses, so that results can be compared.
Indicators are grouped into three basic types — biological, physical and chemical — and the grouping is not merely tidy. Each type reports on a different aspect of the soil, on a different timescale, and no one type is sufficient alone. For continuity with the water indicators of section 4.7, they are taken up below in the order physical, chemical, biological.
4.8.2 Physical Indicators
Physical indicators describe the architecture of the soil — how its particles are arranged, how much pore space exists between them, and how freely water, air and roots can move through it. They report on the soil’s structure rather than its chemistry.
The two used here as examples are soil aggregate stability and infiltration rate. Aggregate stability measures how well soil particles are bound into crumbs that survive wetting; infiltration rate measures how fast water actually enters the soil surface. Both are treated in detail in sections 4.8.7 and 4.8.8.
What physical indicators tell you is whether the soil can perform its physical functions. Poor values mean roots cannot penetrate, water runs off the surface instead of soaking in, air does not reach the root zone, and the soil is vulnerable to erosion. They respond quickly to how land is worked — to repeated tillage, to heavy machinery driven over wet soil, to the removal of vegetation cover — and they are largely what a farmer is judging by feel when picking up a handful of earth.
4.8.3 Chemical Indicators
Chemical indicators describe the soil’s chemical environment — its acidity, its salt content, and the availability of the nutrients that plants require. They report on whether the soil can supply what a crop needs, and whether it holds anything that will harm one.
The two used here as examples are soil pH and soil salinity, the latter measured as the electrical conductivity of a soil–water suspension, on exactly the principle described in section 4.7.1. Soil pH is treated in section 4.8.6.
Salinity indicates the concentration of soluble salts in the soil. Salts accumulate where irrigation water is applied in a hot climate with inadequate drainage: the water evaporates and the dissolved salts it carried remain behind, building up season after season. High salinity makes it physically harder for roots to draw water from the soil, so a plant can wilt in visibly damp ground. It is one of the principal ways in which productive irrigated land is lost, and it is measured easily.
What chemical indicators tell you is whether the nutrient supply and the chemical conditions are within the range crops and soil organisms can work in. They also change relatively slowly and predictably, which makes them good for tracking long-term direction.
4.8.4 Biological Indicators
Biological indicators describe the living fraction of the soil — the bacteria, fungi, protozoa, arthropods and earthworms that decompose organic matter, release nutrients into available forms, and bind particles together. This fraction is a small part of the soil by mass and a very large part of what makes it work.
The two used here as examples are soil respiration and soil organism population, the latter assessed by counting the larger organisms present, such as earthworms in a measured block of soil, or by estimating microbial numbers. Soil respiration is treated in section 4.8.5.
What biological indicators tell you is whether the soil is functioning as a living system. They are the most sensitive of the three types — a biological measure often registers a change in management within a single season, long before the chemical or physical indicators shift — which makes them valuable as early warning. Their weakness is the other side of the same property: they vary strongly with temperature, moisture and season, so a single measurement is nearly meaningless and comparisons must be made under similar conditions.
Table 4.10 – The three basic types of soil indicator | Type of indicator | Two examples | What this type of indicator tells you | |—|—|—| | Biological | Soil respiration; population of soil organisms such as earthworms and microorganisms | Whether the soil is biologically active — decomposing organic matter, cycling nutrients and supporting life; responds first to a change in management | | Physical | Aggregate stability; infiltration rate | Whether water, air and roots can move through the soil; how vulnerable it is to compaction, crusting and erosion | | Chemical | Soil pH; soil salinity, measured as electrical conductivity | Whether the chemical environment allows nutrients to be available to plants, and whether acidity or accumulated salt is limiting growth |
The three types answer three different questions about the same handful of earth: is it alive, is it open, and is it chemically hospitable. A soil can pass on one and fail on another — a chemically well-supplied field can still be so compacted that nothing grows in it — which is why a single test is never a soil assessment.
4.8.5 Soil Respiration
Soil respiration is the release of carbon dioxide from the soil surface, produced by the respiration of soil microorganisms decomposing organic matter and by the respiration of plant roots and soil animals.
(Soil respiration is the total quantity of carbon dioxide released from a given area of soil surface in a given time, arising from the respiration of soil microorganisms, roots and soil fauna.)
It is reported as a mass of carbon dioxide per unit area per unit time, for example in kilograms per hectare per day (kg/ha/day) or in grams per square metre per day (g m⁻² day⁻¹).
Its value as an indicator is that it measures biological activity directly. A soil rich in organic matter and well populated with microorganisms respires vigorously; a depleted, compacted or lifeless soil respires little. Because the organisms doing the respiring are also the ones releasing nutrients into plant-available forms, a soil that respires well is generally a soil that feeds crops well.
Several factors control it, and all must be known before a reading can be interpreted: the number and activity of soil microorganisms, the amount and quality of soil organic matter available to them, temperature, soil moisture, and aeration. Microbial activity roughly accelerates with rising temperature over the normal range, and falls away in very cold soil. It also falls in soil that is too dry for organisms to be active and in soil that is waterlogged, where the absence of oxygen halts aerobic decomposition.
This dependence produces the one genuine ambiguity in the indicator, and students should be careful with it. A high respiration rate normally means a healthy, active soil. But an unusually high rate immediately after ploughing means something else entirely: tillage has broken open the soil aggregates and exposed organic matter that had been protected inside them, and the burst of decomposition that follows is organic matter being lost from the soil, not evidence of its abundance. Sustained high respiration with declining organic matter is a symptom of depletion.
measuring soil respiration Principle: Carbon dioxide released by soil organisms accumulates in a closed volume of air over the soil surface; the amount accumulating in a known time is proportional to the rate of respiration. What it measures: Carbon dioxide evolved per unit area of soil per unit time, in kg/ha/day or g m⁻² day⁻¹. Procedure in brief: In a simple classroom demonstration, place a measured quantity of fresh, moist soil in a jar together with a small open dish of limewater, and seal it; set up an identical jar containing soil that has been dried and heated in an oven, and a third containing no soil at all; leave all three for a day in the same place and compare how far the limewater in each has turned milky, since carbon dioxide turns limewater milky. In field use, a chamber of known area is sealed over undisturbed soil and the accumulated carbon dioxide is measured by an instrument. Limitation: The jar comparison shows that respiration is occurring and allows two soils to be ranked; it does not give a quantitative rate. Any measurement is valid only for the temperature and moisture at the time, so readings taken on different days are not comparable.
Managing soil respiration
Soil respiration is not raised or lowered for its own sake; it is managed by managing the biological life of the soil. Adding organic matter — farmyard manure, compost, green manure, crop residues left on the field — supplies both food and habitat for soil organisms and raises activity. Keeping the soil covered with mulch or a growing crop moderates temperature and conserves the moisture organisms need. Reducing the frequency and depth of tillage protects the aggregates in which organic matter is stored, so that carbon is retained rather than burnt off in a post-ploughing flush. Correcting waterlogging by drainage restores the oxygen that aerobic decomposition requires. Heavy repeated applications of chemical fertiliser, and of pesticides, can suppress soil organisms; moderate applications combined with organic inputs do not.
4.8.6 Soil pH
Soil pH is the acidity or alkalinity of the soil solution, measured on the same logarithmic scale defined in section 4.7.3. It is often called the master variable of soil chemistry, because it controls the availability of almost every nutrient.
(Soil pH is the negative logarithm to base 10 of the hydrogen ion concentration of the soil solution, indicating the degree of acidity or alkalinity of the soil.)
The reason for its importance is that nutrients dissolve and precipitate at different pH values. Most plant nutrients are available across the greatest range at a pH near neutral to slightly acidic, which is why most crops perform best in that range. In strongly acidic soils, iron, manganese and aluminium become highly soluble — aluminium to the point of being toxic to roots — while calcium, magnesium and potassium are readily leached away and phosphorus is locked up in unavailable forms. In strongly alkaline soils, iron, manganese, zinc and phosphorus become insoluble and unavailable, and a plant can starve in a soil that contains plenty of them. Soil pH also determines which microorganisms thrive, and therefore how fast organic matter decomposes and nitrogen is released.
Its natural value depends on the parent rock, on rainfall (heavy rainfall leaches basic ions and leaves soils acidic), and on the vegetation. It is shifted by management: prolonged use of certain nitrogenous fertilisers acidifies soil, while irrigation with alkaline water and poor drainage push it the other way.
determining soil pH Principle: The hydrogen ion concentration of the soil solution can be detected by an indicator dye that changes colour with pH, or by a glass electrode that develops a voltage proportional to it. What it measures: Soil pH, on the dimensionless 0–14 scale. Procedure in brief: Shake a known mass of air-dried, sieved soil with a stated volume of distilled water in a fixed ratio — commonly one part soil to two and a half parts water — allow the suspension to settle, then dip pH paper or universal indicator into the clear liquid and match the colour against the chart, or read the value with a calibrated pH meter. Limitation: The reading depends on the soil-to-water ratio used, so results are comparable only when the same ratio is used throughout. Colour-matching with paper resolves the value only to about half a pH unit, which is coarse on a logarithmic scale.
Correcting soil pH
An acidic soil is corrected by liming — applying agricultural lime, that is calcium carbonate, or dolomite, which neutralises acidity and supplies calcium and magnesium at the same time. The quantity required depends on how acidic the soil is and on how strongly it resists change, which is greater in clay-rich and organic soils.
An alkaline or sodic soil is corrected by applying gypsum, calcium sulphate, whose calcium displaces sodium from the soil particles so that the sodium can be washed out — a treatment that works only where drainage is adequate to carry the displaced salts away. Adding organic matter helps in both directions, because it buffers the soil against sudden change. Where the water supply is itself alkaline, the longer-term correction lies in the irrigation practice rather than in the amendment.
4.8.7 Soil Aggregate
Soil particles do not sit as loose grains. They are bound together into crumbs, called aggregates, by the sticky substances that microorganisms and decomposing organic matter produce, by fine roots and fungal threads that hold them physically, and by clay and carbonate acting as cement.
(A soil aggregate is a cluster of soil particles bound together more strongly to one another than to the surrounding particles; aggregate stability is the ability of such a cluster to resist breaking apart when wetted or disturbed.)
Aggregation is what creates soil structure, and structure is what creates pore space. The large pores between aggregates conduct water downward and admit air; the small pores within them hold water for plants to draw on. A well-aggregated soil therefore drains and breathes and stores water at the same time, which a structureless one cannot do.
When aggregates are weak, they collapse on wetting. The fine particles released clog the pores below, and as the surface dries it sets into a hard crust. Rain then runs off the crust instead of entering, seedlings struggle to push through it, and the water that runs off carries the loosened particles with it. Poor aggregate stability is thus the beginning of both erosion and drought on the same field.
assessing soil aggregate stability by the slake test Principle: Aggregates that are well bound by organic matter and biological activity hold together when suddenly immersed in water; poorly bound aggregates disintegrate as trapped air escapes and the binding fails. What it measures: The relative stability of soil aggregates in water — a comparative rather than an absolute measure. Procedure in brief: Take similar air-dried lumps of soil about the size of a marble from two fields being compared, place each gently in a clear glass of water without stirring, and observe over about ten minutes; record how quickly each breaks up and how cloudy the water becomes. Limitation: It ranks soils against one another but yields no numerical value, and the result depends on how dry the lumps were at the start, so both samples must be treated identically.
Improving aggregate stability
Everything that builds aggregates is biological, so the measures are the same ones that support soil life. Regular addition of organic matter supplies the binding substances directly. Keeping the ground covered — with a growing crop, a cover crop or mulch — protects the surface from the impact of raindrops, which is what physically shatters exposed aggregates, and keeps roots and fungal threads growing through the soil. Reducing tillage leaves aggregates intact instead of breaking them each season. Avoiding traffic and grazing on wet soil prevents compaction, which crushes the pore space between aggregates. Growing crops in rotation, including deep-rooted and grass species, builds structure at different depths.
4.8.8 Infiltration Rate
Infiltration rate is the speed at which water enters the soil through its surface. It is expressed as a depth of water entering per unit time, most often in millimetres per hour (mm/h).
(Infiltration rate is the depth of water that enters the soil surface per unit time, expressed in mm/h.)
\[i = \frac{d}{t}\]
where \(i\) is the infiltration rate in mm/h, \(d\) is the depth of water that has entered the soil in millimetres (mm), and \(t\) is the time taken in hours (h).
The rate is highest when a dry soil is first wetted and falls steadily as the pores fill, settling after some time to a steady value — the figure that actually matters, because it determines what happens in a long storm.
Infiltration rate is arguably the most practically important physical indicator, because it decides where rainfall goes. Where infiltration is fast, rain enters the soil, is stored for the crop, and recharges the groundwater below. Where it is slow, the same rain runs off the surface, taking topsoil with it, arriving in the stream as a flood peak and as turbidity rather than as a stored resource. Two neighbouring fields receiving identical rainfall can therefore experience one as a good season and the other as erosion followed by drought.
The rate depends on soil texture — coarse sandy soils admit water fast, clay soils slowly — and, much more responsively to management, on structure, aggregate stability, surface crusting, compaction and the amount of organic matter and surface cover present.
measuring infiltration rate with a ring infiltrometer Principle: Water ponded inside a ring driven into the soil can escape only downward into the soil beneath, so the rate at which the water level falls measures the rate at which water is entering the soil. What it measures: Infiltration rate, in mm/h. Procedure in brief: Drive an open-ended metal ring or a large tin with both ends removed a few centimetres vertically into level, undisturbed ground; place a ruler inside; pour in water to a marked depth and start a stopwatch; record the fall in water level at intervals until successive readings show the same rate; calculate the rate from the depth infiltrated and the time taken. Limitation: A single ring allows some water to spread sideways beneath it, which overstates the vertical rate; the reading is strongly affected by how wet the soil already was, so measurements on different days are not directly comparable.
Working out an infiltration rate
A ring infiltrometer is set up on a field. The water level inside is at the 90 mm mark on the ruler when timing starts, and at the 66 mm mark 30 minutes later. In the following 30 minutes it falls from 66 mm to 48 mm.
For the first half hour, the depth infiltrated is \(90 - 66 = 24\) mm in \(0.5\) h, so
\[i = \frac{24}{0.5} = 48 \text{ mm/h}\]
For the second half hour, the depth infiltrated is \(66 - 48 = 18\) mm in \(0.5\) h, so
\[i = \frac{18}{0.5} = 36 \text{ mm/h}\]
The rate has fallen because the soil pores are filling as the soil becomes wetter, exactly as described above. The steady rate reached after the fall has stopped is the value to compare between fields, and it is the figure that tells you whether an hour of heavy rain — which in the monsoon can easily exceed 30 mm — will soak in or run off.
Improving infiltration
The measures overlap almost entirely with those for aggregate stability, which is unsurprising, since surface structure is what controls entry. Maintaining organic matter and keeping the surface covered with mulch or vegetation prevents crusting and protects aggregates from raindrop impact. Relieving compaction — by keeping heavy traffic off wet soil, and by breaking an existing compacted layer where one has formed — reopens the pores. Reduced tillage preserves the continuous channels made by roots and earthworms, through which water enters far faster than through the soil mass itself. On sloping land, contour bunding, terracing and vegetative barriers slow the water down so that it has time to enter rather than running off, and these also trap the soil it would otherwise carry away.
Q5. Water in a river and soil in a field cannot be judged by eye either. Which measurements tell you that they are in poor condition?
A. For water, a set of indicators is used together: electrical conductivity, turbidity, pH, dissolved oxygen, faecal indicator organisms, temperature, hardness, nitrate and sulphate — with dissolved oxygen the key sign of organic pollution, and Biochemical and Chemical Oxygen Demand measuring how much oxygen the water’s contents will consume. For soil, three types of indicator are used: biological ones such as soil respiration and the population of soil organisms, physical ones such as aggregate stability and infiltration rate, and chemical ones such as soil pH and salinity. In both cases no single number is an assessment; the condition is read from the set.