Arsenic in Drinking Water

Arsenic is a naturally occurring element found in bedrock across much of the United States, and small amounts regularly dissolve into groundwater that feeds public water supplies. Federal regulations set a Maximum Contaminant Level of 10 µg/L (micrograms per liter) for arsenic in drinking water — and when a system's measured results reach or exceed half that limit, the law requires specific health-effects language to appear in the Consumer Confidence Report (CCR).

Where arsenic comes from

Most arsenic in drinking water comes from natural geological sources. Arsenic-bearing minerals weather over time and leach into groundwater, particularly in the western United States, New England, and parts of the Midwest where arsenic-rich bedrock is common. Human activity adds to the load in some areas: historic and active hard-rock mining operations, agricultural application of arsenic-based pesticides (common before the 1970s), and industrial waste streams from glass and semiconductor manufacturing have all contributed arsenic to surface and groundwater in specific regions.

Systems drawing from groundwater wells are more likely to detect arsenic at meaningful concentrations than systems drawing from surface reservoirs, though neither source type is exempt from monitoring obligations under 40 CFR § 141.23.

Health effects

Arsenic is a known human carcinogen. Long-term exposure through drinking water has been associated with cancers of the bladder, lung, and skin, as well as cardiovascular effects and skin damage including keratosis and hyperpigmentation.

The Maximum Contaminant Level Goal (MCLG) for arsenic is zero — meaning EPA has determined there is no concentration of arsenic in drinking water that carries zero health risk (40 CFR § 141.51). The enforceable MCL of 10 µg/L is set above that goal because EPA must balance the scientifically determined risk level against the technical and economic feasibility of treatment.

Mandatory CCR health-effects boilerplate

When a community water system detects arsenic at or above 0.005 mg/L (5 µg/L) — the regulatory mid-range trigger — it must include the following language verbatim in its Consumer Confidence Report, per 40 CFR § 141.154(b):

"Arsenic is known to cause cancer in humans. Arsenic also may cause other health effects such as skin damage and circulatory problems."

In addition, the CCR must include a companion disclosure:

"[NAME OF UTILITY] meets the EPA arsenic drinking water standard, also known as a Maximum Contaminant Level (MCL). However, you should know that EPA's MCL for arsenic balances the scientific community's understanding of arsenic-related health effects and the cost of removing arsenic from drinking water."

The report must also state: "The highest concentration of arsenic found in [YEAR] was [INSERT MAX ARSENIC LEVEL] ppb."

This boilerplate is required whenever the highest detected arsenic level in the reporting year is at or above 0.005 mg/L, even if the result is below the MCL of 0.010 mg/L. A state's primacy agency may approve alternative educational language in place of the example text, but the substitute must convey the same information.

The MCL and how it's monitored

The 2001 Arsenic Rule

For decades, the federal arsenic MCL was 50 µg/L, a standard set in 1942. EPA finalized the current 10 µg/L MCL on January 22, 2001, publishing the rule at 66 FR 6976. Compliance became effective on January 23, 2006, giving systems five years to install treatment or make supply changes. The MCL is codified at 40 CFR § 141.62 and applies to all community water systems and non-transient, non-community water systems.

The MCLG remains at zero, reflecting EPA's conclusion that arsenic is a human carcinogen with no demonstrated threshold below which risk is absent (40 CFR § 141.51).

Monitoring requirements

Monitoring frequency under 40 CFR § 141.23 depends on source type and detected levels:

• Groundwater systems typically monitor annually or less frequently once they establish a compliance history at low levels. New sources or systems with previously elevated results monitor quarterly.
• Surface water systems follow a comparable graduated schedule based on prior results and source characteristics.
• Entry-point monitoring is the standard method: samples are collected at each point where water enters the distribution system after treatment.

Systems that demonstrate arsenic consistently well below the MCL may apply to their state primacy agency for a reduced monitoring schedule. Systems that exceed the MCL must take corrective action and return to more frequent monitoring.

California's Public Health Goal: stricter than federal

California's Office of Environmental Health Hazard Assessment (OEHHA) published a Public Health Goal (PHG) for arsenic of 0.004 µg/L (4 parts per trillion) in April 2004. The PHG is based on the excess cancer risk level OEHHA considers de minimis — roughly one additional cancer case per million people over a 70-year lifetime — calculated primarily from bladder and lung cancer data.

The PHG is a risk-based health target, not a regulatory limit. California water systems are not required to achieve 0.004 µg/L; the enforceable standard remains the federal MCL of 10 µg/L. The PHG informs state regulatory decisions and serves as a benchmark when California evaluates whether to adopt a stricter state MCL. As of 2026, the California MCL for arsenic mirrors the federal standard of 10 µg/L.

The gap between the federal MCL (10,000 ppt) and California's PHG (4 ppt) is among the largest in the state's drinking water program, and it reflects a genuine scientific tension: the PHG captures theoretical carcinogenic risk at very low concentrations, while the MCL acknowledges the practical limits of treatment and detection.

Treatment options when arsenic exceeds the MCL

When monitoring results exceed 10 µg/L, systems have several EPA-recognized treatment techniques available. EPA identifies the following as best available technologies under 40 CFR § 141.62:

• Coagulation/filtration — adds a coagulant (typically iron or aluminum salt) that causes arsenic particles to aggregate and settle; effective primarily for arsenic in the pentavalent (As V) oxidation state. Pre-oxidation is typically required to convert arsenite (As III) to arsenate (As V) before this step.
• Anion exchange — passes water through a resin bed that selectively removes arsenic ions from solution; well-suited to systems with moderate arsenic levels and stable water chemistry.
• Activated alumina — an adsorption process where arsenic binds to aluminum oxide media; widely used for small systems because of its relatively low capital cost.
• Reverse osmosis (RO) — a membrane process that removes arsenic along with a broad range of other dissolved solids; the highest removal efficiency but also the highest cost and waste-brine management requirement.

Pre-oxidation with chlorine, permanganate, or ozone is frequently needed upstream of coagulation/filtration and activated alumina when influent arsenic is predominantly in the trivalent form, since As III is less effectively removed by most processes than As V.

Selection among these options depends on arsenic concentration, co-occurring contaminants, system size, water chemistry, and disposal constraints for treatment residuals.

What utilities must disclose on the CCR

Every community water system that detects arsenic must include it in the regulated contaminants table of its annual CCR, regardless of whether the result exceeds the MCL. The table must show the detected level (using the Running Annual Average or, for some monitoring designs, the highest single result), the MCL of 0.010 mg/L, the MCLG of zero, and the likely source.

When the highest detected result in the reporting year is at or above 0.005 mg/L, the additional health-effects boilerplate described above — the verbatim cancer and health-effects language, the MCL-balancing statement, and the disclosure of the highest detected level — must appear in the report body (40 CFR § 141.154(b)).

If results exceed the MCL of 0.010 mg/L, the system is also required to report the violation, including the nature of the violation, the potential adverse health effects, the steps taken or being taken to address it, and what consumers should do — under 40 CFR § 141.153(d).

These disclosure requirements flow from the statutory chain established by the Safe Drinking Water Act (SDWA) 1996 amendments (Pub. L. 104-182), reinforced by the America's Water Infrastructure Act of 2018 (AWIA, Pub. L. 115-270), and implemented through the 2024 CCR Rule Revisions (89 FR 46013).

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