Arsenic in Yavapai County well water

Arsenic here is geological, not industrial. It comes out of the rock, it has no taste or smell, and the only way to know what your well is doing is to send a sample to a laboratory.

The limit, and who it binds

EPA's maximum contaminant level for arsenic is 0.010 mg/L, effective 23 January 2006. That is the same number as 10 µg/L and the same number as 10 ppb.

It is an enforceable standard for public water systems. It is not enforceable anywhere else. ADEQ does “not regulate the water quality of private wells,” and the University of Arizona's AZ1486f is blunter: “No one else will test your water for you.”

The World Health Organization uses the same 10 µg/L figure but calls it provisional, “because of practical difficulties in removing arsenic from drinking-water.” That is worth holding onto, because it tells you something the marketing will not: removal is genuinely hard.

What has actually been measured here

These are published figures from state and federal work, not estimates.

• Paulden — 2,900 ppb. Arizona Cooperative Extension's AZ1503 states that “the highest concentration of arsenic in groundwater in Arizona was found near Paulden in the Verde Valley, with a concentration of 2,900 parts per billion in a private, domestic well.” That is 290 times the limit, in this county.
• Cornville — every well sampled. A 2004 Arizona Department of Health Services health consultation sampled six private wells. All six exceeded the MCL. “The analyzes detected arsenic ranging from 15 to 952 µg/L.”
• Walker and Lynx Creek — the old mining ground. ADHS sampled 30 private wells near Prescott across 2000 and 2001 and found arsenic from 0 to 0.058 mg/L, alongside cadmium and sulfate exceedances. The report notes the Walker Mining District was established in the 1860s.
• City of Prescott — below the limit. The city's 2024 report shows arsenic detected at 1.8 to 8.0 ppb against the 10 ppb MCL. Chino Valley's 2024 report shows a high of 7.7 ppb. On a public system, somebody is checking.

A 2026 Arizona State University sampling of 47 Yavapai County private wells reportedly found 11 percent with arsenic above EPA limits, according to the Verde Independent. That is a newspaper account of a public presentation rather than a published paper, so treat it as an indication of scale rather than a hard rate.

Why it is in the rock

The University of Arizona Water Resources Research Center explains the mechanism: arsenic “is associated with a variety of mineralized granitic and volcanic rocks and sedimentary rocks derived from those mineralized zones,” and contamination of aquifers “often occurs naturally.” Yavapai County is made of exactly that. Two wells a quarter mile apart can pull from different units and return different answers.

There is also a mining legacy here. The Iron King Mine–Humboldt Smelter Superfund site in Dewey-Humboldt holds about four million cubic yards of tailings with high levels of arsenic and lead. That is a soil contamination site, not a documented groundwater plume, and it is worth being precise about the difference. But it is a reason that a well in an old mining district justifies a wider panel than a well in open country.

What long-term exposure does

Stated plainly, from the agencies. WHO: “Skin lesions and skin cancer are the most characteristic effects,” and “long-term exposure to arsenic may also cause cancers of the bladder and lungs.” EPA adds kidney, nasal passages, liver and prostate, plus non-cancer cardiovascular, pulmonary, immunological, neurological and endocrine effects.

This is chronic-exposure science, measured over years of drinking. It is not an emergency. It is a reason to know your number.

What removes arsenic, and what does not

Arizona Cooperative Extension's AZ1650 (Artiola and Wilkinson, 2015) is the document to read before buying anything.

Works

• Reverse osmosis — up to 95 percent removal, but “efficiencies can vary dramatically.” In a study of five Arizona homes, one home's RO reduced arsenic by 85 percent and the result was still above 10 ppb. That single sentence is the most useful thing in the publication.
• Distillation — can produce water “free of salts, metals, and other constituents, including removal of arsenic to a level much below” 10 ppb.
• Adsorptive iron media — iron filters designed to adsorb arsenic, with effectiveness depending on your specific water chemistry.

Does not work

• Water softeners. AZ1650: softeners “remove calcium and other minerals from hard water but do not remove arsenic from water.”
• Carbon filters, pitcher filters, particle filters, chlorination, boiling. All listed as ineffective. Boiling concentrates it.
• Activated alumina, usually, here. AZ1650 notes it is “usually not effective to remove arsenic from water with pH above 7, common in Arizona groundwater.” Which is why your pH result matters when someone quotes you a system.

Most households treat at the tap rather than the whole house. AZ1650: “Point-of-use treatment can often treat enough water needed for drinking and cooking, while keeping installation and maintenance costs lower.”

Then test it again

AZ1650 is direct about this: “test your treated well water for arsenic every year using a state-certified laboratory.” Given that one of the five Arizona homes in their own study had a functioning RO system that still failed to get under the limit, an untested treatment system is a belief, not a result. Arizona's licensed laboratories are listed by the Arizona Department of Health Services.

Common questions

Is arsenic in Yavapai County well water common?

It is common enough to be worth testing for. Arizona Cooperative Extension records the highest arsenic concentration ever found in Arizona groundwater near Paulden, at 2,900 parts per billion in a private domestic well. An Arizona Department of Health Services consultation in Cornville found all six sampled private wells above the 10 ppb limit, ranging from 15 to 952 micrograms per liter. Because arsenic is geological, results vary sharply well to well, so a neighbor's number does not predict yours.

Can I taste or smell arsenic in my water?

No. Arsenic is a naturally occurring element dissolved in groundwater, and it gives water no taste, odor or color. There is also no handheld device that measures it at the relevant concentration. EPA Method 200.8 uses inductively coupled plasma mass spectrometry, which is laboratory equipment. A certified laboratory test is the only way to find out.

Does a water softener remove arsenic?

No. Arizona Cooperative Extension publication AZ1650 states that water softeners remove calcium and other minerals from hard water but do not remove arsenic. Carbon and pitcher filters, chlorination and boiling do not remove it either. Reverse osmosis, distillation and adsorptive iron media do, though reverse osmosis performance varies enough that the treated water should be retested annually by a certified laboratory.