Arsenic and manganese are widespread in Pennington County wells.
2 widespread, 1 likely overstated, 3 rare out of 6 health checks.
From 100 wells tested in Pennington County since 2001; your own well needs its own test. How to read this
*Uranium: All of these tests come from the US Geological Survey in one small part of the county, so it may not match the rest of the county.
Rare: under 5%Worth checking: 5 to 15%Widespread: 15%+
Lab results: Water Quality Portal (USGS and EPA)
Who drinks from a well here
1 in 7 people 15,241 people
Nobody else tests these wells. Only you do.
People on wells: EPA researchers' 2020 estimate
How deep wells go here
Deep wells: fewer surface germs, more minerals from the rock.
Well depths: US Groundwater Well Database
Radon risk (a gas from rock)
Possible here. A home air test is inexpensive.
Radon zones: EPA map
Here's how Pennington County compares with South Dakota and the counties around it.
Each result puts Pennington County next to the 7 counties that border it, on the chart's 0 to 50% scale.
These 2 are widespread in Pennington County wells.
Arsenic6 of 17 wells over
Too few tested to show: Haakon, Jackson, Lawrence, Meade, Oglala Lakota, Weston, WY.
Manganese7 of 32 wells over
Too few tested to show: Custer, Haakon, Jackson, Lawrence, Oglala Lakota, Weston, WY.
This one has turned up often here, mostly from one small area.
Uranium2 of 10 wells over
All of these tests come from the US Geological Survey in one small part of the county, so it may not match the rest of the county.
These 3 are rare in Pennington County.
Nitrate3 of 95 wells over
* Likely overstated, so we don't rank it.
Too few tested to show: Custer, Haakon, Jackson, Lawrence, Oglala Lakota, Weston, WY.
Lead0 of 10 wells over
Fluoride0 of 32 wells over
Too few tested to show: Custer, Haakon, Jackson, Lawrence, Oglala Lakota, Weston, WY.
Every well needs a bacteria test every year.
No bacteria tests from Pennington County wells are in the public data. Test every year anyway.
Iron reached staining levels in 5 of 32 tested wells, and it isn't a health risk.
Iron stains5 of 32 wells had it
Too few tested to show: Custer, Haakon, Jackson, Lawrence, Oglala Lakota, Weston, WY.
Most of the 100 wells tested in Pennington County were last tested before 2010.
An old result shows the water as it was then, so treat these 100 wells as a starting point and test your own.
Most Pennington County wells in these records were tested for research or because someone suspected a problem, which can push the shares above what a typical home well would show (how we read the numbers).
One mail-in lab test covers the 5 things to check in Pennington County.
- Bacteria
- Nitrate
- Arsenic
- Manganese
- Uranium
What South Dakota offers well owners
- Free testingDANR points well owners to a free private well assessment, including water testing, offered by Midwest Assistance Program. See the program
- Certified labsSouth Dakota's list of certified labs
- Well recordsSouth Dakota's well record search
Half the household wells here are shallower than 165 feet.
That comes from 2,660 household wells in South Dakota's drilling records. Most wells of every kind here are 32 to 288 feet deep. Wells as deep as Pennington County's are usually better protected from fertilizer and bacteria at the surface, but they can pick up more of what's in the rock, like arsenic and uranium. Typical household wells here are shallower than in 41 of the 65 other South Dakota counties with enough drilling records.
Look up your own well's depth in South Dakota's well record search, or see how to find it.
Well depths: US Groundwater Well Database
Here's what we know about radon and PFAS in Pennington County.
Pennington County is in the EPA's middle radon zone. An indoor air test is cheap and tells you whether to look at your water. Radon guide
Between 2023 and 2025 the EPA had 6 town water systems in Pennington County test for PFAS. 2 of them had at least one test above 4 ng/L, the new EPA limit for PFOA and PFOS. The highest single result was 11 ng/L of PFOS. No well in Pennington County has public PFAS results, so the town systems are the best clue to what's in the groundwater nearby. PFAS guide
Radon zones: EPA Map of Radon Zones. PFAS: EPA tests of town water systems, 2023 to 2025
Questions people ask about well water in Pennington County
Is well water in Pennington County safe to drink?
6 of 17 wells tested in Pennington County had arsenic over the limit, about 1 in 3. Across South Dakota it's about 1 in 4. So test your own well for arsenic before you count on it.
Can I get my well tested for free in Pennington County?
DANR points well owners to a free private well assessment, including water testing, offered by Midwest Assistance Program. If not, use a lab South Dakota has certified for drinking water. Ask for bacteria, nitrate, arsenic, manganese and uranium, the tests that matter most in Pennington County.
How deep are wells in Pennington County?
Half the household wells here are shallower than 165 feet. Typical household wells here are shallower than in 41 of the 65 other South Dakota counties with enough drilling records.
How does Pennington County compare with the counties around it?
For arsenic, the share of tested wells over the limit: Custer, the one neighbor with data, came in higher than Pennington County, at 36% against 35%. Groundwater doesn't stop at county lines, so Custer's 36% is worth knowing about if you live near it.
How often should I test my well in Pennington County?
Pennington County has no public bacteria results for wells, so follow the CDC's advice: test for bacteria and nitrate every year, and right away after a flood, a repair, or any change in taste or smell.
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Where the Pennington County numbers come from
- People on wells: 15,241 in Pennington County, from EPA estimates built on the 2020 Census (Murray et al.).
- Lab tests: 377 well water samples from 100 wells in the Water Quality Portal, run by the USGS and EPA, 2001 to 2023.
- Well depths: 2,660 South Dakota drilling records for Pennington County, gathered in the US Groundwater Well Database (Lin and others, 2024).
- Neighboring counties: the 7 counties that share a border with Pennington County, from the Census Bureau's county adjacency file.
- Results marked likely overstated: our own check of who tested the wells, when and where.
- PFAS: EPA tests of 6 town water systems in Pennington County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
- Radon: Pennington County is zone 2 on the EPA Map of Radon Zones.
- State programs and links: Department of Agriculture and Natural Resources (DANR), checked November 1, 2026.