Arsenic in Arizona Well Water
Arizona has the highest rate of arsenic contamination in groundwater of any state in the continental US. Over 20% of wells exceed the EPA limit — three times the national average. Arsenic is colorless, odorless, and tasteless. You cannot detect it without testing.
Arsenic is invisible. Unlike iron (rust staining), hardness (scale), or sulfur (rotten egg smell), arsenic gives no sensory warning. You can drink arsenic-contaminated water for years without knowing it. The only way to detect arsenic is to test.
The Scale of the Problem
Why Arizona?
Arizona's arsenic problem is geological. The Basin and Range province that covers most of the state creates ideal conditions for arsenic accumulation in groundwater:
- Volcanic and granitic source rocks — Arizona's mountains and volcanic fields contain arsenic-bearing minerals that dissolve slowly into groundwater
- Long groundwater residence times — water moves slowly through basin-fill aquifers, spending centuries in contact with arsenic-bearing rock
- Arid climate — minimal precipitation means minimal recharge and dilution. What rain does fall evaporates or transpires, concentrating dissolved minerals
- High pH conditions — alkaline groundwater (common in Arizona) mobilizes arsenic more effectively than acidic water
- Irrigation practices — decades of agricultural irrigation have mobilized arsenic from soil and rock into groundwater in some basins
Where Are the Hot Spots?
Arsenic contamination is widespread across Arizona but concentrated in specific regions:
| Area | Arsenic Situation |
|---|---|
| Yavapai County (Prescott, Chino Valley, Paulden, Dewey-Humboldt) | 40%+ of samples exceed 10 ppb. Cornville wells tested up to 952 ppb. |
| Pinal County (Queen Creek, Apache Junction, Casa Grande) | 40%+ exceedance rate. Basin-fill aquifer with volcanic influence. |
| Central and Southern Arizona basins | Most exceedances statewide are in central and southern regions. |
| Western Basin and Range | Belt of basins along western AZ predicted to have highest concentrations. |
| Cave Creek / Carefree | Municipal wells too high for potable use without treatment. |
| Green Valley | Naturally occurring from Sonoran Desert geology. |
Health Effects
Arsenic is a known human carcinogen. Long-term exposure to arsenic in drinking water increases the risk of:
- Cancer — lung, bladder, skin, kidney, and liver
- Skin damage — thickening and discoloration, particularly on palms and soles
- Cardiovascular disease — heart disease and stroke
- Neurological effects — peripheral neuropathy
- Developmental effects — reduced birth weight, impaired cognitive development in children
The EPA lowered the arsenic MCL from 50 ppb to 10 ppb in 2001, effective 2006. This single regulatory change put 35% of Arizona water-supply wells out of compliance. The lower standard reflects growing evidence that arsenic is harmful at lower concentrations than previously thought.
The MCL Isn't a Safety Threshold
The EPA's Maximum Contaminant Level of 10 ppb is not a "safe" level — it's a regulatory compromise between health risk and treatment feasibility. The EPA's Maximum Contaminant Level Goal (MCLG) for arsenic is zero. Any exposure carries some risk. The 10 ppb standard represents a balance between reducing risk and the cost of treatment for water systems.
Testing for Arsenic
AZDHS recommends testing your private well for arsenic at least every five years. Given Arizona's contamination rates, we'd recommend testing more frequently — every two to three years, and immediately when you purchase a property.
Arsenic testing typically costs $25-$75 at a certified lab. It's one of the cheapest tests you can get, and one of the most important in Arizona.
See our testing guide for AZDHS-certified labs and details.
Treatment
The good news: arsenic removal technology is effective and affordable at the household level.
| Treatment | Effectiveness | Notes |
|---|---|---|
| Point-of-use reverse osmosis (kitchen sink) | 90-95% removal | Most cost-effective first step. $300-$600 installed. Treats one tap for drinking and cooking. |
| Adsorptive media (iron-based) | 95%+ removal | Whole-house or point-of-use. Specialty media designed for arsenic. Higher upfront cost but lower maintenance than RO. |
| Whole-house reverse osmosis | 90-95% removal | $4,500-$20,000+. For high arsenic levels where all water contact is a concern. |
| Distillation | 95%+ removal | Effective but slow and energy-intensive. Not practical for whole-house use. |
Standard carbon filters, water softeners, and pitcher filters do NOT effectively remove arsenic. A Brita filter will not protect you. You need reverse osmosis or arsenic-specific adsorptive media. Don't assume your existing filter handles arsenic — check the manufacturer's specifications.
Arsenic Species Matter
Arsenic exists in two forms in groundwater: arsenite (As III) and arsenate (As V). Most treatment systems are more effective against arsenate. If your arsenic is primarily arsenite, pre-oxidation (converting arsenite to arsenate) may be needed for optimal removal. A qualified water treatment professional can advise based on your specific water chemistry.
Sources
- University of Arizona WRRC — Arsenic in Groundwater Poses Ongoing Challenge
- Jones & Vogt (2020) — Arsenic Concentrations in Ground and Surface Waters Across Arizona Including Native Lands
- USGS — Arsenic and Drinking Water
- USGS — Predicted Nitrate and Arsenic in Basin-Fill Aquifers of the Southwestern United States
- AZDHS — Test Your Well Water for Arsenic
- ADEQ — Groundwater Quality in Arizona: A 15-Year Overview
- University of Arizona Cooperative Extension — Arizona Drinking Water Well Contaminants (AZ1503)