Well water in Frio County, Texas
In Frio County, 10% of 79 tested well samples exceeded the EPA limit for nitrate (10 mg/L), based on public data from Texas Water Development Board Groundwater Database and USGS Water Quality Portal.
119 individual samples from 57 wells, 1928 to 2026. Data refreshed September 9, 2026.
TWDB samples an ambient monitoring network of wells on a rotation, so these describe aquifers, not a survey of homeowners' tests.
What individual samples found
Each row summarizes every public sample for that contaminant inside Frio County. Above limit means above the EPA maximum contaminant level; where EPA has no enforceable limit, the row shows a health guideline or nuisance level instead.
| Contaminant | Samples | Detected | Above limit | Median | Highest | Limit | |
|---|---|---|---|---|---|---|---|
| Nitrate 1942 to 2026 | 79 | 46% | 10% | 0 | 66 | 10 mg/L EPA limit | |
| Fluoride 1940 to 2026 | 105 | 98% | 0% | 0.4 | 2.98 | 4 mg/L EPA limit | |
| Arsenic 1990 to 2026 | 77 | 4% | 0% | 0 | 3.08 | 10 ug/L EPA limit | |
| Lead 1990 to 2026 | 77 | 3% | 0% | 0 | 8.11 | 15 ug/L EPA limit | |
| Copper 1990 to 2026 | 77 | 39% | 0% | 0 | 0.035 | 1.3 mg/L EPA limit | |
| Uranium 2010 to 2026 | 35 | 20% | 0% | 0 | 6.76 | 30 ug/L EPA limit | |
| Radium (226 + 228) 1990 to 2015 | 18 | 83% | 0% | 1.5 | 4 | 5 pCi/L EPA limit | |
| Gross alpha 1990 to 2014 | 17 | 41% | 0% | 0 | 9.4 | 15 pCi/L EPA limit | |
| Chloride 1928 to 2026 | 119 | 100% | no limit | 145 | 1,910 | 250 mg/L nuisance level | |
| Sodium 1928 to 2026 | 115 | 100% | no limit | 107 | 878 | 20 mg/L health guideline | |
| Iron 1928 to 2026 | 96 | 75% | no limit | 254 | 5,330 | 300 ug/L nuisance level | |
| Manganese 1990 to 2026 | 77 | 83% | no limit | 21.1 | 212 | 300 ug/L health guideline |
Sampled fewer than 10 times, too few to summarize: E. coli (2), Radon (2).
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What this means
The contaminants most often above their limit here are nitrate (10% of 79 samples). A well next door can differ sharply from yours because bedrock fractures, well depth and plumbing all matter, so these rates describe the area, not your tap.
Chloride, Iron, and Manganese are widespread in local groundwater. They are mainly a staining and taste problem, but high manganese also carries a health advisory for infants.
What to test for in Frio County
- Volatile organic compounds (VOCs)20 leaking underground storage tank sites on record locally.
- Nitrate10% of 79 local samples were above the limit.
- Coliform bacteriaRecommended for every private well, every year.
- RadonMedian local reading is 84 pCi/L.
- ChlorideDetected in 100% of local samples.
- FluorideDetected in 98% of local samples.
- ManganeseDetected in 83% of local samples.
- IronDetected in 75% of local samples.
Contamination sources on record in Frio County
TCEQ records 20 leaking underground storage tank sites in Frio County, among 24 regulated sites in total. Groundwater does not follow property lines, so a well within a few hundred feet of one of these is worth testing for what that site could release.
- 20
- leaking underground storage tank sitesGasoline and heating oil that leaked from tanks move through groundwater as benzene, MTBE and other volatile organic compounds.
- 4
- lined landfills and transfer facilities
Treatment that fits the local profile
- Nitrate
- Point-of-use reverse osmosis; Anion exchange; Distillation
- Fluoride
- Point-of-use reverse osmosis; Activated alumina; Distillation
- Chloride
- Point-of-use reverse osmosis; Distillation
- Iron
- Oxidizing filter whole-house; Water softener (low levels); Aeration plus filtration
- Manganese
- Oxidizing filter (greensand, birm) whole-house; Water softener (low levels); Point-of-use reverse osmosis
Get your Frio County water tested at home
A licensed local water treatment company tests your tap water in your home, walks you through the numbers, and quotes treatment only if something needs fixing. Free, about 30 minutes, no obligation. Tell us what you need and we match you with a company that serves Frio County.
Common questions
Is there arsenic in well water in Frio County?
Arsenic was detected in 4% of 77 public well samples in Frio County, and 0% were above the EPA limit of 10 ug/L. Arsenic has no taste or smell; a lab test is the only way to know your level.
How high is radon in well water in Frio County?
The median radon level across 2 public samples in Frio County was 84 pCi/L, with a high of 100 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.
How often should I test a private well in Frio County?
Test for bacteria and nitrate every year. Test once for arsenic, uranium, radon, fluoride, manganese and PFAS, then again every three to five years or after any change in taste, color, or plumbing.
Sources
- Texas Water Development Board Groundwater Database: 883 samples in Frio County. Water quality samples from the Texas Water Development Board Groundwater Database, collected by TWDB, groundwater conservation districts and cooperators. Retrieved September 9, 2026.
- USGS Water Quality Portal: 13 samples in Frio County. Data from the Water Quality Portal, a service of USGS, EPA and USDA; groundwater samples collected by USGS and cooperators. Retrieved September 3, 2026.
- TCEQ regulated sites: 24 regulated sites located in Frio County. Leaking petroleum storage tank sites, state Superfund sites and municipal solid waste landfills from the Texas Commission on Environmental Quality. Retrieved September 9, 2026.
Non-detects count as samples and are treated as zero when computing medians. Exactly at the limit counts as not above it. A sample belongs to the town containing the well's reported coordinates.