Haddam's bedrock is mostly Monson Gneiss, and the town holds more than half of Connecticut's mapped area of the lower member of the Middletown Formation, a unit where USGS found 25.0% of 12 statewide samples above the EPA uranium limit (USGS Open-File Report 2020-1111; CT DEEP). CT DPH recommends testing every private well for arsenic, uranium and radon at least once (CT DPH).
● Key Takeaways
About 44% of Haddam is Monson Gneiss (statewide: 0% of 115 over the 10 µg/L arsenic limit, 8.5% of 94 over the 30 µg/L uranium limit). About 13% is Hebron Gneiss (13.4% of 290 over the arsenic limit) and about 13% the lower Middletown Formation (25.0% of 12 over the uranium limit). No town-level results are published. Test for uranium, arsenic and radon at least once.
What Rock Is Under Haddam?
Monson Gneiss covers the largest share of Haddam. Hebron Gneiss, the members of the Middletown Formation and the Collins Hill Formation make up most of the rest.
| Rock unit | Share of town (CT DEEP map) | USGS samples above 10 µg/L arsenic | USGS samples above 30 µg/L uranium |
|---|---|---|---|
| Monson Gneiss | About 44% | 0% of 115 | 8.5% of 94 |
| Hebron Gneiss | About 13% | 13.4% of 290 | 4.5% of 199 |
| Lower member of Middletown Formation | About 13% | 0% of 5 | 25.0% of 12 |
| Collins Hill Formation | About 12% | 0% of 23 | 4.5% of 22 |
| Middletown Formation | About 8% | 0% of 38 | 4.4% of 45 |
| Upper member of Middletown Formation | About 6% | 0% of 20 | 11.1% of 9 |
| Brimfield Schist (queried on the map) | About 2% | 0% of 6 | 0% of 6 |
The share of town is the area of each unit inside Haddam's boundary on the CT DEEP bedrock map (CT DEEP); units under 1% are left out. The percentages are from USGS Table 4 and describe wells in that rock unit statewide, not wells in Haddam (USGS Open-File Report 2020-1111). Units with only a handful of samples tell you very little.
The units split along the two elements. In the Monson Gneiss and the Middletown Formation and its members, no statewide samples exceeded the arsenic limit, but uranium exceedances ranged from 4.4% to 25.0%. In the Hebron Gneiss, the most-sampled unit in the whole study, 13.4% of 290 samples exceeded the arsenic limit and 4.5% of 199 the uranium limit. A Haddam well's result can depend on which of these it is drilled into.
What Has Testing Found in Haddam?
Neither USGS report breaks out Haddam; the 2017 report gave town-level figures only for Stamford, Weston and Wilton. Statewide, the 2013 to 2018 data found uranium at 1 µg/L or more across about 44.9% of Connecticut (USGS Open-File Report 2020-1111).
Because radon forms from uranium, CT DPH recommends a radon in water test in every home on a well, separate from the uranium test (CT DPH, revised February 2025).
What Should Haddam Well Owners Test, and How Often?
CT DPH's schedule applies to every private well in Connecticut, whatever the town (CT DPH, updated May 2026):
- Basic indicators every year (bacteria, nitrate, nitrite, sodium, chloride, iron, manganese, hardness, pH and others), and after any well, pump or pipe repair or a flooded wellhead.
- Arsenic, uranium and radon at least once, ideally every 5 years. Levels can fluctuate.
- Lead and volatile organic compounds at least once, and lead again when planning a pregnancy or with a child under 6.
- Fluoride every 5 years when a child under 12 lives in the home.
Since October 1, 2022, new wells have been tested for arsenic and uranium when drilled (CT DPH arsenic; CT DPH uranium). If your well is older and has no arsenic or uranium result on file, that is the gap to close first. CT DPH also recommends a radon in water test for every home on a well, even if indoor air tested low (CT DPH, revised February 2025).
Use a laboratory certified by CT DPH for the tests you need, and compare results with CT DPH Action Levels and EPA limits: 10 µg/L for arsenic, 30 µg/L for uranium, and CT DPH's recommended 5,000 pCi/L action level for radon in water. If a result is high, CT DPH says to retest, stop drinking the water until it is resolved, and contact your local health department (find yours) or the CT DPH Private Well Program at (860) 509-8401. The full schedule, lab tips and how to read a report are in our Connecticut private well testing guide.
What This Page Does Not Tell You
It does not tell you what is in your well. USGS cautions that rock-unit exceedance rates "may not represent the actual hazard for existing and future wells," and the study's lead author said bedrock geology "is not always predictive" (USGS Open-File Report 2020-1111; USGS, April 2021). Even neighboring towns can differ sharply, as Weston and Wilton do in USGS's 2013 to 2015 data. Only a certified lab test of your water answers the question.
Test your Haddam well
A water specialist can arrange certified lab testing for your well, starting with arsenic, uranium, radon and a basic indicators panel.
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
- Arsenic and uranium in Connecticut wells, by region
- Radon in Connecticut well water
- Newtown well water testing
- Uranium in eastern Connecticut wells
How we built this page: rock unit shares come from overlaying the CT DEEP bedrock geology map (Rodgers, 1985, digitized at 1:50,000) on the CTDOT municipal boundary for Haddam. Exceedance percentages and sample counts are copied from USGS Open-File Report 2020-1111, Table 4. This page describes geology and published sampling. It is not a measurement or rating of any home's well.
Sources: Gross and Brown, USGS Open-File Report 2020-1111; Flanagan and Brown, USGS Open-File Report 2017-1046; USGS news release (April 2021); CT DEEP, Bedrock Geology of Connecticut; CTDOT Municipalities; CT DPH, Educational materials: testing recommendations for private wells (updated May 2026); CT DPH, Arsenic in private well water; CT DPH, Uranium in Private Well Water (April 2024); CT DPH, Radon in Your Well Water (revised February 2025).
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.
