Nitrate-Nitrite (Combined) in drinking water
1,387 active public water systems in 18 states had a Nitrate-Nitrite (Combined) violation in the past three years, and 41 had one in the latest quarter of EPA's data.
You are on Tapcensus, an independent guide to the EPA compliance records of every public water system in the United States. This page explains Nitrate-Nitrite (Combined) and lists the water systems whose records include a related violation. It does not show measured levels.
Is Nitrate-Nitrite (Combined) in the records for your water system?
Enter a ZIP code, town, or street address to see the water systems that serve your area, their status, and which rules and contaminants appear in their violations.
Enter a ZIP code, town, or street address.
What it is and where it comes from
This code tracks the combined total nitrogen from both nitrate and nitrite. While each has its own MCL (nitrate: 10 mg/L, nitrite: 1 mg/L), the combined sum is also limited to 10 mg/L as nitrogen. The combined measurement applies when both forms are present, particularly in partially treated or mixed sources where nitrification may be incomplete.
Health effects
The combined limit protects against methemoglobinemia — the same risk as nitrate alone and nitrite alone. In infants under 6 months, gut bacteria convert these nitrogen compounds to forms that block oxygen transport in blood ('blue baby syndrome'). Pregnant women and individuals with certain enzyme deficiencies are also at elevated risk.
The standard and what a violation means
EPA limit: 10 mg/L as nitrogen (NO₂-N + NO₃-N combined).
Water systems with a Nitrate-Nitrite (Combined) violation, by state
Active public water systems that reported at least one related violation to EPA in the past three years. Each state links to the list of its affected systems.
| State | Systems with violations, 3 yr | Share of active systems | In violation, latest quarter |
|---|---|---|---|
| Montana | 338 | 14% | 10 |
| Oklahoma | 236 | 18% | 8 |
| Virginia | 180 | 6% | 1 |
| Missouri | 159 | 6% | 1 |
| Georgia | 145 | 6% | 0 |
| Wyoming | 107 | 14% | 7 |
| Nebraska | 76 | 6% | 11 |
| Mississippi | 52 | 4% | 0 |
| New York | 36 | 0% | 0 |
| New Mexico | 25 | 2% | 2 |
| Wisconsin | 19 | 0% | 1 |
| North Dakota | 5 | 1% | 0 |
| Michigan | 2 | 0% | 0 |
| South Dakota | 2 | 0% | 0 |
| Utah | 2 | 0% | 0 |
| California | 1 | 0% | 0 |
| Indiana | 1 | 0% | 0 |
| Kansas | 1 | 0% | 0 |
Largest water systems with a Nitrate-Nitrite (Combined) violation
The ten biggest systems, by people served, whose records include a related violation in the past three years.
| System | Serves | Status | Rules violated, 3 yr | Unresolved |
|---|---|---|---|---|
|
Midwest City Oklahoma County |
55,935 | 3 | 1 | |
|
Roswell Municipal Water System Roswell · Chaves County |
54,025 | 4 | 1 | |
|
Greenlawn Water District Huntington (t) · Suffolk County |
42,000 | 3 | 0 | |
|
Riverhead Water District Riverhead (t) · Suffolk County |
35,000 | 1 | 0 | |
|
West Bend Waterworks West Bend · Washington County |
32,149 | 5 | 0 | |
|
Hastings, City of Hastings · Adams County |
24,927 | 1 | 0 | |
|
Lee County Utilities Authority Leesburg · Lee County |
20,194 | 3 | 0 | |
|
Mustang, City of Canadian County |
18,576 | 5 | 2 | |
|
Artesia Municipal Water System Artesia · Eddy County |
15,176 | 2 | 0 | |
|
Okmulgee Pwa Okmulgee County |
13,495 | 6 | 0 |
About this data
A system is counted once if any of its violations in the past three years involved Nitrate-Nitrite (Combined), including monitoring and reporting lapses. Rules violated and status in the systems table are for the whole system, not Nitrate-Nitrite (Combined) alone. Counts cover active systems in all 50 states and the District of Columbia.
Source: the U.S. EPA's Enforcement and Compliance History Online system (ECHO), which publishes the federal Safe Drinking Water Information System. EPA's terms and values are shown as published. The records are from EPA's data refresh of Oct 7, 2026; EPA refreshes this data quarterly, and states report to EPA with a lag of three to six months, so the newest violations may not appear yet.