By SCWS Team
February 1, 2026 · 12 min read
You can't see it. You can't smell it. You can't taste it. Yet arsenic—one of the most dangerous contaminants found in private wells—could be flowing from your tap right now. This naturally occurring element is linked to multiple cancers, heart disease, and developmental problems, making it a serious concern for California well owners. The only way to know if your water is safe? Testing. Here's everything you need to know about protecting your family.
⚠️ Critical Warning
Boiling water does NOT remove arsenic—it actually concentrates it, making the water more dangerous. If you suspect arsenic contamination, use bottled water for drinking and cooking until your well has been tested and, if necessary, a proper arsenic treatment system is installed.
Natural Sources of Arsenic in Groundwater
Unlike many well water contaminants that come from human activities, arsenic in groundwater is primarily natural. Arsenic is the 20th most abundant element in Earth's crust, found in over 200 mineral forms. When groundwater flows through arsenic-bearing rock formations, the element dissolves into the water supply.
Several geological conditions increase arsenic levels in groundwater:
- Volcanic rock formations: Areas with volcanic geology often have elevated arsenic. Much of California, including parts of San Diego and Riverside Counties, sits on volcanic or metamorphic rock that can release arsenic.
- Sedimentary basins: Arsenic accumulates in ancient lake and river sediments. The Central Valley, Mojave Desert, and many inland valleys have arsenic-rich alluvial deposits.
- Geothermal areas: Hot springs and geothermal activity bring arsenic-rich water to the surface. Nearby wells often show elevated levels.
- Mining regions: Historic gold, silver, and copper mining areas may have elevated arsenic from disturbed ore deposits and mine tailings.
- Agricultural areas: Past use of arsenic-based pesticides (banned since the 1980s) can leave residual contamination in soil and shallow groundwater.
The depth of your well matters significantly. Arsenic levels can vary dramatically between shallow and deep aquifers, and even between wells on neighboring properties. This variability makes individual well water testing essential—you cannot assume your well is safe based on nearby wells.
Health Risks of Arsenic Exposure
Arsenic is classified as a Group 1 carcinogen—meaning there is definitive evidence it causes cancer in humans. The health effects of arsenic depend on the concentration and duration of exposure, but even low levels over many years pose significant risks.
Cancer Risks
Long-term arsenic exposure is strongly linked to several types of cancer:
- Skin cancer: The most well-documented association, including both melanoma and non-melanoma types
- Bladder cancer: Risk increases significantly with chronic exposure
- Lung cancer: Even without smoking, arsenic exposure increases lung cancer risk
- Kidney cancer: Elevated risk with long-term exposure
- Liver cancer: Associated with high arsenic intake
Non-Cancer Health Effects
Beyond cancer, chronic arsenic exposure causes numerous health problems:
| System Affected | Health Effects |
|---|---|
| Cardiovascular | Heart disease, high blood pressure, thickening of blood vessels, irregular heartbeat |
| Nervous System | Numbness in hands and feet, cognitive impairment, decreased IQ in children |
| Skin | Characteristic skin lesions, darkening, warts, thickening of palms and soles |
| Digestive | Nausea, vomiting, diarrhea, abdominal pain (acute exposure) |
| Metabolic | Increased risk of diabetes, hormonal disruption |
| Developmental | Low birth weight, developmental delays, immune system effects in children |
Children are particularly vulnerable to arsenic's effects. Their developing bodies absorb more arsenic relative to body weight, and exposure during critical development periods can cause lasting harm to cognitive development and immune function.
California's Arsenic Standard: The 10 ppb MCL
California adopted the federal Maximum Contaminant Level (MCL) of 10 parts per billion (ppb) for arsenic in drinking water. This standard, which took effect in 2006, replaced the previous limit of 50 ppb that had been in place since 1942.
Understanding Arsenic Limits
- Maximum Contaminant Level (MCL): 10 ppb — The enforceable standard for public water systems
- Public Health Goal (PHG): 0.004 ppb — California's health-based goal with negligible cancer risk
- Detection Limit for Reporting: 2 ppb — The level at which labs must report arsenic presence
Note: While the MCL is 10 ppb, the PHG of 0.004 ppb reflects that there is no truly "safe" level of arsenic—the MCL represents a balance between health protection and treatment feasibility.
Private wells are not regulated under these standards—testing and treatment are the homeowner's responsibility. However, when selling a property, California law requires disclosure of known water quality issues, and many buyers request well water arsenic testing during real estate transactions.
Well Water Arsenic Test Requirements
Testing is the only way to know if your well water contains arsenic. The California Department of Public Health recommends testing private wells for arsenic at these times:
- When you first acquire a property: Test before relying on the well for drinking water
- After drilling a new well: Baseline testing should include arsenic
- Every 3-5 years: Routine monitoring for all private wells
- Annually if in a high-risk area: More frequent testing if arsenic has been detected nearby
- After changes to the well: Test after well repairs, deepening, or pump replacement
- If neighbors have arsenic: Test if nearby wells show elevated levels
How to Test for Arsenic
Arsenic testing requires laboratory analysis—home test kits are not accurate enough for the low levels that matter for health. For comprehensive guidance on testing procedures, see our complete well water testing guide.
Arsenic Testing Basics
- Cost: $25-$50 for arsenic alone; $100-$200 for comprehensive panels
- Turnaround: 5-10 business days typically
- Sample requirements: Use lab-provided containers; follow collection instructions precisely
- Speciation testing: Optional test ($40-$80 extra) that distinguishes between arsenic(III) and arsenic(V)—useful for selecting treatment
Areas at Higher Risk for Arsenic
While arsenic can occur anywhere, certain regions have significantly higher rates of contamination:
California High-Risk Areas
- Central Valley: One of the nation's highest arsenic areas, with many wells exceeding 10 ppb
- Mojave Desert region: Elevated arsenic in alluvial groundwater basins
- Owens Valley: Volcanic geology contributes to high arsenic levels
- Former mining areas: Mother Lode region, parts of San Diego County backcountry
- Coachella Valley: Some areas show elevated arsenic in deeper wells
- Inland valleys: Scattered elevated readings in Ramona, Julian, and other inland communities
The USGS estimates that 2.1 million people in the United States use private wells with arsenic above 10 ppb. California has among the highest rates of naturally occurring arsenic contamination in the country.
Choosing Your Treatment
The right arsenic treatment depends on your water chemistry. Speciation testing ($40-$80) determines whether your water contains arsenic(III) or arsenic(V)—critical information since most treatment methods work better on arsenic(V).
Arsenic Treatment Options
Several proven technologies effectively remove arsenic from well water. The best choice depends on your arsenic level, water chemistry, water usage, and budget.
Adsorptive Media Filtration
Adsorptive media systems use specialized materials—typically iron-based—that bind arsenic as water passes through. This is often the most cost-effective whole-house arsenic removal method.
Adsorptive Media Details
- How it works: Iron oxide or hydroxide media attracts and holds arsenic molecules
- Effectiveness: Removes 95%+ of arsenic(V); less effective for arsenic(III) without pre-oxidation
- Cost: $1,500-$4,000 installed for whole-house systems
- Media replacement: Every 1-3 years depending on arsenic levels and water usage ($300-$800)
- Pros: No wastewater, low maintenance, treats entire house
- Cons: Media is a consumable cost; less effective for arsenic(III)
Reverse Osmosis (RO)
Reverse osmosis forces water through a semi-permeable membrane that blocks arsenic and many other contaminants. RO is highly effective and commonly used for point-of-use (drinking water) treatment.
Reverse Osmosis Details
- How it works: Pressure pushes water through a membrane that blocks arsenic molecules
- Effectiveness: Removes 85-95% of both arsenic(III) and arsenic(V)
- Cost: $200-$600 for under-sink units; $1,500-$3,000 for whole-house
- Maintenance: Filter and membrane replacement every 1-3 years ($100-$300)
- Pros: Highly effective, removes many other contaminants, relatively affordable point-of-use
- Cons: Produces wastewater (3-4 gallons per 1 gallon treated), slow flow rate, wastes water
Ion Exchange
Ion exchange systems use specialized resin that swaps arsenic ions for harmless chloride ions. This technology is effective but can be affected by other minerals in your water.
Ion Exchange Details
- How it works: Strong-base anion exchange resin captures arsenate ions
- Effectiveness: 90-95% removal of arsenic(V); requires pre-oxidation for arsenic(III)
- Cost: $2,000-$5,000 installed
- Maintenance: Resin regeneration or replacement; ongoing salt costs if regenerating
- Pros: High capacity, well-established technology
- Cons: Sulfate and nitrate compete for resin capacity; generates brine waste
Arsenic(III) vs. Arsenic(V): Why It Matters
Arsenic exists in two forms in water: arsenic(III) (arsenite) and arsenic(V) (arsenate). Most treatment methods work better on arsenic(V). If your water contains significant arsenic(III), you may need a pre-oxidation step—typically using chlorination, ozone, or air injection—to convert it to the more easily removed arsenic(V) form.
Speciation testing ($40-$80) can determine which form predominates in your water, helping you select the most effective treatment approach.
Cost of Arsenic Treatment Systems
| Treatment Type | Initial Cost | Annual Maintenance |
|---|---|---|
| Point-of-use RO (under sink) | $200-$600 | $100-$200 |
| Whole-house adsorptive media | $1,500-$4,000 | $200-$500 |
| Whole-house RO | $1,500-$3,000 | $200-$400 |
| Ion exchange system | $2,000-$5,000 | $200-$600 |
| Annual arsenic testing | $25-$50 | $25-$50 |
For budget-conscious homeowners, a point-of-use RO system for drinking and cooking water is often the most practical starting point. This protects against the primary exposure route (ingestion) at modest cost, while bathing and other uses pose much lower risk since arsenic is poorly absorbed through skin.
Ongoing Monitoring
Installing a treatment system isn't the end of the process. Ongoing monitoring ensures your system continues working effectively:
- Test treated water annually: Verify your system is still removing arsenic effectively
- Test raw (untreated) water every 2-3 years: Arsenic levels can change over time
- Follow maintenance schedules: Replace filters, media, or membranes as recommended
- Monitor system pressure: Declining pressure may indicate clogged media or filters
- Test after any well work: Pump replacement or repairs can change water chemistry
- Keep records: Document all test results and maintenance for future reference
Frequently Asked Questions
What is a safe level of arsenic in well water?
California has set the Maximum Contaminant Level (MCL) for arsenic at 10 parts per billion (ppb), which matches the federal EPA standard. However, there is no truly "safe" level of arsenic—even low levels pose some cancer risk. California's Public Health Goal (PHG), which represents the level with negligible health risk, is just 0.004 ppb. If your well tests above 10 ppb, treatment is legally required for public water systems and strongly recommended for private wells.
How do I test my well water for arsenic?
Arsenic testing requires a water sample analyzed by a state-certified laboratory. Home test kits exist but are not reliable for the precision needed at low levels. Collect a sample in a laboratory-provided container, following their instructions carefully. Standard arsenic tests cost $25-$50 and results typically take 5-10 business days. Test when you first buy a property, after any changes to your well, and every 3-5 years for ongoing monitoring—or annually if you're in a high-risk area.
What is the best treatment for arsenic in well water?
The best treatment depends on your arsenic levels and water chemistry. For most residential applications, adsorptive media filters (using iron-based media) are the most cost-effective option for arsenic removal, especially for arsenic(V). Reverse osmosis systems are highly effective at the point-of-use (under sink). Ion exchange systems work well but may be affected by competing contaminants. For high arsenic levels or complex water chemistry, a combination of treatment methods may be needed. Professional water testing should guide your treatment choice.
How much does arsenic treatment cost for a private well?
Point-of-use reverse osmosis systems (treating drinking water only) cost $200-$600 installed. Whole-house adsorptive media systems range from $1,500-$4,000 installed, with media replacement every 1-3 years costing $300-$800. Ion exchange systems run $2,000-$5,000 installed with ongoing regeneration costs. Annual arsenic testing costs $25-$50. Total first-year costs typically range from $500 for basic point-of-use treatment to $5,000+ for comprehensive whole-house systems.
Can boiling water remove arsenic?
No—boiling water does NOT remove arsenic. In fact, boiling concentrates arsenic because water evaporates but arsenic remains behind. This makes boiled water more dangerous, not safer. Unlike bacteria, which boiling kills, arsenic is an element that cannot be destroyed or removed by heat. Only proper filtration methods like reverse osmosis, adsorptive media, or ion exchange can effectively remove arsenic from drinking water.
Concerned About Arsenic in Your Well Water?
If you haven't tested your well for arsenic, or if you've received elevated results and need help selecting a treatment system, we're here to help. Our experienced team serves San Diego and Riverside Counties with comprehensive well water testing guidance and treatment solutions. Protect your family's health with proper arsenic testing and treatment.