City of Missoula looks to address stormwater runoff, protect aquifer

Martin Kidston

(Missoula Current) The remnants of Glacial Lake Missoula sit 30 feet below the valley floor and rest several hundred feet deep. Even now, that hidden lake remains Missoula's only source of clean drinking water.

Given the aquifer's importance, the City of Missoula hired Morrison-Maierle in 2023 to develop a new stormwater quality plan – itself an update on a plan created in 2018. The new plan looks to implement actions to protect both surface and groundwater quality, and it comes with a broad range of recommendations.

“It's our job to manage the quantity, quality and the routing of stormwater runoff,” said Tracy Campbell, the city's stormwater superintendent. “We know the community values our clean water resources, and we're committed to protecting them.”

The plan covers the city's drainage characteristics, including its creeks, runoff basins and development patterns, which often extend nonpermeable surfaces. The resulting runoff is not treated at the city's Wasterwater Treatment Plant but instead, it's discharged into area rivers via outfalls or injected into the ground through dry wells.

The resulting pollution remains the leading cause of water quality impairment in Montana and across the U.S.

“It's basically rainwater and snowmelt off our impermeable surfaces, things like roofs, roads and sidewalks,” said Campbell. “It picks up pollutants like oil, chemicals and debris, and deposits them in our rivers and streams.”

The plan evaluated 30 unique drainages that discharge to high-priority outfalls. The outfalls, which drain large urban areas, were ranked based on their ability to contribute to water pollution.

Campbell said eight downtown drainage areas include high-priority outfalls that place runoff into the Clark Fork River. The Bitterroot outfall, which includes Pattee Creek and most of the South Hills, was identified as the largest drainage basin in the valley.

While outfalls feed runoff and all that it contains directly into the rivers, drywells also inject runoff into the ground. They work as a sump, allowing stormwater to filtrate to the aquifer.

“Most of the valley has a high infiltration rate, sometimes in the hundreds of inches per hour, and these facilities manage a large volume of runoff,” said Campbell. “But the high infiltration rate also indicates that we're rapidly directing stormwater directly into the aquifer with no water quality treatment.”

The plan identified roughly 140 dry wells needing replacement, but the city doesn't have the funding capacity to address them all. Rather, the plan identified the top 10 dry wells needing replaced and is seeking funding to carry out the work.

Campbell said advances in stormwater runoff could also be applied, including green infrastructure like bioswales and retention ponds. Both use soils to help bind and neutralize pollutants.

“Instead of discharging runoff directly to the aquifer and river, it can be filtered as it flows through vegetation and soil,” Campbell said. “Most people now recognize that water pollution can have a cumulative impact on human health and the environment.”

The Missoula aquifer was identified by the EPA in 1988 as a sole-source aquifer, meaning it's the city's only source of clean water. The Clark Fork River accounts for roughly 77% of the aquifer's recharge, making the aquifer somewhat resilient.

Still, Campbell said dilution is no longer seen as the solution to pollution.

“Dry wells risk contamination due to accidental releases, as well as chronic discharge of untreated stormwater underground,” Campbell said.

She added that Spokane has an aquifer similar to Missoula. But while Washington has an “underground injection control program,” Montana does not.

“In Washington, all dry wells surrounded by pavement are considered an endangerment to the drinking water source,” said Campbell. “They require pre-treatment with bioswales or other vegetative facilities. Based on Washington's guidance for dry wells, we could develop a similar framework for requiring pre-treatment in Missoula.”