Nitrate in Drinking Water

Nitrate is a nitrogen compound found in almost every groundwater aquifer in the United States, and it is the contaminant most likely to force a California water system off the safe-drinking-water roster. When a sample result crosses 10 mg/L as nitrogen, federal regulations trigger two simultaneous obligations: a mandatory health-effects statement in the annual Consumer Confidence Report and a 24-hour Tier 1 public notification to every person served.

Where nitrate comes from

Nitrate enters source water through three main pathways. Agricultural fertilizer — synthetic and organic — is the largest contributor in most regions. Fertilizer applied to fields dissolves in irrigation return flows and rainwater, percolating downward into shallow aquifers. Septic systems and leaking sewer lines contribute a second pathway, releasing nitrate from human and animal waste directly to groundwater. The third pathway is natural: geologic deposits containing nitrate minerals erode over time, releasing the compound into aquifers at background concentrations that typically stay well below the regulatory limit.

In the San Joaquin Valley and on the Central Coast, decades of intensive agriculture have raised groundwater nitrate to levels where dozens of small community systems routinely detect concentrations at or near the MCL. The contamination is not episodic — it is structural, tied to land use patterns that change slowly. That chronic character is why nitrate generates more CCR health-effects disclosures than almost any other regulated contaminant.

Health effects

The health risk from nitrate is specific to one population group: infants under six months of age. Adults and older children metabolize nitrate without harm at drinking-water concentrations. Infants cannot.

When a young infant ingests nitrate, gut bacteria convert it to nitrite, which binds to hemoglobin and displaces oxygen — a condition called methemoglobinemia. Oxygen delivery to tissues falls. In mild cases the infant appears lethargic; in severe cases the skin turns bluish-gray, particularly around the mouth and fingertips. This presentation gives methemoglobinemia its common name: "blue baby syndrome." Without treatment, severe methemoglobinemia can be fatal.

Appendix A to Subpart O of 40 CFR Part 141 — the table that governs what health-effects language must appear in Consumer Confidence Reports — lists the following required text for nitrate:

"Infants below the age of six months who drink water containing nitrate in excess of the MCL could become seriously ill and, if untreated, may die. Symptoms include shortness of breath and blue baby syndrome."

— 40 CFR Part 141, Appendix A to Subpart O (MCL: 10 ppm; MCLG: 10; major sources: runoff from fertilizer use, leaching from septic tanks and sewage, erosion of natural deposits)

This language must appear verbatim in the CCR whenever nitrate is detected at or above the MCL. The maximum contaminant level (MCL) and maximum contaminant level goal (MCLG) for nitrate are both set at 10 mg/L as nitrogen under 40 CFR § 141.62(b).

The infant-health trigger and Tier 1 public notification

The nitrate MCL generates two separate regulatory responses depending on whether the system exceeds or merely approaches the limit.

Mid-range trigger (above 5 mg/L but below 10 mg/L). Section 141.154(c) of 40 CFR requires systems detecting nitrate above 5 mg/L but below the MCL to include the following statement in their CCR:

"Even though [NAME OF UTILITY] meets the EPA nitrate drinking water standard, also known as a Maximum Contaminant Level (MCL), if you are caring for an infant and using tap water to prepare formula, you may want to use alternate sources of water or ask for advice from your health care provider. Nitrate levels above 10 ppm pose a particularly high health concern for infants under 6 months of age and can interfere with the capacity of the infant's blood to carry oxygen, resulting in a serious illness. Symptoms of serious illness include shortness of breath and blueness of the skin, known as 'blue baby syndrome.' Nitrate levels in drinking water can increase for short periods of time due to high levels of rainfall or agricultural activity, therefore we test for nitrate [INSERT APPLICABLE SAMPLING FREQUENCY]. The highest level for nitrate found during [YEAR] was [INSERT MAX NITRATE LEVEL per § 141.153(d)(4)(iv)] ppm."

— 40 CFR § 141.154(c)

The primary agency may approve alternative language under § 141.154(c)(2), but the alternative must convey the same infant-health warning.

MCL exceedance — Tier 1 public notification. When a sample shows nitrate at or above 10 mg/L as nitrogen, the system enters Tier 1 public notification territory under 40 CFR § 141.202. Appendix B to Subpart Q of 40 CFR Part 141 specifies the standard health-effects language water systems must include in that notice:

"Infants below the age of six months who drink water containing nitrate in excess of the MCL could become seriously ill and, if untreated, may die. Symptoms include shortness of breath and blue baby syndrome."

— 40 CFR Part 141, Appendix B to Subpart Q

Tier 1 notification must be issued "as soon as practical but no later than 24 hours" after the system learns of the exceedance (40 CFR § 141.202(a)). The system must simultaneously initiate consultation with the primacy agency within 24 hours to determine whether additional or repeated notifications are required. Tier 1 delivery methods include broadcast media, conspicuous posting in public places, hand delivery, or any other method the primacy agency approves as reaching all persons served.

A second Tier 1 trigger exists alongside the MCL exceedance itself: if a system fails to collect a confirmation sample within 24 hours of receiving the first sample showing a nitrate MCL exceedance, that procedural failure is independently classified as a Tier 1 violation under § 141.202.

The same 24-hour framework applies to nitrite (MCL: 1 mg/L as nitrogen, 40 CFR § 141.62(b)) and to combined nitrate-plus-nitrite (MCL: 10 mg/L as nitrogen, same citation).

The MCL and how it's monitored

Nitrate is regulated under the Phase II and Phase V National Primary Drinking Water Regulations. The current values:

| Parameter | MCL / MCLG | Units | Citation | |---|---|---|---| | Nitrate | 10 mg/L (MCL and MCLG identical) | as nitrogen (NO3-N) | 40 CFR § 141.62(b) | | Nitrite | 1 mg/L | as nitrogen (NO2-N) | 40 CFR § 141.62(b) | | Total nitrate + nitrite | 10 mg/L | as nitrogen | 40 CFR § 141.62(b) |

The 10 mg/L nitrate MCL is sometimes expressed as 45 mg/L NO3 (nitrate as the full molecule rather than just the nitrogen fraction). Both figures describe the same limit; the as-nitrogen convention is what appears in the CCR table.

Monitoring frequency is set by 40 CFR § 141.23. Surface water systems and systems using groundwater under the direct influence of surface water typically sample at least annually. Groundwater systems that have never detected nitrate above 50 percent of the MCL may qualify for reduced monitoring — once every three years or less — with primacy agency approval. Systems that detect nitrate above 5 mg/L but below the MCL must confirm whether elevated levels are seasonal or persistent, and the monitoring frequency language they insert into the § 141.154(c) CCR boilerplate must accurately reflect the approved schedule.

The treatment technique classification does not apply to nitrate — nitrate has a numerical MCL, not a treatment technique standard. Any result above the MCL triggers a violation, not a treatment failure.

Nitrate in California's Central Valley

California has more small water systems with chronic nitrate problems than any other state, and the geography is concentrated: the San Joaquin Valley and the Salinas Valley account for a disproportionate share of systems that regularly detect nitrate near or above 10 mg/L.

The State Water Resources Control Board (SWRCB) identifies nitrate contamination as the leading reason that small, rural, disadvantaged community water systems in California lose access to safe drinking water. The Safe and Affordable Funding for Equity and Resilience (SAFER) program — established under Senate Bill 200 (2019) and funded at up to $130 million per year through 2030 — prioritizes nitrate remediation in the Central Valley and Central Coast. SAFER funding has supported well-testing programs, emergency bottled water distribution, and consolidation of small systems into regional water networks that can blend high-nitrate groundwater with cleaner sources.

California water systems affected by nitrate face a dual regulatory burden: EPA's federal nitrate MCL under 40 CFR § 141.62 and any additional monitoring or reporting requirements imposed by the Division of Drinking Water (DDW) under the SWRCB's primacy authority. California does not set a stricter nitrate MCL than EPA's 10 mg/L standard, but it does enforce the reporting timeline more actively and requires immediate DDW notification when a nitrate sample triggers Tier 1.

The CV-SALTS (Central Valley Salinity Alternatives for Long-Term Sustainability) program, approved by the Central Valley Regional Water Quality Control Board, addresses the agricultural runoff side of the problem through managed aquifer recharge, land management changes, and a nitrate offset system for new dischargers. That long-term remediation work does not change current CCR obligations — a system detecting nitrate above 5 mg/L today must include the § 141.154(c) boilerplate in its next annual report, regardless of whether aquifer remediation is funded or underway.

For a full account of California-specific CCR requirements beyond the federal baseline, see the California CCR requirements page.

Treatment options

When a system's nitrate levels exceed or approach the MCL, three treatment technologies are in common use. None of these are treatment techniques in the regulatory sense — nitrate compliance is measured by the MCL, not by the method used to meet it.

Ion exchange. Strong-base anion exchange resins selectively remove nitrate from water. The process is well-understood, relatively low-cost at small to medium scale, and produces a concentrated brine waste stream that must be managed and disposed of in compliance with state regulations. Ion exchange is the most common choice for small groundwater systems in the Central Valley.

Reverse osmosis (RO). Membrane treatment removes nitrate alongside other dissolved ions, making it effective for systems with multiple groundwater quality problems. Operating costs and concentrate disposal logistics make RO more common at larger treatment facilities or where co-contamination (e.g., arsenic plus nitrate) justifies the capital investment.

Biological denitrification. Microorganisms convert nitrate to nitrogen gas under anoxic conditions. Biological denitrification is standard in wastewater treatment but is less common in drinking water systems because maintaining an active microbial population in a potable water train requires rigorous process control and post-treatment to remove bacterial biomass. Some larger systems in Europe and in California pilot programs use this approach.

Blending is not a treatment technology — it is a compliance strategy. A system blending high-nitrate and low-nitrate sources to achieve a blended result below 10 mg/L must monitor the blend and ensure no single-source result exceeds the MCL without triggering the confirmation and notification requirements.

What utilities must disclose on the CCR

Every community water system that detects nitrate must include it in the CCR's regulated contaminants table (40 CFR § 141.153) with the measured level or range, the MCL, the MCLG, and the source-of-contamination footnote. The table covers results regardless of whether they exceed the MCL.

The additional disclosure obligations depend on the result:

A violation reported in the CCR must include a description of the violation, the potential health effects, the population at risk, whether alternative water supplies are available or needed, what the system is doing to correct the problem, and when it expects to return to compliance (40 CFR § 141.153(h)(1)–(2)).

The statutory chain grounding these requirements runs from SDWA 1996 (Pub. L. 104-182, codified at 42 U.S.C. § 300g-3(c)(4)) through AWIA 2018 (Pub. L. 115-270) through the May 2024 CCR Rule Revisions (89 FR 46013, effective June 24, 2024). The CCR obligations for nitrate itself — the contaminants table, the § 141.154(c) boilerplate, and the violation-reporting requirements — predate 2024 and were not modified by the May 2024 revision. The 2024 changes affect the mandatory summary section (§ 141.156), delivery mechanics, and biannual reporting for larger systems, not the nitrate-specific disclosure language.

Operators who want to compare nitrate disclosures against what a neighboring system reported — or against Gemini Group's template language — should note that nitrate is one of the contaminants where CCR text is highly templated by state primacy agencies. California DDW provides model language; systems that substitute the primacy agency's approved model satisfy § 141.154(c)(2). The underlying obligation and the health-effects mechanism remain the same.

For related context on how lead disclosures work — a contaminant with a different regulatory structure (action level + treatment technique rather than a numerical MCL) — see the lead contaminants page.

Last reviewed: 2026-05-03. Next scheduled review: every 6 months.