Which of the following is not a threat to groundwater pollution?

Rainwater itself is not a direct threat to groundwater pollution because it is a natural phenomenon that typically contributes to the replenishment of groundwater supplies. When rainwater falls, it usually infiltrates the soil, filtering through various layers of the earth and helping to replenish aquifers. In its pure form, rainwater does not contain harmful substances that could pollute groundwater.

In contrast, the other choices — PFAS (per- and polyfluoroalkyl substances), sewage, and agricultural runoff — are associated with various forms of pollution. PFAS are chemicals often found in industrial applications and can seep into groundwater sources, causing contamination. Sewage includes waste materials that can introduce pathogens and nutrients into groundwater, leading to pollution. Agricultural runoff includes fertilizers, pesticides, and sediments that can wash into water sources, polluting groundwater supplies when rainwater or surface water transports these substances into the soil.