Arsenic in San Fernando Valley Tap Water: What the EPA 10 ppb Standard and LADWP Annual Data Show

What is the EPA arsenic limit for drinking water?

The current federal standard is 10 micrograms per liter (10 ppb), set by the EPA's Arsenic Rule published in January 2001 and enforceable starting January 23, 2006. Before 2006, the limit was 50 ppb, a standard that dated to the 1970s. The Safe Drinking Water Act required EPA to revise outdated MCLs using the best available science, and the downward revision to 10 ppb reflected updated epidemiological data linking arsenic exposure to lung, bladder, and skin cancers.

The Maximum Contaminant Level Goal (MCLG) for arsenic is 0. The MCLG is EPA's health-based target with no recognized safe exposure threshold for carcinogens. Because treatment to 0 is not technologically feasible at the distribution-system scale, the enforceable MCL was set at 10 ppb as the lowest limit achievable with standard treatment technology. This gap between the MCLG (0) and the MCL (10 ppb) means that any arsenic present in tap water at levels below the MCL is still a legal compliance matter, not a finding of zero risk. The EPA National Primary Drinking Water Regulations table documents the current MCL and MCLG side by side.

Why does the San Fernando Valley have naturally occurring arsenic?

Arsenic is a naturally occurring metalloid found in soils and rocks throughout the western United States, particularly in areas with volcanic rock, geothermal activity, or mineral-rich sedimentary formations. The U.S. Geological Survey's national arsenic in groundwater assessment documents elevated arsenic concentrations across much of California, with concentrations driven by the geology of the underlying aquifer formations rather than by human activity.

The San Fernando Valley Basin, which LADWP draws from as a local groundwater source alongside its imported Metropolitan Water District supply, sits within the broader Los Angeles Basin geology. Local groundwater makes up a portion of the LADWP blend depending on the season and availability. The Colorado River, one of MWD's two major import sources, flows through geologically diverse terrain in the Rocky Mountain and Basin and Range regions, areas where naturally occurring arsenic in river sediments is documented in USGS Colorado River basin water quality data. The State Water Project, the other major import source, draws primarily from the Sacramento-San Joaquin Delta and Sierra Nevada snowpack, which carries a different arsenic profile.

Industrial or agricultural arsenic contamination is a separate issue from naturally occurring arsenic and is tracked through a different regulatory pathway. In the SFV, the aerospace and defense manufacturing history is more closely associated with perchlorate and volatile organic compound contamination than with arsenic. For most homes on LADWP or Las Virgenes MWD service, the arsenic source is geological rather than industrial. The annual CCR reports each source of the detected contaminant; check the current LADWP CCR at ladwp.com/water-quality for the reported typical source for arsenic in your service area.

Does LADWP's water meet the arsenic standard?

LADWP is required by law to maintain arsenic levels at or below 10 ppb and to report any violation in its annual Consumer Confidence Report and in direct customer notifications under EPA's Safe Drinking Water Act notification rules. The CCR table for inorganic chemicals lists the highest level detected during the reporting year, the range of detections across distribution system monitoring points, and the MCL. Customers can confirm the current reported level at ladwp.com/water-quality.

The Environmental Working Group's Tap Water Database at ewg.org/tapwater compiles reported arsenic data from LADWP and other utilities and presents it alongside EWG's own health benchmarks, which are typically lower than the federal MCL. EWG benchmarks are not regulatory standards, but they can be a useful reference for households that want to reduce exposure below the legal limit. A compliance finding from LADWP means the utility is operating within federal law; it does not mean the water is arsenic-free. The MCLG of 0 reflects that distinction explicitly.

Las Virgenes Municipal Water District, which serves Calabasas, Agoura Hills, and Westlake Village, publishes a separate Consumer Confidence Report at lvmwd.com/water-quality. Homes in those communities should check the LVMWD CCR for their service area-specific arsenic figures rather than the LADWP report. The two utilities draw from the same MWD import system but have different local blending and groundwater contributions.

What does California's arsenic regulation add beyond the federal rule?

California adopted the 10 ppb federal MCL when it took effect in 2006, replacing the prior 50 ppb state standard. The California Division of Drinking Water, operating under the State Water Resources Control Board, maintains the current MCL table at waterboards.ca.gov. For arsenic, California does not currently enforce a standard stricter than the federal 10 ppb limit, unlike its approach to some other contaminants such as perchlorate (California: 1 ppb, well below the federal 18 ppb) or hexavalent chromium.

California does maintain a separate notification level and a response level process for contaminants detected below the MCL. For arsenic, the State Water Resources Control Board also uses the Primary MCL compliance framework as the primary enforcement mechanism. Homeowners wanting California-specific data for their utility can also search the State Water Resources Control Board's SAFER (Safe and Affordable Drinking Water) tool at waterboards.ca.gov, which aggregates violation and monitoring data for California water systems.

Which home filtration systems reduce arsenic in SFV tap water?

Three treatment technologies carry NSF/ANSI 58 certification for arsenic reduction at the point of use:

• Reverse osmosis (RO): NSF/ANSI 58-certified RO systems that carry a specific arsenic reduction performance claim have been independently tested and verified to reduce arsenic concentration at the point of use. Not every certified RO system is tested for arsenic; the system must carry an explicit arsenic claim in its NSF listing. RO also reduces total dissolved solids, nitrates, perchlorate, and many other contaminants simultaneously, making it a common multi-contaminant solution for SFV homes. Reverse osmosis installations in the San Fernando Valley are sized based on household water use and daily purified water volume requirements.
• Activated alumina: An adsorbent medium that reduces arsenic through a selective adsorption process, also listed under NSF/ANSI 58. Performance is pH-dependent: activated alumina works most effectively at water pH below 8.5. An in-home water test that covers pH is the appropriate first step before selecting this treatment path.
• Strong-base anion exchange: Ion exchange systems using strong-base anion resin also carry NSF/ANSI 58 certification pathways for arsenate (As(V)) reduction. Arsenite (As(III)), the other primary form, requires oxidation to arsenate before anion exchange is effective; municipal treatment systems typically handle this oxidation step, but it is worth confirming with your CCR data.

Standard activated carbon block filters, which many SFV households use for taste and odor improvement, do not reliably reduce arsenic. Activated carbon is effective for chloramine residual, some volatile organic compounds, and taste compounds, but it does not adsorb arsenic at the concentrations typical in treated tap water. Whole-home filtration systems that use carbon media alone are not rated for arsenic removal. If arsenic reduction is a priority, an NSF/ANSI 58-certified point-of-use RO or activated alumina system at the kitchen tap is the standard approach for SFV homes.

Confirm any system's certification at nsf.org/certified-products before purchasing. The NSF listing will show which specific contaminants each product was tested and certified to reduce, so you can verify that the arsenic claim is explicitly included.

Should I get my SFV tap water tested for arsenic?

The LADWP Consumer Confidence Report gives system-wide averages, not readings at any individual address. For most households on LADWP or LVMWD service, the CCR figures are the best available starting point for understanding arsenic exposure from the distributed water supply.

A tap-specific test is appropriate when a household relies on a private well rather than a utility supply, when there is reason to suspect local contamination upstream of the meter, or when the household contains members with elevated sensitivity to arsenic exposure (including infants and young children, for whom independent testing may be worth considering). Well water in the Calabasas foothill area, Agoura Hills, or Chatsworth areas sits outside LADWP and LVMWD distribution and is not subject to utility CCR monitoring. Well owners should test independently.

For certified laboratory arsenic testing, the California State Water Resources Control Board maintains a list of ELAP-certified labs at waterboards.ca.gov/drinking_water/certlic/labs/. Field in-home consultations that test for hardness, TDS, and chloramine residual are not a substitute for certified laboratory arsenic analysis. A free in-home water consultation is the right starting point for understanding the overall water quality profile at your address and identifying whether a certified lab arsenic test is the appropriate next step.

The San Fernando Valley water filtration decision starts with knowing what is actually present. For homes on utility service, the CCR covers the distribution system. For homes with well water or in locations adjacent to former industrial sites, a certified lab test adds the address-level specificity the CCR cannot provide.

Sources used in this article: EPA National Primary Drinking Water Regulations; 40 CFR 141.62 (Arsenic MCL); California Division of Drinking Water MCL Table; USGS Arsenic in Groundwater of the United States; NSF International: Certified Drinking Water Treatment Systems (NSF/ANSI 58); Environmental Working Group Tap Water Database; LADWP Consumer Confidence Report; Las Virgenes MWD Consumer Confidence Report; California ELAP-Certified Drinking Water Laboratories.