Colorado Private Well Water Risk: Arsenic & Nitrate
These figures are USGS statistical estimates for private (domestic) wells in Colorado. They describe the area's modeled risk — not any single well.
Quick answer: Yes — arsenic occurs naturally in Colorado groundwater. USGS modeling estimates about 14,342 Colorado residents rely on private wells with arsenic above the EPA limit of 10 µg/L. The highest modeled exposure is in Alamosa County (~2,831), El Paso County (~1,547), Rio Grande County (~1,382). For nitrate, about 0.7% of Colorado's ~341,000 private-well residents are modeled above the 10 mg/L limit. These are area estimates, not a test of your well — arsenic has no taste or smell, so only a certified lab test tells you your level.
This is a modeled probability for the area, not a measurement of your well. USGS statistical models estimate how many private wells in this area are likely to exceed the federal limit — they cannot tell you what is in your water. Two neighbors can have very different results. The only way to know your well is to test it.
Nitrate
0.7%
of the private-well population is modeled above the 10 mg/L nitrate limit
~2,500
people on wells estimated above the limit
~341,000
total residents on private wells statewide
Source: USGS machine-learning nitrate model, Ransom et al. (2022), domestic-well depth zone.
Find out what's actually in your well
Colorado has one of the higher modeled arsenic exposures in the country — but that number is an area estimate, not your well. Arsenic, nitrate and coliform bacteria are the three most common private-well problems, and none of them has a taste or smell. A certified lab test checks all of them at once; the CDC and EPA recommend testing every well at least once a year.
Basic screening only. Confirm arsenic, nitrate and bacteria with a certified lab.
Find a certified drinking-water labAmazon links are affiliate links — we may earn a commission at no extra cost to you.
Arsenic
Across Colorado, an estimated ~14,342 people rely on private wells with modeled arsenic above the 10 µg/L EPA limit. The highest modeled exposure is in Alamosa County (~2,831 people).
Counties with the highest modeled arsenic exposure
| County | Est. well residents over arsenic limit | 90% range |
|---|---|---|
| Alamosa County | ~2,831 | 2,442–3,208 |
| El Paso County | ~1,547 | 1,181–2,031 |
| Rio Grande County | ~1,382 | 1,209–1,530 |
| Elbert County | ~1,257 | 1,049–1,507 |
| Douglas County | ~997 | 839–1,190 |
| Adams County | ~697 | 573–851 |
| Jefferson County | ~513 | 404–653 |
| Yuma County | ~470 | 376–587 |
| Conejos County | ~361 | 300–431 |
| Arapahoe County | ~352 | 299–410 |
| Park County | ~302 | 231–403 |
| Garfield County | ~268 | 204–354 |
| Kit Carson County | ~248 | 208–295 |
| Teller County | ~245 | 173–344 |
| Washington County | ~200 | 163–252 |
| Bent County | ~186 | 152–226 |
| Fremont County | ~185 | 140–246 |
| Saguache County | ~163 | 135–192 |
| Baca County | ~153 | 131–178 |
| Logan County | ~149 | 107–237 |
| La Plata County | ~137 | 110–176 |
| Weld County | ~117 | 93–147 |
| Moffat County | ~109 | 86–140 |
| Phillips County | ~101 | 81–126 |
40 additional Colorado counties have lower modeled arsenic exposure (fewer than 100 estimated well residents over the limit). Of these, 25 have measured USGS well samples: Boulder, Chaffee, Clear Creek, Crowley, Delta, Denver, Eagle, Gilpin, Grand, Gunnison, Hinsdale, Lake, Larimer, Lincoln, Mesa, Montezuma, Montrose, Morgan, Otero, Prowers, Pueblo, Rio Blanco, Routt, San Juan, Summit.
Source: USGS domestic-well arsenic model, Lombard et al. (2021); county population product (2017).
Measured well tests by county
The USGS has published laboratory results from wells in 43 Colorado counties (at least 5 wells each). These are real measurements of other wells — domestic, public-supply and monitoring wells — not a test of yours. The tables rank counties by the share of sampled wells over each federal threshold.
Nitrate (as N): counties with the most wells > 10 mg/L (EPA MCL)
| County | Wells over | Median | Sampled |
|---|---|---|---|
| Weld County | 50% | 10.2 mg/L | 66 wells |
| Alamosa County | 48% | 8.9 mg/L | 23 wells |
| Logan County | 43% | 6.1 mg/L | 7 wells |
| Rio Grande County | 34% | 6.8 mg/L | 32 wells |
| Morgan County | 33% | 9.7 mg/L | 21 wells |
| Denver County | 23% | 2 mg/L | 57 wells |
| Mesa County | 19% | 1.5 mg/L | 36 wells |
| Arapahoe County | 18% | 0.9 mg/L | 99 wells |
| Montrose County | 18% | 0.2 mg/L | 45 wells |
| Adams County | 17% | 4.8 mg/L | 42 wells |
Manganese: counties with the most wells > 300 µg/L (EPA health advisory)
| County | Wells over | Median | Sampled |
|---|---|---|---|
| Denver County | 71% | 1,618 µg/L | 42 wells |
| Logan County | 43% | 95 µg/L | 7 wells |
| Mesa County | 40% | 160 µg/L | 35 wells |
| Montrose County | 35% | 235 µg/L | 65 wells |
| Delta County | 35% | 165 µg/L | 23 wells |
| Routt County | 29% | 60 µg/L | 24 wells |
| Elbert County | 26% | 21 µg/L | 27 wells |
| Grand County | 26% | 4 µg/L | 19 wells |
| Pueblo County | 24% | 6 µg/L | 33 wells |
| Arapahoe County | 23% | 40 µg/L | 107 wells |
Source: USGS ScienceBase 5f36baa782cee144fb3873e8 (manganese, pH, arsenic) and McMahon et al. (2019), Environ. Sci. Technol. 53:29–38 (nitrate, iron); aggregated by county with Census 2020 boundaries. USGS well samples; not a test of your well.
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Colorado well water: FAQ
Is there arsenic in well water in Colorado?
Yes. USGS modeling estimates about 14,342 people in Colorado use private wells with arsenic above 10 µg/L, across 64 modeled counties. Arsenic comes from natural bedrock and varies well to well, even between neighbors, so a lab test is the only way to know your own level.
Which Colorado counties have the most arsenic in well water?
By modeled number of well residents over the arsenic limit: Alamosa County (~2,831), El Paso County (~1,547), Rio Grande County (~1,382). Counties with more private-well users rank higher, so a smaller county can still have a high share of affected wells.
How do I remove arsenic from well water in Colorado?
Test first. If arsenic is present, a point-of-use reverse-osmosis system under the kitchen sink removes 90%+ of it; boiling does not remove arsenic (it concentrates it), and carbon pitchers and water softeners do not remove it either. Whole-house adsorptive arsenic media is used when levels are high. Re-test after installing and then annually.
Is nitrate a problem in Colorado well water?
USGS models about 0.7% of Colorado's private-well population (~2,500 people) above the 10 mg/L nitrate limit. Nitrate is highest in shallow wells near fertilized cropland, septic systems or livestock, and matters most for infants and pregnancy.
Do I need an acid neutralizer for my well in Colorado?
Acidic well water is uncommon in Colorado, but it does occur. In USGS lab results from 1,807 wells across 38 Colorado counties, 3% had pH below 6.5. The most acidic sampled counties are Summit County (45% of 11 wells), Bent County (14% of 7 wells), Douglas County (11% of 35 wells). You only need a neutralizer if your own well tests below about 6.5 — signs include blue-green stains on fixtures and pinhole leaks in copper pipe. A calcite (or calcite/Corosex) neutralizer tank handles mildly acidic water (roughly pH 6–6.5); very acidic water usually needs soda-ash injection. Test first-draw lead and copper too, since acidic water dissolves both from plumbing.
High manganese in Colorado well water — what should I do?
In USGS lab results from 1,313 wells across 35 Colorado counties, 16% were above the EPA's 300 µg/L health advisory for manganese and 32% were above the 50 µg/L level where it stains fixtures and laundry black or brown. Sampled counties with the highest share over the health advisory: Denver County (71% of 42 wells), Logan County (43% of 7 wells), Mesa County (40% of 35 wells). If your own test is over 300 µg/L, don't use the water for infant formula and use another source for drinking and cooking until it is treated — high manganese is linked to neurological effects, especially in babies and children. Boiling does not remove it. For the whole house, an oxidizing filter (greensand, Birm or air-injection) handles manganese, usually alongside iron; a water softener can remove low levels. For drinking water, a point-of-use reverse osmosis system reduces manganese at the kitchen tap. Retest after treatment to confirm it worked.