Arsenic in Well Water California: Testing, Health Risks & Treatment

Arsenic is colorless, odorless, and one of the most dangerous contaminants found in California groundwater. If you have a private well, there's no government agency testing your water—that responsibility falls entirely on you. Here's everything you need to know to protect your family.

California has some of the highest natural arsenic levels in the United States, with certain regions routinely exceeding federal safety limits. Unlike public water systems that must test and treat for arsenic, private well owners are responsible for their own water quality—and many have never tested for this invisible threat.

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Important: You Cannot Detect Arsenic Without Testing

Arsenic has no taste, no smell, and no color. Your water can look and taste perfectly clean while containing dangerous arsenic levels. The only way to know is through laboratory testing.

Understanding California's Arsenic Standards

The regulatory framework for arsenic in drinking water involves several important numbers that every well owner should understand:

Key Arsenic Limits

MCL (Maximum Contaminant Level): 10 µg/L (10 ppb) — The enforceable standard for public water systems
PHG (Public Health Goal): 0.004 µg/L — California's health-based goal (2,500x lower than MCL)
Previous Standard: 50 µg/L — The old limit until 2006 (5x higher)
Detection Limit: 2 µg/L — The reporting threshold for labs

The significant gap between the MCL (10 µg/L) and the Public Health Goal (0.004 µg/L) reflects an important reality: California regulators acknowledge that even the current legal limit carries health risks, but balancing technical feasibility and cost led to the 10 µg/L standard.

For private well owners, this means the legal limit shouldn't be viewed as "safe"—it's a practical regulatory threshold. Many health experts recommend treating water if arsenic levels exceed 5 µg/L, particularly if children or pregnant women drink the water.

Where Arsenic Comes From in California Groundwater

Understanding the sources of arsenic helps explain why it's so prevalent in certain areas and why your neighbor's well might have different levels than yours.

Natural Geological Sources

The vast majority of arsenic in California groundwater comes from natural geological deposits. Arsenic is the 20th most abundant element in the Earth's crust and is found in many rock types:

Volcanic rocks: Common throughout California, these release arsenic as they weather
Marine sedimentary deposits: Ancient seabeds now underground contain arsenic-bearing minerals
Geothermal areas: Hot springs and geothermal activity concentrate arsenic
Alluvial deposits: River sediments can carry arsenic from source rocks

When groundwater comes into contact with these formations, arsenic dissolves into the water. The process is influenced by pH, oxygen levels, and how long water remains in contact with the rock. Deep wells and wells in certain geological formations are often at higher risk.

Human-Caused Contamination

While natural sources dominate, human activities can also contribute to arsenic contamination:

Historical pesticides: Lead arsenate and other arsenic-based pesticides were used extensively in orchards until the 1940s
Mining operations: Gold and copper mining can release arsenic from ore
Industrial activities: Wood treatment, glass manufacturing, and electronics production
Agricultural runoff: Some fertilizers and animal feed additives contain arsenic

High-Risk Areas in California

Arsenic contamination is not evenly distributed across California. Certain regions have significantly higher risks due to underlying geology:

Region	Risk Level	Notes
San Joaquin Valley	High	Many wells exceed MCL; Kern, Tulare, Fresno counties
Mojave Desert	High	Natural volcanic and geothermal sources
Coachella Valley	Moderate-High	Desert geology; some agricultural impact
Owens Valley	High	Volcanic activity; geothermal influence
San Diego Backcountry	Moderate	Variable by location; granite and metamorphic rocks
Imperial Valley	Moderate-High	Salton Sea region; geothermal activity
Sacramento Valley	Moderate	Alluvial deposits; varies significantly

Important: Even in "low risk" areas, individual wells can have elevated arsenic. Geological conditions can vary dramatically over short distances. Always test your specific well rather than relying on regional assumptions.

Health Effects of Arsenic Exposure

Arsenic is classified as a Group 1 carcinogen by the World Health Organization—the highest certainty category. Long-term exposure through drinking water is associated with serious health effects:

Cancer Risks

Chronic arsenic exposure is conclusively linked to several cancers:

Bladder cancer: The strongest association, with risk increasing with concentration and duration
Lung cancer: Even in non-smokers, arsenic exposure increases lung cancer risk
Skin cancer: Including squamous cell carcinoma and basal cell carcinoma
Kidney and liver cancers: Moderate associations documented in studies

Non-Cancer Health Effects

Beyond cancer, arsenic exposure causes numerous other health problems:

Cardiovascular disease: Increased risk of heart disease, stroke, and peripheral arterial disease
Diabetes: Arsenic exposure is linked to increased Type 2 diabetes risk
Developmental effects: Children exposed to arsenic may experience reduced cognitive function and IQ
Skin changes: Characteristic skin lesions, pigmentation changes, and keratosis
Neurological effects: Peripheral neuropathy and cognitive impairment
Immune suppression: Reduced immune function and increased susceptibility to infections

Vulnerable Populations

Children, pregnant women, and the elderly are especially vulnerable to arsenic's effects. Children absorb arsenic more readily and are more susceptible to developmental impacts. During pregnancy, arsenic can cross the placenta and affect fetal development. If your household includes vulnerable individuals, consider treating water even at levels below the MCL.

Testing Your Well Water for Arsenic

Regular testing is the only way to know if your well water contains arsenic. Here's how to approach it:

When to Test

Initial purchase: Always test when buying property with a well
Annual testing: Arsenic levels can change over time
After well work: Following any drilling, pump replacement, or significant repairs
After nearby changes: New construction, mining, or agricultural changes nearby
Water level changes: During drought when drawing from deeper aquifers

How to Test

For accurate arsenic testing, use a certified laboratory. California requires laboratories analyzing drinking water to be certified by the State Water Resources Control Board. Options include:

State-certified labs: Search the ELAP (Environmental Laboratory Accreditation Program) database
County health departments: Many offer testing services or referrals
Professional well services: We can coordinate testing as part of our well inspection services

Testing costs typically range from $25-75 for arsenic alone, or $100-250 for comprehensive water quality panels that include arsenic along with other contaminants.

Home Test Kits: A Caution

While home test kits exist for arsenic, most are designed for screening rather than accurate measurement. They may indicate the presence of arsenic but cannot reliably measure concentrations near the MCL. For decisions about treatment, always use a certified laboratory.

Treatment Options for Arsenic Removal

If your well water tests positive for arsenic above safe levels, several effective treatment technologies are available:

Reverse Osmosis (Most Recommended)

Reverse osmosis is the gold standard for residential arsenic treatment. It forces water through a semi-permeable membrane that blocks arsenic along with many other contaminants.

Effectiveness: 95-99% arsenic removal
Types: Point-of-use (under sink) or whole-house systems
Cost: $200-500 for point-of-use; $1,500-4,000+ for whole-house
Maintenance: Filter replacement every 6-12 months; membrane every 2-3 years
Pros: Highly effective, well-proven technology, removes other contaminants
Cons: Produces wastewater (3-4 gallons waste per gallon treated), removes beneficial minerals

Adsorptive Media Filters

These systems use iron-based media that attracts and binds arsenic. They're particularly effective for arsenic(V), the more common form in well water.

Effectiveness: 90-99% removal (depends on arsenic species and water chemistry)
Types: Point-of-use or whole-house
Cost: $300-600 for point-of-use; $1,000-3,000 for whole-house
Maintenance: Media replacement every 6-12 months depending on usage and levels
Pros: No wastewater, maintains beneficial minerals
Cons: Less effective for arsenic(III); may require pre-oxidation

Anion Exchange

Similar to water softeners, anion exchange systems remove negatively charged contaminants including arsenic(V).

Effectiveness: 90-95% for arsenic(V)
Cost: $1,000-3,000 for whole-house
Maintenance: Regular regeneration with salt; media replacement every 5-10 years
Pros: Good for whole-house treatment; long media life
Cons: Not effective for arsenic(III); competes with other anions in water

Oxidation + Filtration

This approach converts arsenic(III) to arsenic(V), then removes it through filtration. It's often used when water contains significant arsenic(III).

Effectiveness: 80-95% depending on system design
Cost: $2,000-5,000+ for complete systems
Methods: Chlorination, ozonation, or aeration followed by filtration
Pros: Addresses both arsenic species; can treat high concentrations
Cons: More complex; may require multiple treatment stages

Choosing the Right Treatment

The best treatment option depends on several factors:

Arsenic concentration: Higher levels may need more robust systems
Arsenic species: Arsenic(III) vs. arsenic(V) affects treatment choice
Other water quality issues: Iron, manganese, pH can affect treatment
Treatment scope: Point-of-use (drinking water only) vs. whole-house
Budget: Initial cost vs. ongoing maintenance

Reducing Arsenic Exposure Beyond Treatment

While water treatment is the primary solution, additional steps can reduce exposure:

Use treated water for cooking: Arsenic isn't removed by boiling; it actually concentrates. Use treated water for all cooking.
Consider bottled water: As an interim measure while installing treatment
Test food-garden water: If using well water for vegetables, arsenic can accumulate in plants, especially leafy greens and rice
Don't use hot water for cooking: Hot water may contain higher contaminant levels from water heaters

Frequently Asked Questions

What is the arsenic limit for drinking water in California?

California's Maximum Contaminant Level (MCL) for arsenic is 10 µg/L (micrograms per liter), which equals 10 parts per billion (ppb) or 0.01 mg/L. This matches the federal EPA standard set in 2006. However, California's Public Health Goal (PHG) is much lower at 0.004 µg/L, recognizing that any arsenic exposure carries some cancer risk. Private well owners are not regulated but should test annually and treat if levels exceed 10 µg/L.

How do I know if my well water has arsenic?

Arsenic is colorless, odorless, and tasteless—you cannot detect it without testing. The only way to know if your well water contains arsenic is through laboratory analysis. Test kits from certified labs cost $25-75 and provide accurate results. Test your well water when you first move in, annually thereafter, and after any changes to your well system. California has many arsenic hotspots, particularly in the Central Valley, desert regions, and some Southern California areas.

What are the health effects of arsenic in drinking water?

Long-term arsenic exposure through drinking water is linked to several serious health effects. It's a known human carcinogen associated with bladder, lung, and skin cancers. Arsenic can also cause cardiovascular disease, diabetes, developmental effects in children including reduced IQ, skin lesions and discoloration, and immune system suppression. Even levels below the MCL may pose health risks with prolonged exposure, which is why treatment is recommended when arsenic is detected.

What is the best treatment for arsenic in well water?

Reverse osmosis (RO) is the most effective and commonly recommended treatment for arsenic in residential well water, removing 95-99% of arsenic. Point-of-use RO systems cost $200-500 and treat drinking water at the tap. Whole-house systems cost $1,500-4,000+. Other effective options include adsorptive media filters using iron-based materials, anion exchange systems, and oxidation/filtration systems. The best choice depends on your arsenic level, water chemistry, and whether you need whole-house or point-of-use treatment.

Where in California has high arsenic in groundwater?

Arsenic contamination in California groundwater is widespread but concentrated in certain regions. High-risk areas include the San Joaquin Valley (particularly Kern, Tulare, and Fresno counties), the Mojave Desert region, parts of the Coachella Valley, the Owens Valley, some areas of San Diego County's backcountry, and the Salton Sea region. Arsenic occurs naturally in these areas due to geological formations. Even in lower-risk regions, individual wells can have elevated arsenic, so testing is essential everywhere.

Concerned About Arsenic in Your Well Water?

Southern California Well Service can help with water testing coordination, treatment system recommendations, and complete well system maintenance. We serve San Diego and Riverside Counties with expert well services.

Call (760) 463-0493 for Expert Advice

Arsenic in Well Water California