Soil and Water Pollution
This domain examines how fertilizers, pesticides, effluents and sewage degrade soil and water, and how standards and indicators such as nitrate, BOD and eutrophication are applied.
Core rules
- Nitrate leaches readily because it is anionic and not held on the exchange complex; ammonia, by contrast, is retained and nitrified in situ.
- High nitrate in drinking water causes methaemoglobinaemia in infants. BIS 10500 limits: nitrate 45 mg/L, fluoride 1.0–1.5 mg/L, arsenic 0.01 mg/L, lead 0.01 mg/L, cadmium 0.003 mg/L, pH 6.5–8.5.
- Eutrophication is driven by N and P entering water bodies; BOD measures biodegradable oxygen demand, COD measures total chemical oxidisable matter, and COD is always greater.
- Heavy metals enter soils from phosphatic fertilizers (cadmium), sewage sludge and industrial effluent; they do not degrade and can biomagnify.
- Soil degradation pathways include salinisation from over-irrigation, acidification from ammonium fertilizers, and structural decline from sodium.
Worked example
A field receives 120 kg N/ha; the crop takes up 60 kg N/ha and 20% of the remainder leaches. Leached N = (120 − 60) × 0.20 = 12 kg N/ha. With 100 mm of drainage (1 mm = 10 m³/ha, so 1000 m³ = 1,000,000 L), concentration = 12,000,000 mg/1,000,000 L = 12 mg N/L. As nitrate, 12 × 62/14 = 53 mg/L, above the 45 mg/L limit - note the conversion.
Traps that cost marks
- Reporting nitrate as N when the standard is expressed as NO3, or vice versa.
- Using BOD and COD interchangeably.
- Treating dilution as a solution; nitrate in drainage volume depends on the amount of water, not just the fertilizer rate.