Pennington County, South Dakota: drinking water report. Pennington County in western South Dakota has a population of about 115,000, centered on Rapid City.
Pennington County in western South Dakota has a population of about 115,000, centered on Rapid City. The county's water sources include the Madison Aquifer, Rapid Creek, and Pactola Reservoir in the Black Hills. Ellsworth Air Force Base, home to the 28th Bomb Wing (B-1B Lancers), operates northeast of Rapid City and has direct implications for local groundwater quality.
Ellsworth AFB has confirmed PFAS contamination from AFFF firefighting foam use. According to the Air Force's 2024 environmental investigation, monitoring wells near the base's fire training areas detected PFOS at concentrations exceeding 100 ppt. The contamination plume extends off-base, and the Air Force has tested private wells in the Box Elder and Ellsworth areas. Several wells have been provided with point-of-use treatment systems.
Rapid City's municipal supply from the Madison Aquifer and surface sources has tested clean for PFAS, but the city's proximity to the base means that groundwater in the eastern suburbs draws from potentially affected zones. A 2024 South Dakota DENR report documented PFAS detections in two water systems in the county.
Box Elder and communities near Ellsworth AFB are in the direct path of the documented PFAS plume. If you are on a private well in this area and have not been contacted by the Air Force for testing, requesting a test is reasonable.
Check your water to see data for your area. For PFAS, reverse osmosis is the most effective household treatment. Our water filter guide identifies systems rated for military-concentration PFAS removal. Pull your detailed report for trends, and visit our South Dakota page for statewide context.