March 10 to 16 is National Groundwater Awareness Week. Stormwater is intimately tied to groundwater. About one-quarter of all U.S. rainfall becomes groundwater, according to the National Groundwater Association (NGWA). Groundwater also feeds streams. According to the U.S. Geological Survey (USGS), 30% of U.S. streamflow is from groundwater, though this number varies by location. Finally, groundwater is an important source of water. USGS estimates that nearly half of America’s population depends on groundwater regularly for its water supply. Get more groundwater stats.

Overall, U.S. groundwater quality is acceptable for most uses. However, pollutants can leach through the soil, particularly as rainwater infiltrates the ground. Leaking septic tanks and collection lines or landfill leachate also contribute to groundwater pollution. NGWA emphasizes the testing of private wells to ensure a safe water supply. In 2012, the U.S. National Academies of Science released a report titled Alternatives for Managing the Nation’s Complex Contaminated Groundwater Sites. The report estimates that 126,000 sites nationwide still have contaminated groundwater, and their closure is expected to cost at least $110 billion to $127 billion. Contaminated sites include current and former dry-cleaning facilities, Superfund sites, some military installations, and hazardous waste facilities.