Arsenic and uranium in Connecticut wells come from the rock the well is drilled into. A U.S. Geological Survey study with CT DPH found arsenic above the EPA limit across 3.9% of the state and uranium above its limit across 4.7%, with much higher rates in particular rock units (USGS Open-File Report 2020-1111).
● Key Takeaways
2,433 well samples were analyzed for arsenic and 2,191 for uranium (2013 to 2018). Spatially weighted, 3.9% of the state exceeded 10 µg/L arsenic and 4.7% exceeded 30 µg/L uranium. 44 of 115 rock units had at least one exceedance. The highest rates sit in specific units in southwestern, northwestern and eastern Connecticut and the lower Connecticut River valley. USGS warns these rates may not represent the hazard for any one well. Test yours.
What Did the Statewide Study Find?
From 2013 to 2018, CT DPH collected samples from private wells drilled into bedrock and USGS analyzed them: 2,433 for arsenic and 2,191 for uranium, from wells in 115 geologic units (USGS Open-File Report 2020-1111). The headline results:
- Arsenic: 102 samples (4.2%) were above the EPA limit of 10 µg/L. Weighted so every part of the state counts equally, high arsenic was present in 3.9% of Connecticut.
- Uranium: 118 samples (5.4%) were above the EPA limit of 30 µg/L, or 4.7% of the state spatially weighted. Low-level uranium, 1 µg/L or more, was found across 44.9% of the state.
- Geology: 44 of the 115 rock units studied had at least one sample above a limit.
An earlier USGS report on 2013 to 2015 samples found that nearly 1 in 15 samples (7%) from 674 wells exceeded the arsenic or uranium limit, and concluded that a household's chance of a high result "may depend in large part on the particular geologic unit" its well is in (USGS Open-File Report 2017-1046).
CT DPH's fact sheets put it simply: wells with high levels "have been found sporadically all around Connecticut," and testing is the only way to know (CT DPH uranium; CT DPH arsenic).
Which Rock Units Had the Highest Uranium Rates?
The table lists rock units with at least 10 uranium samples where 10% or more exceeded 30 µg/L, from USGS Table 4. The towns are where each unit covers the most ground on the Connecticut bedrock map, so a town listed here also has other rock types (CT DEEP Bedrock Geology).
| Rock unit (USGS) | Samples above 30 µg/L uranium | Where it is mapped (largest areas first) |
|---|---|---|
| Harrison Gneiss | 46.7% of 15 | Greenwich, Stamford, Wilton |
| "Eastford Gneiss phase" of Canterbury Gneiss | 25.9% of 27 | Eastford, Woodstock, Pomfret |
| Ordovician(?) granitic gneiss | 25.6% of 86 | Weston, Redding, Wilton, New Milford, Easton, Ridgefield |
| Lower member of Middletown Formation | 25.0% of 12 | Haddam, Chester, Middletown, Deep River |
| Hoosac Schist | 20.0% of 40 | Barkhamsted, Torrington, New Hartford, Hartland |
| Glastonbury Gneiss | 14.6% of 41 | Glastonbury, Stafford, Portland, Ellington, Vernon |
| Brookfield Gneiss | 12.8% of 78 | Newtown, Litchfield, Monroe, Easton, Bethel, Brookfield |
| Nonewaug Granite | 10.5% of 19 | Harwinton, Woodbury, Watertown, New Hartford, Bethlehem |
Which Rock Units Had the Highest Arsenic Rates?
Same rule: units with at least 10 arsenic samples where 10% or more exceeded 10 µg/L (USGS Open-File Report 2020-1111; CT DEEP Bedrock Geology).
| Rock unit (USGS) | Samples above 10 µg/L arsenic | Where it is mapped (largest areas first) |
|---|---|---|
| Scotland Schist | 16.7% of 96 | Scotland, Franklin, Pomfret, Hampton, Windham |
| Ordovician(?) granitic gneiss | 14.2% of 106 | Weston, Redding, Wilton, New Milford, Easton, Ridgefield |
| Hebron Gneiss | 13.4% of 290 | Hebron, East Haddam, Lebanon, Woodstock, Coventry |
| Lebanon Gabbro | 13.3% of 15 | Lebanon, Windham |
| The Straits Schist | 12.5% of 24 | Granby, Trumbull, Monroe, Easton, Canton |
| Collinsville Formation | 12.1% of 33 | Monroe, Oxford, Watertown, Southbury, Seymour |
| Wepawaug Schist | 10.0% of 10 | Woodbridge, Orange, Milford |
What Does That Look Like by Region?
Grouping those units by where they sit on the map:
- Southwestern Connecticut (Fairfield County). The Ordovician(?) granitic gneiss is the only unit on both lists, and most of its area is in Fairfield County, led by Weston, Redding and Wilton. The Harrison Gneiss (Greenwich, Stamford, Wilton) and Brookfield Gneiss (Newtown and neighbors) are on the uranium list.
- Northwestern Connecticut (Litchfield County). The Hoosac Schist and Nonewaug Granite are on the uranium list.
- Eastern Connecticut (Windham, Tolland and New London counties). The Scotland Schist, Hebron Gneiss and Lebanon Gabbro are on the arsenic list; the Eastford Gneiss phase of the Canterbury Gneiss is on the uranium list. The Hebron Gneiss had more samples than any other unit (290 for arsenic).
- Lower Connecticut River valley (Middlesex County). The lower member of the Middletown Formation, mostly in Haddam and Chester, is on the uranium list; the Hebron Gneiss extends into East Haddam.
- Central New Haven County and the Naugatuck valley. The Straits Schist, Collinsville Formation and Wepawaug Schist are on the arsenic list.
The record also has striking individual results. The 2017 USGS report noted the highest arsenic sample, 470 µg/L, from a well in Oxford, and the highest uranium sample, 766 µg/L, from a well in Brookfield (USGS Open-File Report 2017-1046).
Why a Rock Unit Is Not a Verdict on Your Well
USGS is explicit: the rock-unit percentages "may not represent the actual hazard for existing and future wells" (USGS Open-File Report 2020-1111). The study's lead author said bedrock geology "is not always predictive" of high or low concentrations (USGS, April 2021), and the 2017 report notes that local groundwater chemistry and how long water stays in contact with rock also matter (USGS Open-File Report 2017-1046). Several units on these lists had only 10 to 40 samples.
Read the tables the other way, too: in every unit listed, most samples were below the limit. A well on a unit that is not listed can still test high, because USGS and CT DPH have found high wells "sporadically" across the state. The tables tell you where testing has turned up the most problems. They do not tell you what is in your water.
What Should a Well Owner Do?
- Test at least once for both, and ideally every five years, through a CT DPH-certified lab (CT DPH testing recommendations). Wells drilled since October 1, 2022 were tested for both at construction; older wells may never have been.
- Compare with the limits. Arsenic: 10 µg/L, the EPA limit and CT DPH Action Level. Uranium: 30 µg/L, the EPA limit.
- If a result is high, retest, then treat the water you drink. CT DPH says both are mainly a concern when swallowed, so a point-of-use system at the drinking and cooking tap is often enough. Reverse osmosis is the most common uranium treatment. For arsenic, CT DPH suggests testing for arsenic III before choosing a system, because it is harder to remove (CT DPH arsenic; CT DPH uranium).
- If uranium is high, test radon too. CT DPH's troubleshooting guide pairs uranium above 30 µg/L with tests for radium and radon. See our radon in Connecticut well water guide.
Find out what is in your well
A water specialist can arrange certified lab testing for arsenic, uranium and radon in your well water.
CheckYourTap is an independent water-quality service, not your town or utility. We connect you with a vetted local partner. Your online report is free. Testing and treatment are paid services, and there's no cost to talk.
Keep Reading
- Private well testing in Connecticut: the complete guide
- Weston, Redding, Wilton, Newtown and Haddam well guides
- Uranium in eastern Connecticut wells
- What a "safe" arsenic result really means
How we built the tables: the percentages and sample counts are copied from USGS Open-File Report 2020-1111, Table 4. The "where it is mapped" towns come from overlaying the CT DEEP bedrock geology map (Rodgers, 1985, digitized at 1:50,000) on CTDOT municipal boundaries and ranking towns by the area of each unit inside them. The town lists describe geology only. They are not measurements of any town's wells.
Sources: Gross and Brown, Arsenic and uranium occurrence in private wells in Connecticut, 2013–18, USGS Open-File Report 2020-1111; Flanagan and Brown, Arsenic and uranium in private wells in Connecticut, 2013–15, USGS Open-File Report 2017-1046; USGS news release, New USGS report shows high levels of arsenic and uranium in some wells (April 2021); CT DEEP, Bedrock Geology of Connecticut; CTDOT Municipalities; CT DPH, Arsenic in private well water; CT DPH, Uranium in Private Well Water (April 2024); CT DPH, Educational materials: testing recommendations for private wells (updated May 2026).
About this guide: CheckYourTap is an independent water-quality service funded by Valiant Energy Solutions, a Connecticut water quality and energy services company established in 1930. Valiant offers paid well testing and treatment, which is what the request button on this page connects you with. You can also contact any CT DPH-certified laboratory directly. The facts on this page come from the public sources cited above.
