Pennington County, SD
drinking water quality
115,979 residents · 158 public water systems · 9 ZIP codes
EPA sampled 6 of 158 public water systems here, together serving about 111,316 people. The highest reading was PFOS at 10.6 ng/L at Nps-Mount Rushmore National Memorial. 2 of the sampled systems were above a federal limit. Last sampled 9/3/2025.
What was found
| Compound | Highest result | Federal limit | Ratio |
|---|---|---|---|
| PFOS | 10.6 ng/L | 4 ng/L | 2.6× |
| PFHxS | 20.8 ng/L | 10 ng/L | 2.1× |
Also detected, with no federal limit set: lithium (33700 ng/L), PFBS (3.2 ng/L).
Health-based violations
Separately from PFAS, EPA records 45 health-based violations across 27 of the county’s water systems since 2020, the most recent in 2026. They involved gross alpha radiation, radium, lead and copper, contaminant 0700, E. coli, contaminant 0400.
What this means
PFAS are industrial chemicals that EPA describes as breaking down only very slowly in the environment. In April 2024 EPA set the first enforceable national limits: 4 ng/L for PFOA and PFOS, 10 ng/L for PFHxS, PFNA and GenX.
These figures are county-level and come from EPA’s sampling of public water systems. They describe the worst result found in the county, not what comes out of any particular tap — that depends on which system serves the address, and on the plumbing in the building. A private well is not covered by this programme at all.
ZIP codes in Pennington County
57701 · 57702 · 57703 · 57725 · 57745 · 57751 · 57775 · 57780 · 57790
Readings are county-level, so every ZIP here shares the figure above. Search your ZIP on the map to see it in context.