Full contaminant data, sample history, and sourcing for Clay County. For readers who want to go beyond the summary.
Contaminant Data — All Analytes
191 well testing events on record, covering
20 analytes.
Data spans 1958 to 2025.
Contaminant
Samples
Years
Detection Rate
Distribution LowModHigh
vs. Limit
vs. MN Avg
Manganese
70
1965–2025
100%
785% of limit
↑ 60% above
Iron
171
1965–2025
97%
392% of limit
↓ 45% below
Arsenic
127
1993–2025
87%
160% of limit
↑ 86% above
Sulfate
166
1965–2025
99%
53% of limit
↑ 183% above
Chloride
171
1965–2025
99%
7% of limit
~ typical
Nitrate
116
1965–2015
71%
0% of limit
↓ 99% below
Lead
127
1993–2025
39%
1% of limit
↓ 98% below
Fluoride
144
1965–2025
99%
6% of limit
~ typical
HFPO-DA (GenX)
municipal
11
2023–2025
0%
—
—
Nitrite
88
2015
6%
1% of limit
↓ 37% below
PFHxS
municipal
8
2023–2025
0%
—
—
PFNA
municipal
11
2023–2025
0%
—
—
PFOA
municipal
11
2023–2025
0%
—
—
PFOS
municipal
11
2023–2025
0%
—
—
Uranium
88
2015
84%
2% of limit
↓ 42% below
Sodium
166
1965–2025
100%
—
↑ 99% above
pH
30
1965–2010
100%
—
~ typical
PFBS
municipal
8
2023–2025
0%
—
—
Hardness
24
1965–1978
100%
—
~ typical
E. coli
2
2018
100%
—
~ typical
Distribution shows the share of samples in each concentration band relative to the EPA Maximum Contaminant Level (MCL): Low = below half the MCL, Moderate = between half and the MCL, High = above the MCL. Analytes without an MCL (e.g. sodium, pH) show — in the limit columns. State average is based on county median values across MN.
Compare your result with Clay County
Of the 127 arsenic tests on record for Clay County (1993–2025),
13% found none, and half were below 14.6 µg/L.
Of the 171 chloride tests on record for Clay County (1965–2025),
1% found none, and half were below 16.3 mg/L.
Of the 144 fluoride tests on record for Clay County (1965–2025),
1% found none, and half were below 246 µg/L.
Of the 171 iron tests on record for Clay County (1965–2025),
3% found none, and half were below 1.13 mg/L.
Of the 127 lead tests on record for Clay County (1993–2025),
61% found none.
Of the 70 manganese tests on record for Clay County (1965–2025),
0% found none, and half were below 392 µg/L.
Of the 116 nitrate tests on record for Clay County (1965–2015),
29% found none, and half were below 8.5 µg/L.
Of the 166 sulfate tests on record for Clay County (1965–2025),
1% found none, and half were below 130 mg/L.
Of the 88 uranium tests on record for Clay County (2015),
16% found none, and half were below 0.4 µg/L.
Data Coverage & Gaps
Well-sampled analytes (15+ samples)
Manganese 70 samples
Iron 171 samples
Arsenic 127 samples
Sulfate 166 samples
Chloride 171 samples
Nitrate 116 samples
Lead 127 samples
Fluoride 144 samples
Nitrite 88 samples
Uranium 88 samples
Sodium 166 samples
pH 30 samples
Hardness 24 samples
Limited data (<15 samples) — interpret with caution
HFPO-DA (GenX) 11 samples
PFNA 11 samples
PFOA 11 samples
PFOS 11 samples
E. coli 2 samples
Public vs. Private Water in Clay County
51Active public water systems
60,378Residents on public water
14%Households on private wells
Public water systems in Clay County are regulated by the EPA and must test and report contaminant levels. Private well owners are responsible for their own testing — there is no routine monitoring of private wells by any government agency.
CDC Health Outcome Correlations
Where contaminants detected in Clay County have established associations with specific health outcomes, we cross-reference CDC PLACES county-level prevalence data. This is a contextual signal, not a causal claim.
This report aggregates data from the following public databases:
Water Quality Portal (WQP) — groundwater sample results for Clay County from EPA, USGS, and state agencies. Browse WQP data →
EPA Safe Drinking Water Information System (SDWIS) — public water system inventory used to estimate the share of residents on private wells. Browse SDWIS →
Census ACS 5-Year Estimates — well and cistern household counts (variable DP04_0074E) used to calculate the private-well percentage for Clay County. Census data explorer →
USGS Aquifer Atlas — principal aquifer type underlying Clay County, which informs which contaminants are geologically likely. USGS Aquifer Atlas →
EPA UCMR 5 (2023–2025) — PFAS occurrence data from public water systems, used as a proxy indicator for aquifer PFAS exposure. Rows marked "municipal" in the table above come from this source. About UCMR 5 →
EPA Toxics Release Inventory (TRI) — industrial facility chemical releases near Clay County, used in contamination risk scoring. TRI Explorer →
EPA Superfund NPL — National Priorities List sites in or near Clay County, used in contamination risk scoring. Superfund site search →
CDC PLACES — county-level health outcome prevalence (kidney disease, cancer, heart disease) correlated with detected contaminants. About CDC PLACES →
Methodology
Raw records are downloaded from the Water Quality Portal and normalized to µg/L (ppb). Records are deduplicated by sample ID and date, and certified outliers are excluded. Analyte names are mapped to EPA canonical forms. Detection rates, distribution bands, and MCL comparisons are computed from the normalized dataset.
Distribution bands use the EPA Maximum Contaminant Level as the threshold: concentrations below 50% of the MCL are classed as Low, between 50% and 100% as Moderate, and above 100% as High. For analytes without an MCL (sodium, hardness, pH), distribution is not computed.
State comparison uses the median of county median values across all counties in MN with at least one sample for that analyte.