What is considered the primary source of groundwater contamination?

Leaching is recognized as the primary source of groundwater contamination because it involves the process through which soluble substances, including pollutants and chemicals, are transported through soil and into groundwater aquifers. When rainwater or irrigation water infiltrates the soil, it can dissolve various contaminants, including pesticides, fertilizers, heavy metals, and other harmful chemicals. These dissolved substances can then seep downward, reaching the groundwater and degrading its quality.

This process can be exacerbated by factors such as soil composition and the intensity of rainfall. Unlike other options, leaching is a direct mechanism that moves contaminants from the surface into the groundwater, making it a significant concern for water quality and public health.

Other options such as runoff, while they can contribute to surface water pollution, primarily affect surface water bodies rather than introducing contaminants deep into the soil and aquifers the way leaching does. Drift may refer to the movement of pesticides through air, but its impact on groundwater is minimal compared to leaching. Contaminated surface water can impact groundwater quality, but it is secondary to the direct introduction of contaminants that occurs through leaching processes.