What contaminants are common in Arizona well water?

Nobody, unless you do. The University of Arizona's well owner's guide says water from domestic wells serving fewer than 15 connections or 25 people "is not subject to EPA drinking water regulations, and undergoes no governmental quality tests for potability in Arizona." Wells with a pump that moves less than 35 gallons per minute and serve a household are private domestic wells. The guide says they are "not required to monitor water quality."

That is why the guide tells owners "to test their water quality against those standards required for community water systems." A city utility has to prove its water meets the federal limits. A well owner is the only one who can find out.

The guide also gives a sense of what testing has found. A nationwide U.S. Geological Survey survey it cites found that about 79% of the wells tested contained one or more contaminants that may be harmful to human health, and 9% to 11% had arsenic and nitrate levels above the EPA limits. A smaller study of wells in seven Arizona counties found that about 90% exceeded at least one contaminant standard, such as nitrates, arsenic or coliform bacteria. The guide describes that Arizona study as covering a small number of wells, so read it as a warning, not a statewide rate.

The guide puts it this way: "In addition to elevated total dissolved solids, the most common constituents found in Arizona groundwater in concentrations above drinking water standards are arsenic, fluoride, gross alpha radiation, and nitrate." Here is each one.

Arsenic. Arsenic occurs naturally in Arizona rock. The guide names three geologic sources, including mineralized veins of ore and some granite bedrock. The EPA limit is 0.010 mg/L, which is 10 parts per billion. The EPA's table of drinking water regulations lists the health effects as "skin damage or problems with circulatory systems, and may have increased risk of getting cancer." The guide warns that changes in an aquifer's chemistry can raise or lower arsenic levels. It gives an example from the Verde Valley, where oxygen entering the aquifer as groundwater levels fell during drought increased arsenic concentrations. Our page on arsenic in Arizona drinking water covers the health limit and municipal water.

Fluoride. Fluoride is a common mineral in volcanic rock. The guide says the highest levels are found in Cochise County, in Mohave, Graham and Greenlee counties, and along the lower Gila River in Yuma County. The limit is 4.0 mg/L. Fluoride at moderate levels supports strong teeth and bones, but the guide notes that "excessive concentrations in drinking water results in tooth mottling and discoloration." A lower guideline of 2.0 mg/L is a secondary standard, set for cosmetic effects.

Radioactive elements. The guide says the most common source of radioactivity in Arizona is dissolved uranium and dissolved radon gas. Gross alpha is a measure of overall radioactivity in water from the decay of uranium, radium or radon. The guide calls it a common naturally occurring contaminant in Arizona bedrock aquifers and in aquifers made of eroded granite. It lists the limit for gross alpha as 15 pCi/L and the uranium limit as 0.030 mg/L. Radon is a colorless, odorless, tasteless gas that dissolves in groundwater, so you cannot detect it yourself.

Nitrate. Nitrate can occur naturally. The guide says it is usually due to "agricultural practices (excessive fertilizer use and/or poor irrigation practices), or failing septic systems that allow contaminated waters to drain into the aquifer." The EPA's regulation table gives the nitrate limit as 10 mg/L, measured as nitrogen. It warns that infants under six months old who drink water with nitrate above that limit could become seriously ill.

Other local metals. The guide also names hexavalent chromium, which it says is known to cause cancer. It has been found in Paradise Valley north of Phoenix and in the Detrital Valley near Kingman. It reports selenium and boron near Kingman and lithium in the Gila Valley near Safford. These show up only in certain geology, which is why a first test should look for more than the basics.

Salts and hardness. Basin and Range aquifers can hold salts that were left behind when ancient lakes dried up. The guide says high total dissolved solids and elevated boron, sodium chloride and gypsum are often found in the deeper basin aquifers. These mainly affect taste and equipment, and our guide on how hard water damages plumbing explains the plumbing side.

Chemistry is not the whole story. The University of Arizona guide reports that in the small seven-county study, 43% of domestic wells sampled were contaminated with waterborne pathogens. The EPA says flooding and other disasters can let contaminants enter a private well, and it advises testing after flooding or land disturbance near the well. The EPA's guidance links each warning sign to a test. Recurring stomach illness points to coliform bacteria. Nearby intensive agriculture points to nitrate, nitrite, pesticides and coliform bacteria. The smell of gasoline or fuel oil points to volatile organic compounds.

The EPA also lists nearby landfills, junkyards, factories, gas stations and dry-cleaning operations as reasons to test for volatile organic compounds, dissolved solids, metals and pH.

The guide says a septic system that fails can be tied to nitrate and also to ammonium and phosphorus, which it says are much less common in Arizona aquifers. If your septic system and well are close together, both deserve attention. Our guide on how often to pump a septic tank covers the septic side.

The EPA says: "Test your private well annually for total coliform bacteria, nitrates, total dissolved solids, and pH levels." Test right away in these cases: known groundwater problems in your area, changes near the well such as flooding or new construction, any repair to the well system, or a change in the water's odor, color or taste. It adds that you may want to test more often if small children, older adults, or someone pregnant or nursing lives in the house.

The University of Arizona guide is more specific to Arizona. It lists these initial tests: hardness, sodium, chloride, fluoride, nitrate, sulfate, radionuclides, iron, manganese, arsenic, mercury and lead. In some locations it adds selenium and hexavalent chromium. Then the annual minimum is total coliform bacteria, total dissolved solids, pH and nitrate. It says annual testing of the naturally occurring chemicals "may not be needed," since their levels usually do not change over time.

Use a certified lab. The EPA says to "only use laboratories that are certified to do drinking water testing." The University of Arizona guide says the Arizona Department of Health Services can provide a list of state-certified water testing laboratories. Follow the lab's sampling instructions exactly, because a bad sample gives a bad result.

Compare the results to the limits. The guide says: "If well water exceeds the MCL for any listed contaminant, your water supply may be a health risk. You should treat your water to avoid the health risk or find an alternative supply." Aesthetic limits, such as iron, manganese and total dissolved solids, are guidelines for taste and staining, not health.

Until you have results you trust, don't guess at a fix. The right treatment depends on what the lab finds, and no single filter removes everything. Our guides on whether reverse osmosis is worth it, reverse osmosis versus a water softener and how to tell you need water treatment explain what each option does. The University of Arizona guide adds one caution. A water softener can change the makeup of the salts and may lower arsenic and other trace chemicals. So the guide says to test at the well head, after any softener, and at the tap.

If your home runs on a well, our guides on how a home well system works in Arizona, why a well pump short cycles and how to choose a well pump cover the equipment side.