The state of America's private wells, 2026

Name: Private well water test results by US county, 2026
Creator: Tested Water
License: https://creativecommons.org/licenses/by/4.0/

By the Tested Water teamData from the USGS and EPA Water Quality PortalUpdated November 8, 2026

The short version

We went through every public lab test of private and monitoring wells in the Water Quality Portal since 2000 and ranked counties by how often their wells came back over the EPA's limits. Below are the counties at the top of each list. Anyone can cite this page or download the full rankings.

How we made these rankings

Each ranking shows the share of tested wells in a county that had at least one result over the EPA limit for public drinking water. We only rank counties with at least 25 tested wells, so a few bad results can't put a county at the top. Many of these wells were tested for research or because a problem was suspected, so the rankings show where problems turn up, not the odds for any one well.

Where nitrate shows up most in wells

#	County	Wells tested	Wells over the limit
1	Malheur County, OR	49	67%^*
2	Iowa County, WI	49	65%^*
3	Adams County, WI	114	63%^*
4	Whatcom County, WA	104	62%^*
5	Stearns County, MN	56	61%
6	Rock County, WI	99	56%^*
7	Kent County, MD	47	55%^*
8	Dunn County, WI	77	55%^*
9	Waushara County, WI	113	50%^*
10	Lancaster County, PA	90	50%^*
11	Pepin County, WI	26	50%^*
12	Calumet County, WI	53	49%^*
13	Portage County, WI	174	49%^*
14	Sauk County, WI	99	47%^*
15	Morrow County, OR	121	46%
16	Green Lake County, WI	78	46%^*
17	Salem County, NJ	25	44%
18	Juneau County, WI	78	44%^*
19	Dane County, WI	138	43%^*
20	Mason County, IL	27	41%
21	Columbia County, WI	114	40%^*
22	Trempealeau County, WI	62	40%^*
23	York County, NE	38	39%^*
24	Queen Anne's County, MD	45	38%
25	Green County, WI	59	37%^*

* Likely overstated, because of who tested these wells, when or where.

Download every county (CSV) · What to know about nitrate

Where arsenic shows up most in wells

#	County	Wells tested	Wells over the limit
1	Malheur County, OR	48	94%^*
2	Kennebec County, ME	58	93%^*
3	Churchill County, NV	177	88%^*
4	Reynolds County, MO	26	69%^*
5	Eagle County, CO	34	62%^*
6	Yuma County, AZ	103	60%
7	Roosevelt County, MT	32	59%^*
8	Benson County, ND	59	59%
9	Owyhee County, ID	56	59%
10	St. Francois County, MO	27	56%^*
11	Cleveland County, OK	65	55%
12	Dixie County, FL	46	54%
13	McLeod County, MN	75	53%^*
14	Washington County, UT	70	53%^*
15	Gallatin County, MT	25	52%^*
16	Clay County, MN	90	50%^*
17	Pope County, MN	69	49%^*
18	Mono County, CA	35	49%
19	Gilliam County, OR	29	48%^*
20	Bradford County, FL	28	46%
21	Silver Bow County, MT	28	46%
22	Twin Falls County, ID	97	46%
23	Montezuma County, CO	26	46%
24	Kings County, CA	33	45%
25	Washington County, ID	36	44%

* Likely overstated, because of who tested these wells, when or where.

Download every county (CSV) · What to know about arsenic

Where uranium shows up most in wells

#	County	Wells tested	Wells over the limit
1	Montrose County, CO	51	67%
2	Stevens County, WA	44	64%^*
3	Carson City, NV	34	35%
4	Doña Ana County, NM	148	31%
5	Arapahoe County, CO	67	28%
6	Fresno County, CA	147	25%
7	Elbert County, CO	29	24%
8	Goshen County, WY	25	24%
9	Orange County, CA	52	23%
10	Adams County, CO	36	22%
11	Kings County, CA	33	21%
12	San Diego County, CA	70	20%
13	Franklin County, WA	28	18%
14	Platte County, WY	28	18%
15	Maricopa County, AZ	96	17%
16	Rio Arriba County, NM	36	17%
17	Pottawatomie County, OK	49	16%
18	Douglas County, CO	31	16%
19	Stanislaus County, CA	140	15%
20	Tulare County, CA	119	14%
21	Madera County, CA	87	14%
22	Sweetwater County, WY	38	13%
23	Sheridan County, WY	34	12%
24	Kern County, CA	179	12%
25	Kennebec County, ME	43	12%

* Likely overstated, because of who tested these wells, when or where.

Download every county (CSV) · What to know about uranium

Where manganese shows up most in wells

#	County	Wells tested	Wells over the limit
1	Washington County, MS	32	91%
2	Lonoke County, AR	28	89%
3	Jackson County, MO	55	85%^*
4	Washington County, OR	27	81%
5	Monroe County, AR	43	74%^*
6	Nobles County, MN	68	72%
7	Multnomah County, OR	46	72%
8	Lancaster County, NE	31	71%^*
9	New York County, NY	25	68%^*
10	Washington County, NE	44	61%
11	Hennepin County, MN	159	61%
12	Douglas County, NE	35	57%
13	Westchester County, NY	44	57%
14	Lake County, IN	37	57%
15	Cecil County, MD	66	56%
16	Harvey County, KS	172	52%
17	Douglas County, OR	58	52%
18	Benson County, ND	86	51%
19	District of Columbia County, DC	38	50%
20	Coos County, OR	30	50%
21	Franklin Parish, LA	28	50%
22	Natchitoches Parish, LA	26	50%
23	Burt County, NE	40	48%
24	Davis County, UT	59	47%
25	Lafayette Parish, LA	38	47%

* Likely overstated, because of who tested these wells, when or where.

Download every county (CSV) · What to know about manganese

Where bacteria show up most in wells

#	County	Wells tested	Wells over the limit
1	Stearns County, MN	30	100%
2	Whatcom County, WA	38	92%
3	Collier County, FL	196	91%
4	Anoka County, MN	58	90%
5	Sedgwick County, KS	52	88%
6	Cumberland County, PA	33	82%
7	Harvey County, KS	172	81%
8	Dauphin County, PA	35	80%
9	Minidoka County, ID	343	78%
10	Washington County, MN	64	75%
11	Montgomery County, PA	28	68%
12	Platte County, WY	28	68%
13	Potter County, PA	46	67%
14	Clinton County, PA	56	64%
15	Eagle County, CO	25	64%
16	Manatee County, FL	25	64%
17	Polk County, FL	71	63%
18	Lee County, FL	60	63%
19	Berkeley County, WV	51	63%
20	Jerome County, ID	36	61%
21	Miami-Dade County, FL	70	60%
22	Albany County, WY	25	60%
23	Goshen County, WY	25	60%
24	Clatsop County, OR	34	59%
25	Marion County, FL	43	58%

Download every county (CSV) · What to know about bacteria

Where lead shows up most in wells

#	County	Wells tested	Wells over the limit
1	Dixie County, FL	54	52%
2	Bradford County, FL	27	44%
3	Lafayette County, FL	64	42%
4	Levy County, FL	135	40%
5	Columbia County, FL	109	39%
6	Westchester County, NY	44	34%
7	Suwannee County, FL	136	34%
8	Gilchrist County, FL	73	33%
9	Union County, FL	28	32%
10	Madison County, FL	69	32%
11	Taylor County, FL	67	31%
12	Shoshone County, ID	494	27%
13	Hamilton County, FL	42	26%
14	Jefferson County, FL	33	24%
15	Whatcom County, WA	29	21%
16	Powder River County, MT	73	16%^*
17	Lake County, CO	93	16%
18	Alachua County, FL	100	16%
19	Reynolds County, MO	26	15%^*
20	Queens County, NY	103	14%
21	Eagle County, CO	34	12%
22	Sussex County, DE	61	11%
23	Kings County, NY	35	9%
24	Miami-Dade County, FL	87	8%
25	Kootenai County, ID	28	7%

* Likely overstated, because of who tested these wells, when or where.

Download every county (CSV) · What to know about lead

Where radon shows up most in wells

#	County	Wells tested	Wells over the limit
1	Stevens County, WA	61	44%^*
2	Baltimore County, MD	59	37%
3	Jefferson County, MT	66	36%
4	Chester County, PA	71	28%
5	Park County, CO	81	23%
6	Sweetwater County, WY	31	16%
7	Madera County, CA	39	13%
8	Wayne County, PA	127	13%
9	San Diego County, CA	70	11%
10	San Bernardino County, CA	83	10%
11	Litchfield County, CT	33	9%^*
12	Dauphin County, PA	36	8%
13	St. Lawrence County, NY	42	7%
14	Sonoma County, CA	47	4%
15	Pike County, PA	95	4%
16	Pima County, AZ	48	4%
17	Pinal County, AZ	58	3%
18	La Paz County, AZ	90	3%
19	Hillsborough County, FL	40	2%
20	Carbon County, WY	40	2%
21	Lycoming County, PA	82	2%
22	Kern County, CA	42	2%
23	Garrett County, MD	43	2%
24	Mohave County, AZ	105	2%
25	Fresno County, CA	60	2%

* Likely overstated, because of who tested these wells, when or where.

Download every county (CSV) · What to know about radon

Where iron shows up most in wells

#	County	Wells tested	Wells over the limit
1	Lonoke County, AR	28	100%
2	Steele County, MN	90	99%
3	Jackson County, MO	55	98%
4	Washington County, MS	32	97%
5	Licking County, OH	26	96%
6	McLeod County, MN	75	96%
7	Ingham County, MI	40	95%
8	Pope County, MN	69	94%
9	Jefferson County, AR	28	93%
10	Washington County, OR	27	93%
11	New York County, NY	25	92%
12	Renville County, MN	97	92%
13	Kandiyohi County, MN	92	91%
14	Preble County, OH	34	91%
15	Monroe County, AR	43	91%
16	Sibley County, MN	92	90%
17	Lake County, IN	37	89%
18	Brown County, MN	92	89%
19	Blue Earth County, MN	100	89%
20	Shelby County, OH	35	89%
21	McHenry County, IL	71	87%
22	Becker County, MN	89	87%
23	Meeker County, MN	94	85%
24	Nicollet County, MN	95	84%
25	Virginia Beach city, VA	78	83%

Download every county (CSV) · What to know about iron

You're welcome to use this data

The rankings are free to use with credit to Tested Water. The underlying test results come from the Water Quality Portal, run by the US Geological Survey and the Environmental Protection Agency.