US Well Water Quality Index 2026: State-by-State Rankings and Contamination Data

Key findings: US private well water quality by the numbers

The following figures are drawn from the USGS National Water-Quality Assessment program, which has monitored domestic well water quality across the United States since 1991. The primary reference is DeSimone et al. (2009), "Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991 to 2004," available through the USGS National Water-Quality Assessment project at usgs.gov/nawqa. Verify all figures with that source before citing them, as the NAWQA program continues to update its findings.

• More than one-third of domestic wells nationally contain at least one contaminant above a human health benchmark, based on USGS NAWQA monitoring data (usgs.gov/nawqa).
• Nitrate exceeds the EPA maximum contaminant level of 10 mg/L in approximately 7% of domestic wells nationally, with the highest rates in Corn Belt agricultural states.
• Arsenic exceeds the EPA MCL of 10 micrograms per liter in approximately 6% of domestic wells, with the highest rates in New England, the Mountain West, and parts of the Midwest.
• Radon exceeds the EPA proposed guidance level of 300 picoCuries per liter in an estimated 22% of domestic wells, primarily in the eastern United States and in granite-heavy western states.
• Total coliform bacteria are detected in a significant share of untreated domestic wells, particularly shallow wells and those in karst or fractured-rock geology.
• PFAS are now detected in private wells near military bases, airports, and industrial facilities in states across the country, based on ongoing EPA and state monitoring programs following the 2024 PFAS MCL rule (epa.gov/sdwa).
• Approximately 43 million Americans, or about 13% of the US population, rely on private wells for drinking water, according to the EPA (epa.gov/privatewells).

These national averages mask significant state-by-state variation. A homeowner in rural Iowa faces a very different contamination risk profile than a homeowner in rural Texas or rural Maine. The state-level index below maps those differences using the best available published data.

State-by-state private well water quality risk index

The table below classifies each state into one of three risk tiers based on USGS NAWQA data and state-level monitoring programs. Risk tiers reflect the pattern of documented contamination in domestic wells, not a single numeric score. Elevated means NAWQA or equivalent state data documents multiple contaminants at health benchmarks in a meaningful proportion of wells. Moderate means one primary documented concern is prominent. Lower means limited NAWQA coverage or few documented widespread concerns, though all private wells in every state require annual testing.

Risk tiers are based on USGS NAWQA published research and represent documented contamination patterns, not predictions for any individual well. Verify with each cited source. All private wells require annual testing regardless of state risk tier.

What are the most common contaminants in private wells?

Five contaminant categories account for the large majority of health-benchmark exceedances documented in USGS NAWQA monitoring of domestic wells. Each has a distinct geographic pattern tied to geology, land use, or proximity to specific industrial activities. Sourced from the USGS NAWQA program at usgs.gov/nawqa and the CDC private well guidance at cdc.gov/privatewells.

Regional breakdown: where private well water problems are most concentrated

USGS NAWQA has identified four broad regions where well water contamination rates are highest. Each region's problems are driven by the underlying geology and dominant land use, not by industrial activity alone. This regional view helps homeowners understand their baseline risk before ordering specific lab tests. All data cited from the USGS NAWQA program (usgs.gov/nawqa) and the EPA (epa.gov/privatewells).

Texas well water quality: Edwards Aquifer, Hill Country, and regional data

Texas has approximately 1.4 million private water wells statewide, according to the Texas Water Development Board (twdb.texas.gov/groundwater). Unlike the New England states where arsenic is the dominant concern, or the Corn Belt where nitrates drive risk, Texas well water quality is highly variable by aquifer and region. The state overlies nine major aquifers, each with a distinct chemistry profile. For homeowners in the San Antonio metro and Hill Country, where True H2O operates, the relevant aquifers are the Edwards Aquifer and the Trinity Aquifer.

The Edwards Aquifer is a karst limestone formation spanning roughly 3,000 square miles beneath central Texas, according to the Edwards Aquifer Authority (edwardsaquifer.org). Water filtering through limestone over geologic time picks up calcium and magnesium, which is why Edwards Aquifer wells are typically very hard at 15 to 25 or more grains per gallon. Hardness is not a health concern but is the single most common water quality problem reported by well owners drawing from the Edwards Aquifer, causing scale buildup, shortened appliance life, and soap efficiency issues. For more detail on how the Edwards Aquifer affects San Antonio water specifically, read our San Antonio water quality guide and our overview of whether San Antonio has hard water.

Beyond hardness, Edwards Aquifer wells in Kendall County and the western Hill Country margin can contain elevated combined radium (radium-226 and radium-228) in some zones. The EPA maximum contaminant level for combined radium is 5 picoCuries per liter for public water systems. Private wells have no federal MCL but the same health reference applies. The Edwards Aquifer Authority publishes groundwater quality monitoring data for the aquifer at edwardsaquifer.org. Iron and hydrogen sulfide (producing a rotten egg odor) are more commonly associated with Trinity Aquifer wells in the Hill Country than with Edwards Aquifer wells.

In West Texas, the Ogallala Aquifer underlies the High Plains agricultural region. Decades of intensive irrigated farming have elevated nitrate concentrations in the Ogallala, making nitrate testing a priority for private well owners in that region. The Texas Water Development Board tracks Ogallala nitrate data through its groundwater quality database at twdb.texas.gov/groundwater.

For Texas Hill Country well owners, the treatment profile typically starts with a whole-home water softener to address hardness, followed by iron and sulfur treatment if those are detected in the specific well, and an under-sink reverse osmosis system for drinking water. The right configuration depends on what a certified laboratory test actually finds in the individual well, not on zone averages. For more on treatment approaches for Texas well water, read our guide to well water treatment options for Texas homeowners. For understanding what signs your home water may have, see our guide to signs of hard water.

True H2O serves private well customers across the San Antonio metro, including the Hill Country communities of Boerne, Helotes, and surrounding Kendall County. For current hardness and water quality data specific to Boerne, see our Boerne, TX water quality article. For New Braunfels, see the New Braunfels water quality guide. Schedule a free in-home well water test at trueh20.net/water-testing.

How to test your private well water

Testing is the only way to know what is in your well water. The CDC recommends annual testing for bacteria and nitrates as a minimum, with a more comprehensive panel every three to five years or after any event that could affect water quality (cdc.gov/healthywater/drinking/private/wells). There are four main approaches:

State-certified laboratory testing is the most reliable option for private well owners. You collect a water sample using the sterile bottles and instructions the lab provides, ship it back via the prepaid label, and receive a detailed report within three to seven business days. A basic bacteria and nitrate panel from a certified lab costs roughly 30 to 60 dollars. A comprehensive heavy metals, arsenic, radon, and VOC panel runs 100 to 200 dollars or more. The EPA maintains a list of state-certified laboratories at epa.gov/privatewells. In Texas, the Texas Commission on Environmental Quality (TCEQ) maintains a database of licensed water testing labs at tceq.texas.gov.

Professional in-home water testing from a water treatment specialist provides an immediate reading for hardness, chlorine (if any), pH, and total dissolved solids at your specific tap using calibrated instruments. The technician interprets the results in the context of your well type, aquifer, and household needs, and recommends treatment only when the data supports it. This is the best first step for Texas Hill Country homeowners who want an expert assessment rather than a mail-in lab number alone. True H2O provides free in-home well water testing across the San Antonio metro and Hill Country communities. Read more about what professional testing involves in our guide on why professional water testing matters.

At-home test strips and meters are useful quick checks but not adequate for treatment decisions. Test strips measure hardness, chlorine, and pH colorimetrically with a resolution too low to size a water softener or confirm safety. TDS meters measure total dissolved solids but do not identify which substances are present. Neither method detects bacteria, arsenic, nitrates, radon, or PFAS. For a full comparison, see our article on how to test water quality at home.

Select your panel based on your region. The state risk index above gives you the starting point. If you are in Maine or New Hampshire, prioritize arsenic and radon. If you are in Iowa or Nebraska, prioritize nitrates and bacteria. If you are in the Texas Hill Country, prioritize hardness, iron, sulfur, bacteria, and radium. Your state health department and the EPA's private wells resources page can help identify any additional locally relevant contaminants to include.

When to treat your well water vs when to connect to municipal supply

Most private well issues are treatable at reasonable cost. The decision to install treatment or connect to a municipal system depends on the specific contaminants found, the severity of contamination, the available treatment technologies, and whether municipal service is accessible.

Treatment is the right choice in most cases. Hardness, iron, sulfur odor, bacteria, nitrates, arsenic at moderate levels, and radon all have well-established treatment approaches. A whole-home water softener addresses hardness and protects appliances. An oxidation filter removes iron and sulfur before the softener. UV disinfection provides chemical-free bacteria inactivation for well water. An under-sink reverse osmosis system reduces nitrates, arsenic, radium, and PFAS at the drinking tap. For most Texas Hill Country homeowners, a softener-plus-RO combination addresses the dominant well water concerns without requiring a connection to municipal supply. For more on how to choose between treatment approaches, see our comparison of reverse osmosis versus whole-house filtration and our guide to water softeners versus water filters.

When to consider municipal connection. Connecting to a public water system is worth evaluating when: (1) multiple serious contaminants are detected that require complex multi-stage treatment, (2) the well has structural issues requiring costly replacement, (3) municipal service is already available at the property boundary and the tap-in fee is competitive with treatment system cost, or (4) the contamination source (agricultural runoff, industrial discharge) is ongoing and unlikely to improve. In many rural Texas areas, municipal service is not available, making treatment the only practical path. Verify availability with your local utility district or the Texas Commission on Environmental Quality at tceq.texas.gov.

True H2O serves well-water homeowners across Bexar County, Kendall County, Comal County, and the surrounding Hill Country communities. We provide free in-home well water assessments and treatment recommendations based on what your actual well test shows. Schedule at trueh20.net/water-testing or see our service areas at Bexar County water testing and Comal County water testing.