Full contaminant data, sample history, and sourcing for St. Johns County. For readers who want to go beyond the summary.
Contaminant Data — All Analytes
668 well testing events on record, covering
23 analytes.
Data spans 1955 to 2023.
Contaminant
Samples
Years
Detection Rate
Distribution LowModHigh
vs. Limit
vs. FL Avg
Iron
299
1997–2024
100%
36% of limit
↓ 82% below
Sulfate
397
1977–2025
100%
68% of limit
↑ 158% above
Manganese
275
1997–2024
100%
19% of limit
↓ 30% below
Chloride
562
1970–2025
100%
11% of limit
↓ 77% below
Arsenic
274
2005–2024
100%
5% of limit
↓ 62% below
Lead
274
2005–2024
100%
0% of limit
↓ 89% below
Fluoride
222
1979–2025
100%
3% of limit
↓ 52% below
HFPO-DA (GenX)
municipal
22
2023–2025
0%
—
—
Nitrate
1
2019
0%
—
—
Nitrite
1
2019
0%
—
—
PFHxS
municipal
22
2023–2025
0%
—
—
PFNA
municipal
22
2023–2025
0%
—
—
PFOA
municipal
22
2023–2025
0%
—
—
PFOS
municipal
22
2023–2025
0%
—
—
Radon
1
2019
100%
58% of limit
↑ 97% above
Uranium
1
2019
100%
0% of limit
↓ 58% below
E. coli
49
2019–2025
98%
—
~ typical
Fecal Coliform
74
2013–2021
100%
—
~ typical
Hardness
91
1970–2019
100%
—
↑ 33% above
PFBS
municipal
22
2023–2025
0%
—
—
pH
534
1977–2025
100%
—
~ typical
Sodium
442
1977–2025
100%
—
↓ 58% below
Total Coliform
122
2013–2025
99%
—
~ 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 FL.
Compare your result with St. Johns County
Of the 274 arsenic tests on record for St. Johns County (2005–2024),
0% found none, and half were below 0.5 µg/L.
Of the 562 chloride tests on record for St. Johns County (1970–2025),
0% found none, and half were below 27.5 mg/L.
Of the 222 fluoride tests on record for St. Johns County (1979–2025),
0% found none, and half were below 135 µg/L.
Of the 299 iron tests on record for St. Johns County (1997–2024),
0% found none, and half were below 107 µg/L.
Of the 274 lead tests on record for St. Johns County (2005–2024),
0% found none, and half were below 50 ng/L.
Of the 275 manganese tests on record for St. Johns County (1997–2024),
0% found none, and half were below 9.27 µg/L.
Of the 397 sulfate tests on record for St. Johns County (1977–2025),
0% found none, and half were below 170 mg/L.
Data Coverage & Gaps
Well-sampled analytes (15+ samples)
Iron 299 samples
Sulfate 397 samples
Manganese 275 samples
Chloride 562 samples
Arsenic 274 samples
Lead 274 samples
Fluoride 222 samples
E. coli 49 samples
Fecal Coliform 74 samples
Hardness 91 samples
pH 534 samples
Sodium 442 samples
Total Coliform 122 samples
Limited data (<15 samples) — interpret with caution
Nitrate 1 sample
Nitrite 1 sample
Radon 1 sample
Uranium 1 sample
No private-well PFAS data for St. Johns County
We have no private well sampling data for PFAS compounds (PFOA, PFOS, PFNA, and related chemicals) in St. Johns County.
PFAS testing for private wells requires a dedicated lab panel (~$300–$500). If you are near a military base, airport, or industrial site, consider testing proactively. Learn more about PFAS →
Public vs. Private Water in St. Johns County
57Active public water systems
221,970Residents on public water
24%Households on private wells
Public water systems in St. Johns 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 St. Johns 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 St. Johns 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 St. Johns County. Census data explorer →
USGS Aquifer Atlas — principal aquifer type underlying St. Johns 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 St. Johns County, used in contamination risk scoring. TRI Explorer →
EPA Superfund NPL — National Priorities List sites in or near St. Johns 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 FL with at least one sample for that analyte.