If your home in Nampa, Caldwell, Sunnyslope, or anywhere else in Canyon or Ada County draws water from a private well, arsenic deserves a spot near the top of your testing list. It has no taste, no color, and no smell. You cannot tell it is there without a lab test. And roughly 15 percent of Treasure Valley private wells we encounter show arsenic levels that exceed the federal safety limit of 10 micrograms per liter (10 mcg/L).
That figure aligns with data compiled by Idaho DEQ from private well sampling across Ada and Canyon counties. In certain hot spots, especially in the Sunnyslope area of Canyon County and parts of Ada County near geothermal zones, exceedance rates climb higher. This article covers where the arsenic comes from, which neighborhoods carry the most risk, what health effects matter, and what actually works to treat it.
Why Arsenic Occurs Naturally in Treasure Valley Groundwater
Arsenic in Treasure Valley wells is not a pollution problem from farms or factories. It is geology. The Snake River Plain sits on ancient volcanic basalt laid down by the Yellowstone hotspot over millions of years. As groundwater percolates through this volcanic rock and associated hydrothermal deposits, it dissolves naturally occurring arsenic minerals, most commonly arsenopyrite and iron arsenates, and carries them into the aquifer.
Geothermal influence is a real factor here. The Boise area sits near active geothermal systems that push warm, mineral-rich water upward through fault zones. That same geothermal heating that gives us cheap district heat also elevates dissolved metals, including arsenic, in certain aquifer zones. The Sunnyslope area of Canyon County, a stretch of hillside between Caldwell and Eagle, is well-documented as a higher-risk zone for both arsenic and uranium because of its proximity to these fault-influenced groundwater pathways.
Gem County wells, particularly in areas north of Emmett, also show elevated arsenic in some surveys, for similar geologic reasons. The pattern follows the volcanic stratigraphy, not property lines.
Treasure Valley Arsenic Risk by Area
Not every well in the valley carries equal risk. Based on Idaho DEQ sampling data and our own testing experience across the region, here is a rough geographic picture:
- Sunnyslope (Canyon County): Highest documented baseline risk. Arsenic and uranium co-occur here frequently. Test first, before assuming your water is safe.
- Nampa and Caldwell (Canyon County): Moderate risk in older wells drawing from deeper basalt layers. Agricultural wells and domestic wells in the same aquifer zone can both show elevated levels.
- Ada County (Boise, Meridian, Eagle, Kuna): Variable. Some wells in the southwest Boise bench and near the Boise Front foothills test above the 10 mcg/L limit. Our article on uranium in Treasure Valley wells covers how arsenic and uranium often co-occur in Ada County at rates around 15 percent for arsenic exceedance overall.
- Star and Middleton: Generally lower risk from arsenic specifically, but nitrates from agriculture are a different concern in those areas.
- Emmett and Gem County fringe: Spot elevations documented; testing recommended for any well over 15 years old.
The takeaway: if your well is in Canyon or Ada County and has not been tested for arsenic in the past three to five years, it is time to test.
What the 2026 Drought and IDWR Moratorium Mean for Arsenic
Idaho declared a statewide drought emergency in spring 2026, with snowpack running at record lows not seen since 1896. In March 2026, the Idaho Department of Water Resources issued a five-year moratorium on new groundwater permit applications in southern Canyon County, freezing 21 pending applications that would have served roughly 7,000 acres. You can read more about the background in our article on Canyon County well water testing in 2026.
Here is why drought conditions matter for arsenic specifically. Aquifer recharge depends on snowmelt and precipitation filtering down through soil and rock. When recharge slows, groundwater levels drop. The same mass of dissolved minerals, including arsenic, is now concentrated in less water volume. The result is higher contaminant concentrations per liter, even if the underlying geology has not changed. We saw this dynamic play out during the 2021 drought cycle, and conditions in 2026 are more severe.
If you tested your well five years ago and it came back clean, that result may not reflect current conditions. Drought-stressed aquifers behave differently. A retest this year is not an overreaction; it is responsible ownership of a private water supply.
Health Effects of Arsenic Exposure
Arsenic is a Group 1 human carcinogen, meaning the evidence for cancer causation in humans is definitive, not theoretical. Long-term exposure to arsenic above 10 mcg/L in drinking water is associated with:
- Bladder cancer (the most consistently linked cancer in drinking water studies)
- Lung and skin cancers
- Kidney disease
- Cardiovascular effects including peripheral arterial disease
- Developmental effects in children exposed during pregnancy or early childhood
- Skin changes including hyperpigmentation and keratosis at higher exposure levels
The health effects are dose-dependent and cumulative over years. A household drinking water at 15 mcg/L for a decade carries meaningfully more risk than one at 8 mcg/L. The federal MCL of 10 mcg/L is set to balance cancer risk reduction against treatment feasibility for public water systems. For private wells, no regulatory enforcement exists. You are your own regulator.
Children and pregnant women face higher risk from arsenic exposure because developing tissues are more sensitive to carcinogens. If you have young children in your household and have not tested your well recently, that is the most pressing reason to act now.
How Arsenic Co-Occurs with Other Treasure Valley Contaminants
In our work across Canyon and Ada counties, arsenic rarely shows up alone. The same geologic and geothermal conditions that release arsenic into groundwater also mobilize uranium. As we covered in our uranium article, roughly 17 percent of sampled Treasure Valley domestic wells exceeded the uranium MCL of 30 mcg/L in USGS and state sampling programs, with Ada County wells reaching 37 percent exceedance in some surveys.
When a well tests positive for elevated arsenic, we always recommend a full contaminant panel before selecting a treatment system. The reason is practical: arsenic and uranium can require different treatment media, and selecting a system optimized for one without accounting for the other can result in an expensive second installation. A comprehensive test upfront costs far less than retrofitting treatment later.
Nitrates are a separate co-concern, especially in Canyon County agricultural areas near Caldwell and Wilder. Nitrates come from fertilizer and septic systems rather than geology, but they matter for the same household: total water quality, not just one contaminant.
Testing Your Well for Arsenic: What to Know
The EPA recommends that private well owners test their water at least annually for bacteria and nitrates, and every three to five years for a full panel that includes arsenic, uranium, heavy metals, pH, and hardness. Idaho DEQ makes the same recommendation for wells in geologically active areas, which includes most of Canyon and Ada County.
A few practical points on testing:
- Use a certified lab. DIY test strips are not reliable enough for arsenic at the 10 mcg/L threshold. Idaho DEQ maintains a list of state-certified laboratories for drinking water analysis.
- Collect properly. Let the tap run for two minutes before collecting a sample. Use the sample bottle provided by the lab; rinse with tap water only, no soap.
- Test at the tap you drink from. If you have a point-of-use filter, sample both before and after the filter to verify it is working.
- Time of year matters. Arsenic concentrations in some wells peak in late summer and fall when water tables are at their lowest. A summer test captures worst-case conditions.
We offer free water testing for Treasure Valley homeowners. Call us at (208) 968-2771 or fill out the contact form below and we will arrange a time to sample your water and provide a full written report.
Treatment Options That Actually Work
Once you have a confirmed arsenic result above 10 mcg/L, the question becomes which treatment system is right for your household. Here is an honest breakdown of what works and what does not.
Reverse Osmosis (Point of Use)
A certified RO system installed under the kitchen sink is the most accessible and cost-effective starting point for most families. Systems certified to NSF/ANSI Standard 58 remove 90 to 99 percent of dissolved arsenic. For a household where the primary concern is drinking and cooking water, this is usually the first recommendation. Installation runs $300 to $700 for the equipment plus labor, and annual filter replacement is straightforward. The limitation: RO only treats water at that one tap. Shower water and laundry water are not addressed, but for arsenic, ingestion is the primary exposure route, so point-of-use treatment covers the main risk pathway.
Adsorptive Media Whole-House Filters
For households with arsenic above 25 to 30 mcg/L, or where whole-house treatment is preferred, iron-based adsorptive media (such as granular ferric oxide or activated alumina) is effective at removing arsenic throughout the home. These systems treat every tap, not just the kitchen, and are well-suited to wells where arsenic co-occurs with iron because the iron chemistry supports arsenic adsorption. The tradeoff is higher upfront cost ($1,500 to $3,500 installed) and periodic media replacement every three to five years depending on water usage and arsenic load.
What Does Not Work
Water softeners do not remove arsenic. Softeners use ion exchange to swap calcium and magnesium for sodium, which has no effect on dissolved arsenate ions. Sediment filters, carbon block filters, and ultraviolet lights also do not address arsenic. If a salesperson suggests a softener will solve your arsenic problem, that is incorrect.
Choosing the Right System
The right treatment depends on your arsenic level, your water's pH, iron content, and competing ions, your household size and water use, and your budget. There is no universal answer. We test first, then recommend. If you call us, we will never sell you equipment before we have seen your water data.
Get Your Well Tested Free
Arsenic has no taste or smell. The only way to know is to test. We serve Boise, Meridian, Nampa, Caldwell, Eagle, Kuna, Star, and the surrounding Treasure Valley.