What does groundwater contamination refer to?

Groundwater contamination refers to the introduction of harmful substances into groundwater, which leads to the degradation of its quality. This can occur through various means, such as pollution from agricultural runoff, industrial discharges, or improperly managed waste disposal. When these unwanted substances infiltrate the soil and reach the aquifers, they can compromise the safety of drinking water supplies and harm ecosystems that rely on groundwater.

Considering the context of the other choices, natural filtration of water is a process where soil and geological formations act as filters for surface water before it becomes groundwater, but it does not represent contamination. Excessive evaporation pertains to the loss of water from surface bodies and soils, but does not address the quality or purity of groundwater itself. Purification of water sources implies the removal of contaminants and enhancement of water quality, which is the opposite of what occurs in groundwater contamination. Thus, the focus on the addition of unwanted substances highlights the core issue in groundwater contamination.