Environmental Chemistry and Soil Pollution
What soil pollution means
Soil pollution is the accumulation of substances in soil beyond its capacity to assimilate them, causing harm to plants, animals or humans. Sources include industrial effluents, sewage sludge, mining spoil, atmospheric deposition, plastics and agrochemicals.
Key pollutants and processes
- Heavy metals (Cd, Pb, Cr, As, Hg, Ni) are non-degradable, accumulate in soil and crops and biomagnify along food chains. Cadmium enters through phosphatic fertilizers and sludge, lead through batteries and old vehicle exhaust, arsenic through arsenical pesticides, mercury through mercurial fungicides.
- Nitrate from excess fertilizer leaches into groundwater and causes methaemoglobinaemia in infants; phosphorus runoff causes eutrophication of lakes and ponds; denitrification releases nitrous oxide, a potent greenhouse gas.
- Persistent pesticides such as DDT accumulate in fat and biomagnify; salinity and waterlogging follow irrigation without drainage.
- Remediation includes bioremediation, phytoremediation with hyperaccumulators, soil washing, addition of organic matter and liming to reduce metal availability.
Worked example
A pond near a fertilized field develops an algal bloom. Analysis shows 0.15 mg/L phosphorus and high nitrate. Reasoning: in freshwater, phosphorus is usually the limiting nutrient, so a small P increase triggers algal growth; control must cut phosphorus (fertilizer rate, timing, vegetated buffer strips), not just nitrogen. The oxygen demand of decaying algae then kills fish.
Traps
Saying heavy metals "degrade" or break down; blaming pesticides alone for eutrophication when N and P are the main cause; confusing biomagnification (increase up the food chain) with bioaccumulation (increase within an organism); assuming pollution is only industrial; forgetting that liming reduces, but does not remove, metal uptake.