Which of the following is NOT a primary source of groundwater contamination?

Direct atmospheric precipitation is not considered a primary source of groundwater contamination. Groundwater contamination typically arises from substances introduced into the environment through human activities or specific industrial processes.

Agricultural runoff, for instance, often carries fertilizers, pesticides, and other chemicals into water bodies, which can then seep into the groundwater. Industrial discharges can introduce hazardous substances and pollutants directly into the ground, affecting the underlying aquifers. Similarly, leaking underground storage tanks can release gasoline, oil, or other harmful chemicals into the soil, which then can migrate into the groundwater.

In contrast, direct atmospheric precipitation refers to rain or snow that falls onto the ground. While precipitation can contribute to the dilution of contaminants, it doesn't introduce new contaminants to groundwater in the same detrimental way as the other sources. As atmospheric precipitation typically collects and transports existing environmental substances rather than introducing new pollutants, it stands apart as a less significant contributor to groundwater contamination.