Water Quality
Arsenic occurs naturally in San Fernando Valley groundwater, introduced by the weathering of volcanic and sedimentary rock formations across the Chatsworth Basin and the upper Santa Clara River watershed. The EPA enforces a Maximum Contaminant Level (MCL) for arsenic in public drinking water of 10 parts per billion (ppb), a threshold California mirrors under the State Water Resources Control Board. The same regulations cite the California Office of Environmental Health Hazard Assessment (OEHHA) Public Health Goal (PHG) for arsenic at 0.004 ppb, a level 2,500 times lower than the enforceable MCL. That gap reflects a policy decision: the MCL balances health protection against the technical and economic cost of removal, while the PHG represents the concentration at which risk to human health is considered negligible. SFV residents on municipal water supplied by LADWP can check their annual Consumer Confidence Report for arsenic readings. Those on private wells in Chatsworth, Sylmar, or Granada Hills have no regulatory agency testing for them and should test independently.
This article draws on EPA National Primary Drinking Water Regulations, OEHHA Public Health Goals, USGS water resource data, California SWRCB published standards, and LADWP Consumer Confidence Reports. For related context, see our overview of whether SFV tap water is safe to drink.
What is arsenic and why does it occur naturally in SFV groundwater?
Arsenic is a naturally occurring metalloid found in the earth's crust. In the San Fernando Valley, arsenic enters groundwater primarily through a geochemical process called mineral weathering. The valley sits within a geologically active region where volcanic and sedimentary rock formations contain arsenic-bearing minerals including arsenopyrite and arsenic-substituted iron oxides. As water percolates through these formations over time, it dissolves and transports arsenic into the underlying aquifer system.
The United States Geological Survey (USGS) has documented arsenic occurrence in groundwater across California, including the Transverse Ranges and adjacent valleys, through its National Water Quality Assessment (NAWQA) program. Groundwater from deeper wells in the Chatsworth Basin and portions of the upper Santa Clara River watershed has shown arsenic at naturally elevated concentrations in regional sampling studies. USGS water resources data and reports are accessible at usgs.gov/mission-areas/water-resources.
Arsenic can also enter water supplies through industrial contamination, but naturally occurring geogenic sources are the primary concern in the SFV context. Reducing conditions deep in an aquifer can mobilize arsenic bound to iron oxides, releasing it into groundwater even without a surface contamination source. EPA and OEHHA both classify arsenic as a known human carcinogen. The distinction between the MCL and the PHG reflects the difference between a regulatory standard set under feasibility and cost constraints and a health-based target set to minimize theoretical lifetime cancer risk.
What is the difference between the EPA MCL and the OEHHA PHG for arsenic?
The EPA Maximum Contaminant Level (MCL) for arsenic in public drinking water is 10 micrograms per liter, commonly expressed as 10 ppb (parts per billion). This standard, which replaced the prior 50 ppb standard in 2006, applies to all community water systems regulated under the Safe Drinking Water Act. California adopted the same 10 ppb MCL. These standards are enforceable: a public water system that delivers water above the MCL is in violation and must notify customers and take corrective action.
The OEHHA Public Health Goal for arsenic is 0.004 ppb, which is 0.004 micrograms per liter. That is 2,500 times lower than the enforceable MCL. OEHHA establishes PHGs under the California Safe Drinking Water and Toxic Enforcement Act. A PHG is not a regulation and carries no enforcement trigger. It is a scientifically derived target representing the concentration in drinking water at which the lifetime cancer risk for an individual is considered negligible, defined by OEHHA as one additional cancer case per one million people exposed over a 70-year lifetime. The OEHHA PHG document for arsenic is available at oehha.ca.gov/water/public-health-goal/arsenic.
The gap between the two numbers is intentional and reflects the structure of California drinking water policy. The Safe Drinking Water Act requires EPA and state agencies to set MCLs as close to the Maximum Contaminant Level Goal (MCLG) as feasible, considering treatment technology availability and cost. The EPA MCLG for arsenic is zero, because arsenic is a known carcinogen for which no safe threshold has been established. The 10 ppb MCL represents a policy judgment that treatment to levels approaching the MCLG would be technically and economically impractical for most water systems. OEHHA's PHG operates under different statutory authority and does not incorporate feasibility considerations. The California State Water Resources Control Board maintains a comparison table of MCLs and PHGs at waterboards.ca.gov.
For a consumer, what this means practically is that water meeting the 10 ppb MCL is legally compliant and meets federal and state drinking water standards, but is not at the level OEHHA considers negligible risk. Whether that distinction matters for a given household depends on exposure duration, individual risk factors, and whether supplementary treatment is feasible and desired.
What do LADWP Consumer Confidence Reports show about arsenic in SFV water?
LADWP publishes an annual Consumer Confidence Report (CCR), also called a Water Quality Report, summarizing monitoring results for regulated contaminants delivered to customers. Under federal and state law, CCRs must report arsenic detections above the detection limit, the range of concentrations detected, and the regulatory limit for comparison.
LADWP draws from two primary source types for the SFV service area: imported surface water from the Metropolitan Water District of Southern California and local groundwater from the San Fernando Basin aquifer. The utility blends these sources and processes them through treatment facilities before distribution. Historical CCR data for the SFV service area has generally shown arsenic in treated water at or below the 10 ppb MCL. Residents can obtain the current and prior year Consumer Confidence Reports from LADWP directly at ladwp.com.
When LADWP wells in the San Fernando Basin detect arsenic or other contaminants approaching regulatory thresholds, the utility has the operational flexibility to reduce withdrawals from those wells and substitute imported Metropolitan Water District supply. This blending capacity provides a buffer against localized groundwater quality events reaching the distribution system. LADWP also operates arsenic treatment at some wellheads where local groundwater serves specific pressure zones.
Other municipal suppliers serving portions of the SFV, including Burbank Water and Power and the City of San Fernando, also publish annual CCRs with arsenic monitoring data. The EPA reference for national primary drinking water regulations, including the arsenic MCL, is at epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations.
Do private wells in the SFV face higher arsenic risk than municipal water users?
Private domestic wells in the San Fernando Valley are not regulated under the Safe Drinking Water Act. The SDWA applies only to public water systems serving 25 or more people or 15 or more service connections. A homeowner with a private well bears sole responsibility for monitoring water quality and for any treatment needed. No state or federal agency routinely tests private well water on a property-by-property basis.
In the SFV, private wells are more common in the western and northern portions of the valley, including areas of Chatsworth, Sylmar, Granada Hills, and the foothill edges of the community. These areas include geology associated with the Chatsworth Basin and the Santa Susana Mountains, formations where naturally occurring arsenic concentrations in groundwater have been documented in regional USGS studies. A private well drilled into an aquifer formation with elevated arsenic will deliver that arsenic to household taps with no warning and no regulatory backstop.
Testing for arsenic requires a laboratory certified for drinking water analysis under the California Environmental Laboratory Accreditation Program (ELAP). County environmental health departments can provide lists of certified laboratories and guidance on recommended analyte panels for residential well water.
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What treatment options are effective against arsenic in drinking water?
For homeowners seeking to reduce arsenic in drinking water below the MCL, or to approach more health-protective concentrations, two treatment technologies have well-documented effectiveness and available NSF/ANSI certifications: reverse osmosis and activated alumina adsorption.
Reverse osmosis (RO) pushes water through a semi-permeable membrane under pressure, physically separating dissolved contaminants from the water stream. A properly maintained point-of-use RO system certified to NSF/ANSI 58 is effective at reducing both inorganic arsenic forms, arsenate and arsenite, in drinking water. A four-stage or five-stage undersink RO unit typically includes a sediment pre-filter, a carbon pre-filter, the RO membrane, and a post-filter carbon stage. The membrane rejects dissolved arsenic along with a broad range of other inorganic contaminants including lead, nitrates, and fluoride. NSF/ANSI 58 certification requires testing against specific contaminant reduction claims, and units certified to this standard with an arsenic reduction claim have been independently verified at stated influent concentrations.
Activated alumina (AA) is an adsorptive media made from aluminum oxide with a high surface area that selectively binds arsenate, the predominant arsenic form in oxygenated groundwater, from water passing through it. Activated alumina filters are used in both point-of-use and whole-house configurations. For activated alumina to perform optimally against arsenate, pH management may be needed, as adsorption efficiency decreases at higher pH levels. Filters using activated alumina media should be certified to NSF/ANSI 53 for arsenic reduction. Media capacity is finite: the media must be replaced at intervals specified by the manufacturer and validated by testing at the influent concentration the system will encounter.
Standard granular activated carbon (GAC) filters, including refrigerator filters and many countertop units, are not effective for arsenic removal and should not be relied upon for this purpose. When selecting any treatment system for arsenic, verify that the product's NSF certification specifically lists arsenic as a covered contaminant at the influent concentration relevant to your water. An NSF/ANSI 42 certification covers only aesthetic effects like chlorine taste and does not address arsenic.
Whole-house treatment addresses arsenic at all household uses. Point-of-use treatment at the kitchen tap covers drinking and cooking water. Water testing before selecting a system establishes the baseline concentration and determines which approach is warranted.
What should SFV residents do about arsenic in their water?
For residents on LADWP or another municipal supply, the starting point is the utility's annual Consumer Confidence Report. This document reports arsenic detections, the range of results, and how the utility's water compares to the regulatory MCL. If the utility's water is at or below the 10 ppb MCL, the water is legally compliant. Residents who want additional reduction toward the OEHHA PHG of 0.004 ppb may choose to install a point-of-use RO system. That is a personal precautionary decision, not a regulatory requirement.
For residents on private wells, the recommendation is direct: test the well. Arsenic testing requires a certified laboratory. A basic drinking water panel at a California ELAP-certified lab will include arsenic and typically covers multiple contaminants in one submission. A result above the 10 ppb MCL threshold indicates the well water would not meet public water system standards and treatment is warranted. Results between the MCL and the PHG place the decision in a precautionary zone where treatment is available and effective if the household elects it.
When testing confirms elevated arsenic, a point-of-use RO system certified to NSF/ANSI 58 with an arsenic reduction claim, or an activated alumina filter certified to NSF/ANSI 53, are the two principal residential treatment options with demonstrated effectiveness. Selecting a system based on actual test results ensures the treatment addresses the specific contaminant present.
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Frequently asked questions about arsenic in SFV water
Is the arsenic in my LADWP tap water safe?
If you are on LADWP service, your tap water is tested and reported in the annual Consumer Confidence Report. Water delivered at or below the EPA and California MCL of 10 ppb complies with federal and state drinking water standards. The OEHHA PHG of 0.004 ppb is a health-based goal that is 2,500 times lower than the enforceable MCL and represents a target rather than a compliance threshold. You can read LADWP's published monitoring results at ladwp.com to see the specific detected levels for the most recent reporting period.
Can you taste or smell arsenic in water?
Arsenic in drinking water is odorless, colorless, and tasteless at concentrations typically found in groundwater, including at levels above the 10 ppb MCL. There is no sensory way to detect arsenic in tap water. Testing by a certified laboratory is the only reliable method to determine whether arsenic is present and at what concentration. This is especially important for private well users who have no utility monitoring to rely on.
Does a standard water filter remove arsenic?
Standard granular activated carbon filters, including most refrigerator filters, pitcher filters, and basic countertop units, are not effective at removing arsenic. The two residential treatment technologies with documented, independently certified effectiveness against arsenic are reverse osmosis systems certified to NSF/ANSI 58 and activated alumina filters certified to NSF/ANSI 53. When evaluating any treatment product for arsenic, verify that the NSF certification specifically lists arsenic as a covered contaminant claim for that unit, not just general VOC or aesthetic reduction.
Are private wells in Chatsworth or Sylmar tested for arsenic?
Private domestic wells are not subject to routine regulatory testing under the Safe Drinking Water Act. No state or federal agency tests private wells on an ongoing basis. Homeowners with private wells in areas like Chatsworth, Sylmar, and Granada Hills, where Chatsworth Basin geology creates conditions for naturally occurring arsenic, should arrange independent testing through a California ELAP-certified laboratory. County environmental health departments can provide referrals to certified labs and guidance on comprehensive analyte panels for residential well water.
What is the OEHHA PHG for arsenic and why is it so much lower than the MCL?
The OEHHA Public Health Goal for arsenic is 0.004 ppb, compared to the enforceable MCL of 10 ppb. OEHHA sets PHGs based solely on health science, targeting the concentration at which theoretical lifetime cancer risk is negligible. The MCL is set under different statutory authority that requires regulators to balance health protection against treatment feasibility and cost. Because arsenic removal to PHG levels is technically difficult and expensive at scale for a public water system, the enforceable standard sits much higher than the health-ideal target. California law requires that PHGs be published alongside MCLs in CCRs so consumers can see both numbers.
