What is the impact of excessive use of fertilizers on groundwater?

The excessive use of fertilizers can lead to the introduction of contaminants into groundwater, which is the correct understanding of the impact. When fertilizers are applied in large amounts, they can leach into the soil and, subsequently, into the groundwater system. This runoff typically contains high levels of nutrients, such as nitrogen and phosphorus.

These nutrients, particularly nitrates, can contaminate groundwater supplies and result in various environmental and health issues. For example, high nitrate levels in drinking water can lead to health risks, including methemoglobinemia, or "blue baby syndrome," which affects the ability of blood to carry oxygen in infants.

In addition, the introduction of these contaminants can affect the overall quality of the groundwater, impairing its usability for drinking and irrigation purposes and potentially disrupting aquatic ecosystems once they reach surface water bodies.

The other alternatives present scenarios that do not directly correlate with the primary impact of excessive fertilizer use on groundwater. While decreased agricultural yield could occur due to nutrient runoff leading to soil degradation or other factors, it isn’t a direct consequence of fertilizers contaminating groundwater in the immediate sense. Similarly, reduced oxygen levels and improved water retention do not accurately describe the specific consequences of fertilizer runoff and contamination.