Excessive use of which type of fertiliser is mainly responsible for the presence of toxic levels of nitrates in groundwater, leading to problems such as blue baby syndrome?

Introduction / Context:
Groundwater quality can be adversely affected by agricultural practices. When fertilisers are applied in large quantities, nutrients can leach down through the soil and contaminate underground water. One well known issue is nitrate contamination, which can cause health problems such as methemoglobinemia or blue baby syndrome in infants. This question asks which type of fertiliser, when used excessively, is primarily responsible for toxic nitrate levels in groundwater.

Given Data / Assumptions:

• The focus is on toxic substances in groundwater related to fertiliser overuse.
• Options include nitrogenous, phosphate, and potassium fertilisers, singly or in combination.
• We assume typical agricultural conditions where fertilisers are applied to fields and some portion may leach downward with water movement.

Concept / Approach:
Nitrogenous fertilisers provide nitrogen in forms such as nitrate (NO3-) or ammonium (NH4+), which plants need for growth. Examples include urea, ammonium nitrate, and ammonium sulphate. Nitrate is highly soluble in water and does not bind strongly to soil particles, so it can easily leach into groundwater if applied in excessive amounts or if irrigation and rainfall are high. High nitrate levels in drinking water are associated with blue baby syndrome, where nitrate is converted to nitrite in the body, interfering with haemoglobin ability to carry oxygen in infants. In contrast, phosphate ions tend to bind strongly to soil particles and usually do not leach as easily; they are more associated with surface water eutrophication when eroded soil enters rivers and lakes. Potassium fertilisers also tend to be adsorbed by soil particles and are less prone to deep leaching than nitrates. Therefore, nitrogenous fertilisers are primarily responsible for nitrate contamination of groundwater when overused.

Step-by-Step Solution:

Verification / Alternative check:
Environmental health reports and soil science references frequently cite nitrate contamination of groundwater as a major issue in areas of intensive agriculture. They link high nitrate levels to overuse of nitrogenous fertilisers and poor management of animal wastes. Drinking water standards normally specify maximum permissible nitrate concentrations, and public health guidelines explain the connection between nitrate rich water and blue baby syndrome in infants. These documents do not identify phosphate or potassium fertilisers as primary culprits for toxic groundwater contamination, reinforcing that nitrogenous fertilisers are the main concern.

Why Other Options Are Wrong:

Phosphate fertilisers only are wrong because phosphates generally bind to soil particles and do not leach as readily into deep groundwater; they are more important for surface water eutrophication than for nitrate toxicity in groundwater.

Potassium fertilisers only are incorrect because potassium ions are not typically associated with severe groundwater toxicity in the same way as nitrates; they can leach but usually do not cause the same health problems.

Phosphate and potassium fertilisers together are also wrong because combining them does not create the specific nitrate contamination that causes blue baby syndrome; nitrogen is missing from this pair.

Common Pitfalls:
Students may assume that any fertiliser can contaminate groundwater and may pick an option that lists multiple nutrients. Another mistake is not remembering which nutrient is specifically associated with blue baby syndrome. To answer correctly, keep in mind that nitrate contamination of groundwater is mainly linked to overuse of nitrogenous fertilisers, not phosphate or potassium fertilisers.

Final Answer:
Thus, excessive use of fertilisers that primarily contaminate groundwater with toxic nitrates involves nitrogenous fertilisers that supply nitrate or ammonium to crops.