If groundwater is contaminated, what is the most likely source of pollution?

The most likely source of pollution to groundwater from the options provided is agricultural practices. Agriculture is known to introduce a variety of contaminants into groundwater through the use of fertilizers, pesticides, and herbicides. When these chemicals are applied to crops, they can leach into the soil and eventually reach the groundwater aquifers, particularly in regions where soils are sandy and allow for rapid movement of water.

Additionally, agricultural practices often involve the application of animal manure, which can also contribute to nutrient loading and pathogens that can infiltrate the groundwater. The excessive use of fertilizers can lead to increased nitrate levels in groundwater, posing risks to human health, particularly for vulnerable populations such as infants.

While the other choices—surface runoff, industrial discharges, and residential waste—are valid sources of groundwater contamination in certain contexts, agricultural practices are particularly pervasive due to the scale and frequency of chemical applications in farming operations, making them a primary contributor to groundwater pollution.