Environmental Pollution and Control IV: Soil Pollution — Soil Properties and Quality Analysis, Industrial Effluents and Soil, Soil Micro-Organisms and the Degradation of Pesticides and Fertilisers, and Control — and Thermal Pollution and Heat Islands, Marine Pollution, its Abatement and Coastal Management, and Radioactive Pollution, Radiation Standards and Protection

When industrial effluents are used for irrigation or discharged on land, their constituents interact with soil components by adsorption on clays, humus and oxides, precipitation, complexation and redox reactions. Tannery effluents, rich in chromium and salts, have left chromium-contaminated soils and groundwater in leather clusters such as those around Vellore in Tamil Nadu and Kanpur in Uttar Pradesh. Distillery spent wash is very high in BOD, potassium and dark colour; textile effluents carry dyes and salts; and effluents high in sodium disperse clays, destroying soil structure and permeability. Sodicity is assessed by the sodium adsorption ratio, SAR = Na⁺ ÷ √[(Ca²⁺ + Mg²⁺) ÷ 2], with concentrations in milliequivalents per litre: water with 20 meq/L of sodium, 3 of calcium and 5 of magnesium has SAR = 20 ÷ √4 = 10. Soils with an exchangeable sodium percentage above about 15 are sodic. Other sources of soil pollution are mining and smelting, excessive agrochemicals (pesticide residues and cadmium from phosphate fertilisers), municipal solid waste and sewage sludge, e-waste recycling, oil spills and plastics.

Soil micro-organisms, bacteria, actinomycetes (which produce many antibiotics and the earthy smell of geosmin), fungi, algae and protozoa, decompose organic matter, cycle nutrients, form soil aggregates and degrade pollutants. Pesticides are degraded when microbes use them as a source of carbon or energy, or by co-metabolism, in which an enzyme made for another substrate transforms the pesticide incidentally: Pseudomonas and other bacteria degrade 2,4-D, Flavobacterium hydrolyses parathion, and Pseudomonas sp. ADP mineralises atrazine. Persistence depends on chemical structure, halogenated and highly branched molecules being resistant, which is why DDT can persist for years while organophosphates break down in weeks; the idea that for every natural compound some microbe can degrade it, Alexander's principle of microbial infallibility, fails for many synthetic, recalcitrant chemicals. Synthetic fertilisers are transformed microbially too: urea is hydrolysed by urease to ammonia, which can volatilise; ammonium is nitrified to mobile nitrate, which leaches; and nitrate is denitrified to nitrogen and nitrous oxide.

Soil pollution is controlled first at the source, by treating effluents before land application, integrated pest and nutrient management, and safe disposal of waste. Contaminated land is remediated by excavation and secure disposal, soil washing, stabilisation and solidification (binding contaminants with cement or lime), thermal desorption, and biological methods, bioremediation and phytoremediation. Amendments reduce the availability of contaminants: liming acid soils raises pH and immobilises most metals, organic matter binds metals and pesticides, and gypsum reclaims sodic soils by replacing exchangeable sodium with calcium, which is then leached away.

Thermal pollution is the degradation of water quality by a change in its temperature, above all the discharge of heated cooling water from thermal and nuclear power stations using once-through cooling, and from steel mills, refineries and other industries. Warm water holds less dissolved oxygen, while it raises the metabolic rate and oxygen demand of fish and invertebrates, so it squeezes them from both sides; it can cause thermal shock and fish kills, disrupt spawning and migration timed by temperature, favour heat-tolerant and invasive species and algal blooms, and increase the toxicity of many pollutants. Sudden cooling when a plant shuts down can be as harmful. It is controlled by closed-cycle cooling with natural- or mechanical-draft cooling towers, cooling ponds and spray ponds, and by recovering waste heat. Under Indian rules, the temperature of cooling water discharged from once-through systems must not exceed that of the receiving water by more than 5 °C, and new thermal power plants have been required to adopt closed-cycle cooling.

An urban heat island is a city that is markedly warmer than its rural surroundings, the difference being greatest on calm, clear nights. Its causes are dark, dry surfaces of asphalt and roofing that absorb sunlight (low albedo) and store heat; the replacement of vegetation and soil, which cool by evapotranspiration, with impervious surfaces; street canyons that trap radiation and reduce wind; waste heat from vehicles, air conditioners and industry; and polluted air that traps outgoing radiation. Its consequences are heat stress and deaths during heat waves, especially among outdoor workers, the poor and the elderly; higher electricity demand for cooling, which adds emissions; faster formation of ozone; and changes in local rainfall. Mitigation uses cool (reflective) roofs and pavements, green roofs, trees and parks, water bodies and better urban design. Ahmedabad's Heat Action Plan (2013), the first in South Asia, combined early warning with public health measures and has been widely copied in Indian cities.

Most marine pollution, about four-fifths by common estimate, comes from the land: sewage, agricultural runoff, industrial discharges and the plastics carried by rivers. Nutrient runoff creates coastal "dead zones" of hypoxic water, the best known in the Gulf of Mexico off the Mississippi, and fuels harmful algal blooms and red tides that poison shellfish. Oil enters the sea from tanker accidents such as the Exxon Valdez (1989) and from well blowouts such as Deepwater Horizon (2010), but more from routine operational discharges, runoff and natural seeps; it smothers seabirds and mammals, destroying the insulation of feathers and fur, and coats shorelines and mangroves. Plastics and microplastics entangle and are ingested by turtles, seabirds and fish; mercury and persistent organic pollutants biomagnify to high levels in top predators such as tuna; ships' ballast water transports invasive species; and warming and acidification bleach corals.

Coastal management aims to balance development with the protection of ecosystems and communities along the coast. Integrated coastal zone management treats the coast as one system of land, estuary and sea and brings the many agencies and users together; in India a World Bank-supported integrated coastal zone management project began in Gujarat, Odisha and West Bengal around 2010, and the Coastal Regulation Zone notifications regulate activities along the shore. Practical measures include protecting and restoring mangroves, coral reefs, seagrass and dunes, which buffer storms and tsunamis; preferring "soft" engineering such as beach nourishment to hard sea walls that shift erosion elsewhere; setback lines for construction; and hazard mapping for cyclones, storm surges and sea-level rise.

Most of the radiation dose people receive is natural: cosmic rays, terrestrial radionuclides of the uranium and thorium series and potassium-40 in rocks, soil and building materials, radionuclides in food and water, and above all inhaled radon and its decay products, which contribute roughly half of the natural dose. The worldwide average natural background is about 2.4 mSv a year, higher in places such as the monazite-sand coast of Kerala. The largest human-made source is medical: diagnostic X-rays, CT scans and nuclear medicine. Smaller contributions come from fallout from past atmospheric weapons tests, the nuclear fuel cycle from mining to waste, industrial radiography and accidents such as Chernobyl (1986) and Fukushima Daiichi (2011).

Ionising radiation damages living cells by breaking chemical bonds in DNA directly and, more often, indirectly through free radicals formed when it splits water molecules. By the law of Bergonié and Tribondeau (1906), cells that divide rapidly and are less differentiated, such as those of bone marrow, the gut lining, gonads and the fetus, are the most radiosensitive. Effects are of two kinds. Deterministic effects (tissue reactions) occur only above a threshold dose and grow more severe with dose: skin burns, cataracts, sterility and acute radiation syndrome, which appears after whole-body doses above about 1 Gy. Stochastic effects, cancer and heritable effects, are assumed to have no threshold, their probability, not their severity, rising with dose; this linear no-threshold assumption underlies radiation protection.

An absorbed dose of 2 mGy from alpha particles is an equivalent dose of 2 × 20 = 40 mSv, the same energy doing twenty times the biological harm of gamma rays. The International Commission on Radiological Protection recommends dose limits of 1 mSv a year effective dose for members of the public from planned sources, over and above natural background and medical exposure, and 20 mSv a year for radiation workers, averaged over five years with no more than 50 mSv in any one year. In India the Atomic Energy Regulatory Board, set up in 1983, enforces these limits. Radiation protection rests on three principles, justification, optimisation (keeping doses as low as reasonably achievable, ALARA) and dose limitation, and on three practical controls: minimising the time of exposure, maximising distance, since dose rate from a point source falls with the square of distance (40 µSv/h at 1 m becomes 10 µSv/h at 2 m), and shielding, lead or concrete for gamma rays, a few millimetres of aluminium or plastic for beta particles, and paper or skin for alpha particles, which are dangerous mainly when inhaled or swallowed.