Arsenic in San Fernando Valley Drinking Water: Inside California's 2,500x PHG-to-MCL Gap

San Fernando Valley tap water meets California's legal arsenic limit of 10 micrograms per liter (µg/L) across all major utilities. What fewer homeowners know is that California's public health science sets the health-based target at 0.004 µg/L, a number 2,500 times lower than the enforceable maximum contaminant level. The gap exists because the MCL must balance cancer risk against treatment feasibility and cost across thousands of water systems, while the OEHHA Public Health Goal represents the concentration the state's scientists consider protective for lifetime exposure. For SFV households served by LADWP, Burbank Water and Power, Glendale Water and Power, or SCV Water, arsenic concentrations appear in each utility's Consumer Confidence Report published each July. The practical question is whether a tested filtration system at the kitchen tap makes sense given what the state's own scientists consider a safe level. Reverse osmosis certified to NSF/ANSI 58 is the primary residential answer.

What Is Arsenic and Why Does It Appear in Drinking Water?

Arsenic (chemical symbol As) is a naturally occurring metalloid found in Earth's crust, particularly in sedimentary and volcanic rock formations throughout the western United States. Groundwater picks up arsenic as it percolates through arsenic-bearing geological layers. In Southern California, arsenic is most commonly detected in groundwater wells drawing from deeper aquifer strata, where water has had prolonged contact with mineral deposits. Surface water sources (Colorado River, State Water Project) typically carry lower arsenic concentrations than groundwater. LADWP blends surface water and treated groundwater across its distribution system, so the final arsenic concentration at your tap depends on the blend ratio in your pressure zone on any given day.

Arsenic exists in two primary inorganic forms: arsenate (As(V), the oxidized form) and arsenite (As(III), the reduced form). This distinction matters for treatment. Arsenate is relatively straightforward to remove with reverse osmosis or activated alumina. Arsenite is more soluble and harder to remove without first oxidizing it to arsenate. Standard utility treatment processes typically pre-oxidize arsenic before filtration, converting arsenite to the arsenate form, so tap water delivered to SFV homes generally contains arsenate as the dominant species.

The primary health concern with long-term arsenic exposure is bladder and lung cancer, along with skin lesions and cardiovascular effects at higher concentrations. These risks are associated with lifetime exposure, not short-term consumption. Regulatory agencies treat arsenic as a carcinogen with no known safe threshold, which is why OEHHA set the PHG at 0.004 µg/L rather than a positive number tied to an acceptable-risk threshold.

How California's Legal Limit Compares to Its Health Goal

The table below summarizes the key regulatory thresholds and the OEHHA Public Health Goal for arsenic in drinking water as of 2026. Verify current values with the sources listed, as standards are subject to revision.

The 2,500x gap between 0.004 µg/L and 10 µg/L reflects a deliberate policy trade-off that has been embedded in California's regulatory framework since the PHG process began in 1983. The PHG is derived from cancer slope factors and is not an enforceable standard. The MCL is the enforceable standard, set at 10 µg/L by EPA in January 2001 (down from the prior 50 µg/L limit that dated to 1975) with an effective compliance date of January 23, 2006. The EPA's risk assessment at the time estimated that a lifetime of drinking water at 10 µg/L would produce approximately 0.4 to 0.7 excess cancer cases per 1,000 people exposed. That residual risk is accepted in exchange for achievable treatment at scale.

The MCLG of zero deserves its own note. Under the Safe Drinking Water Act, EPA sets the MCLG for known or probable human carcinogens at zero because any exposure above zero carries theoretical cancer risk. "Zero" is not an enforceable target; it is a statement that the science identifies no safe floor. The MCL of 10 µg/L is the practical limit, not a declaration that 9 µg/L is safe. More background on MCLG/MCL distinctions is at the EPA SDWA regulation page.

What SFV Utilities Report in Consumer Confidence Reports

Every public water system in the United States must publish an annual Consumer Confidence Report (CCR) by July 1 each year, covering the prior calendar year's monitoring results. The CCR discloses arsenic detections, monitoring methodology, and the range of detected values across sampling sites. For current SFV arsenic data, consult each utility's most recent CCR directly:

• LADWP: ladwp.com, water quality reports page
• Burbank Water and Power: burbankwaterandpower.com
• Glendale Water and Power: glendaleca.gov
• SCV Water: scvwater.org

Arsenic is classified as a regulated contaminant under both federal and California law, so utilities must report any detection, not just detections above the MCL. The CCR will show the detection range, the calendar-year average, the MCL, and the MCLG. Zero as the MCLG signals that no level is considered free of cancer risk, a standard EPA position for known carcinogens under the SDWA.

The California State Water Resources Control Board also maintains a searchable SDWIS database where compliance records for individual water systems can be verified. The SWRCB arsenic page links to system-level monitoring data. Note that CCR detection values represent distribution system averages; your specific tap may differ slightly depending on your pressure zone's source blend on the day of sampling.

Why the San Fernando Valley Faces Elevated Groundwater Arsenic Context

The SFV sits above a deep alluvial aquifer basin that has been a major water supply source since the early 20th century. This groundwater contains naturally occurring arsenic from the volcanic and sedimentary geology of the surrounding mountain ranges, including the Santa Monica Mountains to the south and the San Gabriel and Santa Susana Mountains to the north and east. Unlike the industrial contamination that drives the Valley's Superfund VOC and chromium-6 problems (covered in depth in the SFV groundwater remediation article and the Chromium-6 compliance piece), the arsenic in deeper groundwater is largely geogenic. It was present before human activity, and it occurs across the western United States for the same geological reasons.

LADWP treatment plants blend and process water from multiple sources before distribution. Groundwater drawn from the SFV aquifer is treated at major water reclamation facilities before blending with Colorado River and State Water Project surface water. Utility treatment processes that address other contaminants (iron, manganese, VOCs) typically also reduce arsenic concentrations before the water enters distribution. The net concentration at the tap reflects the treated blend across all sources. Higher groundwater-contribution periods, which typically occur during drought years when surface deliveries are reduced, can result in slightly higher arsenic concentrations in the final blend, though monitored values remain within the 10 µg/L MCL.

For homeowners on private wells in the foothill areas at the edge of the SFV service territory (portions of Granada Hills, Chatsworth, West Hills, and Santa Clarita), arsenic levels in shallow to mid-depth wells can vary more widely than utility-supplied water, and private-well owners are responsible for their own monitoring. The California SWRCB groundwater arsenic guidance covers private well testing requirements.

How Residential Water Treatment Addresses Arsenic

For households where the gap between the PHG and the MCL is a concern, point-of-use treatment at the kitchen tap is the practical intervention. Whole-house arsenic removal exists but is significantly more expensive and is primarily used where private wells consistently detect above 10 µg/L.

Reverse osmosis (NSF/ANSI 58): A properly sized RO system certified to NSF/ANSI 58 removes arsenate (As(V)) at 80 to 95 percent efficiency. LADWP's treatment process pre-oxidizes arsenic, so tap water entering an RO unit in an SFV home is predominantly arsenate. An NSF/ANSI 58 RO unit at the kitchen sink is the primary recommended option for households concerned about the MCL-to-PHG gap. For the current list of certified RO systems, check the NSF certified products database at nsf.org. The same RO unit that addresses arsenic also reduces chromium-6, lead, PFAS, nitrate, and other dissolved contaminants, making it a multi-purpose solution for the SFV water quality profile.

Activated alumina: Point-of-use activated alumina (AA) filters certified to NSF/ANSI 58 reduce arsenic by adsorption: arsenic ions bind to aluminum oxide surfaces and are retained. AA systems work across the pH range of 5.5 to 8.5, covering typical SFV tap water chemistry. They are a valid alternative to RO for households where wastewater from the RO brine stream is a concern (RO units produce reject water at roughly 3 to 4 parts reject per 1 part permeate in residential configurations).

What does not remove arsenic: Standard activated carbon filters, pitcher-style carbon filters, and water softeners do not remove arsenic in meaningful amounts. Arsenic removal requires one of the technologies above, verified by NSF/ANSI 58 certification for arsenic reduction specifically. See the whole-house filtration explainer for a component-by-component breakdown of what each treatment type addresses in the SFV water profile.

The Federal Regulatory History Behind the 10 µg/L Limit

The current arsenic MCL is the product of a multi-decade regulatory effort. The original federal MCL, set in 1975, was 50 µg/L, adopted from a 1942 U.S. Public Health Service standard. In 1996, Congress amended the SDWA to require EPA to revise the arsenic rule. The EPA considered alternatives ranging from 3 µg/L to 20 µg/L before finalizing 10 µg/L in January 2001 under the Arsenic Rule (66 FR 6976), with the effective compliance date of January 23, 2006. The rule is codified at 40 CFR Part 141, Subpart B. The complete technical basis is documented in the EPA arsenic in drinking water rule page.

Congress set the 2006 deadline to give utilities five years to install treatment, arrange financing, and complete capital projects. Small systems with fewer than 25 connections received additional flexibility. The EPA estimated at the time that achieving 10 µg/L would prevent 19 to 25 bladder cancer cases and 6 to 8 lung cancer cases annually across the United States compared to the prior 50 µg/L limit.

California adopted the same 10 µg/L MCL for consistency with the federal standard. Under SDWA preemption rules, state MCLs must meet or exceed federal MCLs. California's OEHHA PHG process, which operates independently from MCL-setting, produced the 0.004 µg/L PHG based on a cancer risk assessment. PHGs are advisory: water systems must achieve the MCL, not the PHG. The ongoing divergence between the PHG and the MCL creates the 2,500x gap visible in the table above.

For context on how this gap compares to other California contaminants: the chromium-6 PHG vs MCL gap is roughly 500x (0.02 µg/L PHG vs 10 µg/L MCL). The arsenic gap is five times wider, reflecting arsenic's classification as a Group 1 carcinogen (IARC) with well-characterized dose-response data. More on the California regulatory framework for all these contaminants is in the CA water quality rules overview.

Frequently Asked Questions

What is the arsenic level in LADWP water?

LADWP monitors arsenic at multiple sampling sites and discloses annual results in its Consumer Confidence Report, published each July for the prior calendar year. Download the most recent CCR from the LADWP water quality page at ladwp.com. The CCR is the authoritative source; values vary by year and by pressure zone depending on the groundwater-to-surface-water blend ratio. Detected values historically appear below the 10 µg/L MCL. For a specific reading at your tap, a certified lab test or an in-home screening test is the most precise option.

Does a whole-house filter remove arsenic from tap water?

Standard whole-house carbon filters and water softeners do not remove arsenic in meaningful amounts. Arsenic removal requires reverse osmosis certified to NSF/ANSI 58, activated alumina, or iron-based adsorption media. In the SFV context, where utility water meets the 10 µg/L MCL, a point-of-use kitchen-tap RO unit is the most cost-effective intervention for households concerned about the PHG-to-MCL gap. Whole-house arsenic removal is more typically justified for private wells detecting above the MCL.

Does boiling water remove arsenic?

No. Boiling eliminates biological contaminants (bacteria, viruses, protozoa) but does not remove arsenic. Boiling actually concentrates arsenic slightly because evaporation reduces water volume while arsenic remains in solution. Arsenic removal requires physical filtration or adsorption technology: reverse osmosis or activated alumina. See the SFV water quality overview for a complete treatment-to-contaminant matrix.

What is the MCLG for arsenic and why is it zero?

The MCLG for arsenic is zero. Under the Safe Drinking Water Act, EPA sets the MCLG for known or probable human carcinogens at zero because any exposure above zero carries some theoretical cancer risk. The MCLG is not enforceable; only the MCL of 10 µg/L is. The MCL is set as close to the MCLG as is feasible given available treatment technology and cost. More information on the MCLG/MCL structure is at the EPA SDWA regulation page.

Will an under-sink RO system remove all arsenic from drinking water?

An NSF/ANSI 58 certified RO system removes arsenate at 80 to 95 percent efficiency, depending on membrane condition and operating pressure. At 10 µg/L incoming arsenic, a 90 percent removal system produces permeate at roughly 1 µg/L. For households on LADWP supply where detected arsenic is typically low, an NSF/ANSI 58 RO unit brings drinking water to the low single-digit or sub-1 µg/L range. Verify that the specific system you purchase lists arsenic reduction (not just total dissolved solids reduction) in its NSF/ANSI 58 certification scope.

Can you taste or smell arsenic in drinking water?

No. Arsenic is odorless and tasteless in drinking water at concentrations near regulatory limits. It cannot be detected by human senses. Testing by a certified laboratory or a verified in-home screening kit is the only way to know actual arsenic levels at your specific tap. The UpTown Cares water testing service includes arsenic screening as part of the standard in-home test.

Last updated: August 3, 2026. Regulatory thresholds are current as of that date. MCLs, PHGs, and MCLG values are subject to revision; verify current arsenic standards at the California SWRCB arsenic page and the EPA arsenic drinking water page. For current utility-specific detection data, consult your utility's most recently published Consumer Confidence Report. OEHHA PHG values verified at oehha.ca.gov/water/public-health-goals.

Related reading: Chromium-6 in SFV Water · PFAS in SFV and Santa Clarita · Lead in SFV Water · TCE, PCE, and 1,4-Dioxane in SFV Groundwater · SFV Utility Water Quality Comparison · CA Water Quality Rules 2026 · How Whole-House Filtration Works · Chloramine in SFV Water · What 2,000 SFV Installs Taught Us · SFV Hard Water by Zip Code · Installation Day Guide · Reverse Osmosis · Water Testing · Whole-House Filtration · SFV Water Quality Overview · Service Areas · FAQ · Contact