US Water Quality by Region: Common Contaminants Across States

Water quality across the United States is not uniform — contaminant profiles vary significantly by geography, geology, land use patterns, and the age of municipal infrastructure. The Water Filtration Listings directory organizes service providers by region precisely because filtration needs in the Colorado River Basin differ fundamentally from those along the Gulf Coast or the Great Lakes watershed. This page maps the primary contaminant categories by region, the federal and state regulatory frameworks that govern acceptable thresholds, and the structural factors that shape where specific water quality problems are most prevalent.

Definition and scope

Regional water quality refers to the measurable chemical, biological, and physical characteristics of drinking water sources — including groundwater aquifers, surface reservoirs, and municipal distribution systems — as they vary across distinct geographic areas of the United States. The Environmental Protection Agency (EPA) establishes Maximum Contaminant Levels (MCLs) under the Safe Drinking Water Act (42 U.S.C. § 300f et seq.), which set the legally enforceable upper limits for over 90 contaminants in public water systems. States may adopt standards equal to or stricter than federal MCLs; California, for instance, maintains state MCLs for contaminants not covered under federal rules through the State Water Resources Control Board (SWRCB).

The scope of regional water quality analysis encompasses:

• Primary contaminants — those posing direct health risks (pathogens, heavy metals, nitrates, arsenic, volatile organic compounds)
• Secondary contaminants — those affecting aesthetic qualities such as taste, odor, and color (iron, manganese, sulfates, total dissolved solids)
• Emerging contaminants — substances not yet subject to enforceable MCLs, including per- and polyfluoroalkyl substances (PFAS) and certain pharmaceuticals

The EPA's Contaminant Candidate List (CCL 5), published in 2022, identifies contaminants under regulatory consideration but not yet regulated under federal rules — a category of growing significance in agricultural and industrial corridors.

How it works

Contaminant distribution follows four primary determinants: source water geology, agricultural and industrial land use, infrastructure age, and treatment technology gaps.

Geology drives baseline contamination. Arsenic occurs naturally in bedrock formations across the Southwest, upper Midwest, and New England. The USGS (U.S. Geological Survey) has mapped arsenic concentrations exceeding the EPA MCL of 10 micrograms per liter (µg/L) in groundwater across parts of Arizona, Nevada, New Mexico, and Michigan. Radon, another geologically sourced contaminant, is elevated across the Appalachian region and the granite belt of New England.

Agricultural land use introduces nitrate contamination through fertilizer and livestock waste runoff. The EPA MCL for nitrate is 10 milligrams per liter (mg/L) as nitrogen (EPA Ground Water and Drinking Water). Exceedances are documented across the Corn Belt — particularly in Iowa, Illinois, Indiana, and Nebraska — where intensive row-crop agriculture loads nitrogen into shallow aquifers and tributary systems feeding municipal intakes.

Industrial land use and legacy sites create localized plumes of volatile organic compounds (VOCs), heavy metals, and PFAS. The EPA PFAS Strategic Roadmap identifies PFAS as present in water systems near military installations, manufacturing facilities, and airports in states including Michigan, New Jersey, and North Carolina.

Infrastructure age introduces lead and copper contamination through service line corrosion. The EPA's Lead and Copper Rule Revisions (LCRR), updated in 2021, requires public water systems to inventory lead service lines. Cities with pre-1986 plumbing infrastructure — concentrated in the Midwest and Northeast — carry the highest exposure risk, as lead pipe was the dominant service line material before Congress banned it in the Safe Drinking Water Act Amendments of 1986.

Common scenarios

The following regional profiles reflect documented contamination patterns drawn from EPA monitoring data and USGS groundwater surveys:

1. Northeast (New England, Mid-Atlantic): Radon, arsenic, and lead in older municipal distribution systems. Private well users — who are not subject to EPA MCL enforcement — face elevated arsenic and radon exposure in granite and metamorphic rock terrain.

2. Southeast (Gulf Coast, Appalachia): Disinfection byproducts (DBPs), including trihalomethanes (THMs), form when chlorine reacts with high organic-matter surface water from humid, vegetated watersheds. The EPA MCL for total THMs is 80 µg/L (EPA Disinfection Byproducts Rule).

3. Midwest (Corn Belt, Great Lakes): Nitrate, atrazine, and agricultural herbicides dominate. The USGS National Water-Quality Assessment (NAWQA) program has documented atrazine in groundwater across more than 20 Midwestern states; the EPA MCL for atrazine is 3 µg/L.

4. Southwest (Arizona, Nevada, California's Central Valley): Arsenic, total dissolved solids (TDS), chromium-6, and uranium in groundwater. California's Division of Drinking Water enforces a state MCL for hexavalent chromium (chromium-6) of 10 µg/L, more stringent than current federal standards.

5. Mountain West (Rocky Mountain states, Pacific Northwest): Naturally occurring fluoride and selenium in groundwater, with localized uranium near phosphate mining districts. Snowmelt-fed surface supplies can carry low-level sediment and microbial loads during spring runoff.

6. Texas and Southern Plains: Radionuclides — including radium-226 and radium-228 — appear in groundwater from the Ogallala Aquifer and Permian Basin formations. The EPA combined radium MCL is 5 picocuries per liter (pCi/L).

Contaminant profiles in private wells diverge sharply from municipal system profiles. The EPA's Private Wells resources note that approximately 43 million Americans rely on private wells not regulated under the Safe Drinking Water Act, making independent testing the sole quality assurance mechanism for those users. The Water Filtration Directory Purpose and Scope page outlines how this distinction shapes the filtration service landscape nationally.

Decision boundaries

Selecting filtration or treatment technology requires matching the contaminant class to the removal mechanism — a distinction that the How to Use This Water Filtration Resource page addresses in the context of navigating provider categories.

Contaminant class versus treatment method:

NSF International (NSF) and the American National Standards Institute (ANSI) jointly publish performance standards — NSF/ANSI 42, 53, 58, and 62 — that define validated removal claims for point-of-use and point-of-entry filtration systems. Equipment certified to these standards has been tested against defined contaminant reduction benchmarks, not manufacturer-stated claims.

Regulatory jurisdiction over point-of-use systems falls to state drinking water programs for municipal connections and to state health departments for private well users. Permitting requirements for whole-house filtration systems, particularly those involving backwash discharge, may require local plumbing permits or wastewater authorization depending on state and county codes. Treatment systems that alter water chemistry — including water softeners using brine regeneration — are subject to discharge restrictions in drought-prone states, including salt-based softener restrictions in 26 California water districts.

The structural divide between regulated public water systems and unregulated private wells defines the primary decision boundary for filtration need assessments. Public system users have access to annual Consumer Confidence Reports (CCRs) mandated by the EPA under 40 C.F.R. § 141.153, which disclose detected contaminant levels versus MCLs. Private well users have no equivalent mandatory disclosure mechanism and must commission independent testing to establish baseline water quality before selecting treatment technology.

References

• U.S. Environmental Protection Agency — Safe Drinking Water Act
• [EPA Maximum Contaminant Levels (MCLs)](https://www.epa.gov/ground