Excessive use of which of the following fertilizers may be responsible for the presence of a toxic substance in groundwater?

Excessive use of nitrogenous fertilizers is primarily responsible for the presence of toxic nitrates in groundwater. Nitrogen, often applied as urea or ammonium-based fertilizers, is highly soluble in water and easily leaches through the soil profile into aquifers [4]. High nitrate levels in drinking water are toxic, specifically causing methemoglobinemia or 'blue baby syndrome' in infants by reducing the oxygen-carrying capacity of hemoglobin [5]. While phosphate and potassium are also major nutrients, they behave differently in the environment. Phosphorus binds strongly to soil minerals and is generally lost through surface runoff and erosion rather than leaching, primarily causing eutrophication in surface waters [5]. Potassium is rapidly absorbed by plants and poses a minimal environmental threat to groundwater [5]. Therefore, nitrogen is the specific fertilizer component associated with toxic groundwater contamination.

Sources

1. [1] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 5: Environmental Pollution > 3) Agricultural sources: > p. 74
2. [4] https://www.dpi.nsw.gov.au/agriculture/soils/guides/soil-nutrients-and-fertilisers/environment
3. [2] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 29: Environment Issues and Health Effects > d) Blue baby syndrome > p. 416
4. [5] https://www.epa.gov/nutrientpollution/sources-and-solutions-agriculture