Missoula, Montana sits in a valley where five mountain ranges converge — and where one of the country’s most significant environmental cleanup stories has played out over the past four decades. The city’s roughly 75,000 residents drink from a groundwater aquifer fed by the Rattlesnake Creek watershed, and for the most part, that water is clean. But the legacy of hard-rock mining upstream along the Clark Fork River has shaped everything about how Missoula thinks about water.
The Milltown Dam Superfund Story
For over a century, mining operations in Butte and Anaconda sent arsenic, copper, zinc, and other heavy metals downstream along the Clark Fork River. Much of that contaminated sediment settled behind the Milltown Dam, built in 1907 at the confluence of the Clark Fork and Blackfoot Rivers — just six miles upstream from Missoula.
The EPA added the Milltown Reservoir Sediments site to the National Priorities List in 1983 after arsenic was detected in private wells near the dam at levels exceeding federal drinking water standards. The contaminated sediment behind the dam contained an estimated 6.6 million cubic yards of material laced with arsenic, copper, zinc, cadmium, and lead — the downstream remnants of decades of copper smelting and mining waste from the Butte-Anaconda corridor.
In 2008, the Army Corps of Engineers began removing the dam itself — one of the first times in U.S. history that a major dam was dismantled specifically for environmental remediation. The $100+ million cleanup involved removing contaminated sediments, restoring the river channel, and reconnecting the Clark Fork and Blackfoot Rivers for the first time in over a century. The dam removal was completed by 2010, and the broader site remediation continued through the mid-2010s.
Where Missoula’s Drinking Water Actually Comes From
Here’s the good news: Missoula’s municipal water supply doesn’t come from the Clark Fork River. The city draws from a deep gravel aquifer recharged primarily by Rattlesnake Creek and snowmelt from surrounding mountains. Mountain Water Company (now owned by the City of Missoula after a lengthy legal battle that concluded around 2017) operates the system, which serves about 70,000 people through a network of wells.
The aquifer produces naturally high-quality water. Missoula is one of the few cities in the western U.S. that doesn’t need to filter its surface water supply — because it’s groundwater, not surface water. The city disinfects with chlorine as a precaution, but the water generally meets or exceeds federal standards straight out of the ground.
That said, the proximity of the Milltown Superfund site to the city’s aquifer recharge zones was a genuine concern for years. Monitoring data from the EPA and Montana DEQ has consistently shown that the contaminated sediments at Milltown did not significantly impact Missoula’s municipal wells, though private wells closer to the dam site did show elevated arsenic levels during the contamination period.
PFAS: The Emerging Concern
Like communities across the country, Missoula is now grappling with per- and polyfluoroalkyl substances (PFAS). The Montana Department of Environmental Quality has been conducting PFAS sampling at various locations statewide, including near airports and military facilities where aqueous film-forming foam (AFFF) was historically used for firefighting training.
Missoula International Airport is one location where PFAS contamination has been investigated. The extent of contamination and its impact on nearby groundwater sources is still being assessed [NEEDS VERIFICATION]. The EPA’s 2024 PFAS drinking water standards set maximum contaminant levels at 4 parts per trillion for PFOA and PFOS individually — levels so low that many water systems across the country are scrambling to test and comply.
What the Data Shows
According to Missoula’s most recent Consumer Confidence Reports, the municipal water supply consistently meets EPA standards for regulated contaminants. The water is moderately hard (typical for mountain aquifer systems) and low in disinfection byproducts since it requires minimal treatment.
The Clark Fork River itself, while dramatically improved since the Milltown Dam removal, still carries the legacy of upstream mining. Copper and arsenic levels in the river have dropped significantly, but the Upper Clark Fork remains one of the largest Superfund complexes in the nation — stretching roughly 120 miles from Butte to Milltown.
For residents on private wells, particularly those near the former Milltown Dam site or along the Clark Fork floodplain, the calculus is different. Well water in these areas should be tested regularly for arsenic, copper, and other metals associated with the mining legacy.
What Missoula Residents Can Do
If you’re on municipal water: You’re drinking from one of Montana’s better aquifer systems. Request a copy of your annual water quality report from the city utility for specific contaminant levels. The water is generally excellent.
If you’re on a private well: Get your water tested — especially for arsenic, copper, zinc, and lead. Wells near the Clark Fork floodplain or the former Milltown area have historically shown elevated metals. The Missoula City-County Health Department can point you toward certified testing labs.
For everyone: Keep an eye on PFAS developments. As Montana DEQ continues its statewide PFAS investigation and the EPA’s new standards take effect, there may be new information about Missoula-area contamination sources.
Water Treatment Options
For arsenic specifically — the contaminant most closely associated with Missoula’s mining legacy — point-of-use reverse osmosis systems are highly effective, typically removing 95%+ of arsenic from drinking water. Whole-house systems using adsorptive media (iron-based filters) are another proven approach for arsenic removal.
For PFAS, activated carbon filtration and ion exchange systems are the most effective residential treatment technologies currently available. If future testing reveals PFAS in Missoula’s water supply, these systems would be the go-to solutions.
If you’re concerned about your water quality, a certified water treatment professional can test your water and advise on solutions.