What’s in Treasure Valley tap water?

Utility-reported hardness ranges from soft in Mountain Home (33–55 mg/L as CaCO3) to 13–219 ppm across Veolia’s Boise system (average 105 ppm in 2025; the report doesn’t state the CaCO3 basis). Eagle describes its city water as “moderately hard”. Nampa, Star, Middleton and Emmett don’t publish a number in the reports we reviewed.

The best regional study is older: IDWR sampling from 1991–1994 found median hardness of 189 mg/L in the shallow Treasure Valley aquifer and 114 mg/L in the deep aquifer. 69% of shallow sites were hard or very hard, while 66% of deep sites were soft or moderately hard. Hardness dropped with well depth and was softer near the Boise River and southwest of Boise.

A 2019 Boise State University / IDWR study found uranium above the 30 µg/L federal limit is present and widespread across the Treasure Valley aquifer system, with higher potential in shallow, oxygen-rich zones. In its compiled dataset, 37% of Ada County domestic wells (54 wells) and 15.5% of Canyon County domestic wells (58 wells) exceeded the limit. Those are raw well samples, not treated tap water, and not a random sample.

The same study notes that fewer than 2% of Ada County domestic wells and about 1% in Canyon County are monitored for uranium by state agencies, and recommends that well owners test regularly. City systems report uranium in their Consumer Confidence Reports; Boise’s 2025 compliance result was 21.5 µg/L and Eagle’s Eastern zone peaked at 22 µg/L, both under the limit.

Arsenic occurs naturally in Treasure Valley groundwater. IDWR’s 1991–1994 sampling listed arsenic among the constituents found above federal limits in some wells. City systems in our guides report values under the 10 ppb limit, from about 1 ppb (Middleton, EWG average) to an 8.6 ppb compliance result in Boise (2025). Statewide, IDWR’s 2021 sampling found 13% of sampled wells above the arsenic limit.

Idaho DEQ calls nitrate the most common and widespread contaminant in Idaho groundwater, which supplies more than 95% of the state’s drinking water. IDWR’s 1991–1994 sampling found nitrate at or above 2.0 mg/L (its threshold for human impact) in 69% of shallow and 32% of deep Treasure Valley sites. In Canyon County, IDWR data through 2003 showed a median of 4.8 mg/L in shallow wells (maximum 29.8) versus 0.6 mg/L in deeper wells; the federal limit is 10 mg/L.

DEQ designates the Northeast Star Nitrate Priority Area as degraded groundwater and offered private well owners there free sampling in September–October 2025.

Chlorine keeps city water safe in the pipes, and it can form trihalomethanes (TTHMs) and haloacetic acids (HAA5). Boise has the highest levels in our guides (2025: TTHMs 45.1 ppb, HAA5 38.5 ppb, against limits of 80 and 60). Well-only systems such as Star, Nampa, Emmett and Middleton show low or non-detected levels in their reports and EWG averages. Whole-home carbon filtration reduces chlorine taste and many byproducts.

Idaho DEQ says PFAS is not currently regulated in Idaho and public water systems are not required by the state to monitor for it, and that Idaho has not seen widespread detections. Department of Defense sampling found PFAS at Mountain Home Air Force Base and Gowen Field in Boise. Veolia’s 2024 testing in Boise found PFOA and PFOS single results below 4 ppt, and Middleton’s 2023 report lists PFAS as not detected.