Missouri Private Well Water Risk: Arsenic & Nitrate
These figures are USGS statistical estimates for private (domestic) wells in Missouri. They describe the area's modeled risk — not any single well.
Quick answer: Yes — arsenic occurs naturally in Missouri groundwater. USGS modeling estimates about 10,233 Missouri residents rely on private wells with arsenic above the EPA limit of 10 µg/L. The highest modeled exposure is in Greene County (~618), Lincoln County (~548), Pettis County (~488). For nitrate, about 0% of Missouri's ~871,000 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%
of the private-well population is modeled above the 10 mg/L nitrate limit
~32
people on wells estimated above the limit
~871,000
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
USGS models flag arsenic in Missouri, but two wells on the same road can test very differently. 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 Missouri, an estimated ~10,233 people rely on private wells with modeled arsenic above the 10 µg/L EPA limit. The highest modeled exposure is in Greene County (~618 people).
Counties with the highest modeled arsenic exposure
| County | Est. well residents over arsenic limit | 90% range |
|---|---|---|
| Greene County | ~618 | 498–770 |
| Lincoln County | ~548 | 421–709 |
| Pettis County | ~488 | 312–768 |
| Franklin County | ~467 | 369–595 |
| Camden County | ~434 | 243–767 |
| Cape Girardeau County | ~394 | 233–657 |
| Butler County | ~384 | 280–523 |
| Newton County | ~340 | 273–424 |
| Jefferson County | ~326 | 264–407 |
| McDonald County | ~245 | 133–463 |
| Miller County | ~208 | 153–287 |
| Morgan County | ~204 | 137–319 |
| Christian County | ~199 | 159–248 |
| Webster County | ~198 | 160–247 |
| St. Charles County | ~196 | 141–274 |
| Scott County | ~195 | 146–261 |
| Bollinger County | ~193 | 145–256 |
| Stoddard County | ~191 | 146–250 |
| Lawrence County | ~181 | 146–227 |
| Polk County | ~181 | 146–227 |
| St. Francois County | ~178 | 144–220 |
| Washington County | ~161 | 131–200 |
| Callaway County | ~148 | 86–252 |
| Phelps County | ~142 | 113–179 |
| Crawford County | ~138 | 109–176 |
| Barry County | ~137 | 99–198 |
| Montgomery County | ~128 | 80–204 |
| Jasper County | ~128 | 104–161 |
| Warren County | ~127 | 96–171 |
| Benton County | ~121 | 89–170 |
| Cooper County | ~121 | 82–178 |
| New Madrid County | ~121 | 91–161 |
| Cole County | ~116 | 86–158 |
| Dallas County | ~115 | 92–143 |
| Howell County | ~115 | 92–143 |
80 additional Missouri counties have lower modeled arsenic exposure (fewer than 100 estimated well residents over the limit). Of these, 56 have measured USGS well samples: Andrew, Atchison, Barton, Bates, Boone, Caldwell, Carroll, Carter, Cass, Cedar, Chariton, Clark, Clay, Dade, Daviess, Dent, Douglas, Dunklin, Gasconade, Gentry, Henry, Hickory, Holt, Howard, Iron, Jackson, Johnson, Laclede, Lewis, Livingston, Madison, Maries, Mercer, Mississippi, Moniteau, Nodaway, Oregon, Osage, Ozark, Pemiscot, Perry, Pulaski, Ray, Reynolds, Ripley, Saline, Shannon, St. Clair, St. Louis, Ste. Genevieve, Stone, Taney, Texas, Vernon, Wayne, Wright.
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 91 Missouri 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 |
|---|---|---|---|
| Barton County | 20% | 0 mg/L | 5 wells |
| Boone County | 17% | 0 mg/L | 29 wells |
| St. Charles County | 11% | 0 mg/L | 70 wells |
Manganese: counties with the most wells > 300 µg/L (EPA health advisory)
| County | Wells over | Median | Sampled |
|---|---|---|---|
| Moniteau County | 92% | 1,202 µg/L | 12 wells |
| Jackson County | 79% | 478 µg/L | 39 wells |
| Clay County | 71% | 761 µg/L | 21 wells |
| Boone County | 57% | 326 µg/L | 28 wells |
| Cape Girardeau County | 36% | 27 µg/L | 11 wells |
| St. Charles County | 20% | 53 µg/L | 66 wells |
| Franklin County | 18% | 1 µg/L | 11 wells |
| Pulaski County | 9% | 1 µg/L | 76 wells |
| St. Francois County | 9% | 34 µg/L | 43 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.
Match the filter to your test result
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Missouri well water: FAQ
Is there arsenic in well water in Missouri?
Yes. USGS modeling estimates about 10,233 people in Missouri use private wells with arsenic above 10 µg/L, across 115 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 Missouri counties have the most arsenic in well water?
By modeled number of well residents over the arsenic limit: Greene County (~618), Lincoln County (~548), Pettis County (~488). 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 Missouri?
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 Missouri well water?
USGS models about 0% of Missouri's private-well population (~32 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 Missouri?
Acidic well water is uncommon in Missouri, but it does occur. In USGS lab results from 2,467 wells across 91 Missouri counties, 2% had pH below 6.5. The most acidic sampled counties are Henry County (33% of 6 wells), St. Clair County (26% of 23 wells), Pemiscot County (20% of 10 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 Missouri well water — what should I do?
In USGS lab results from 375 wells across 15 Missouri counties, 27% were above the EPA's 300 µg/L health advisory for manganese and 47% 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: Moniteau County (92% of 12 wells), Jackson County (79% of 39 wells), Clay County (71% of 21 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.