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Elevated groundwater nitrate suggests new pollution source in West Fork

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By Stephanie Lynn Gallatin River Task Force

Elevated levels of nitrate, a nutrient that can impact river health and drinking water quality, have been found in a groundwater monitoring well above the Big Sky Resort Golf Course, according to a collaborative groundwater study in the Big Sky meadow area.

The study, which took place in June and August this year, sampled six existing monitoring wells and one spring in Big Sky. This data collection resulted from a collaboration between the Montana Bureau of Mines and Geology, Big Sky Water and Sewer District, Gallatin Local Water Quality District, and Gallatin River Task Force.

A well that is located between Two Moons Road and the Town Center stoplight revealed the highest groundwater nitrate levels of 6 to 7 mg/L. Similar nitrate concentrations were measured across Highway 64 in a well near Roxy’s Market a few years earlier.

“We did not expect to see nitrate in the 6 to 7 mg/L range in the Two Moons well,” said Christine Miller, a water quality specialist with the Gallatin Local Water Quality District. “Although the U.S. Environmental Protection Agency Maximum Contaminant Level for nitrate in drinking water is 10 mg/L, 2 mg/L is usually considered background for groundwater. Levels above 6 mg/L, like we saw this season, indicate there are one or more nutrient sources negatively impacting groundwater quality.”

Groundwater can be influenced by human activities on the surrounding landscape when rain and snow carrying pollutants soak into the ground. Human-caused sources of nitrogen include wastewater from septic or sewer systems, household uses such as fertilizer and pet waste, and agriculture.

Once underground, water is directed “down gradient” by elevation, pressure, and impermeable layers of rock. Researchers must consider human activity and the direction of groundwater flow in tandem to determine the cause of contamination.

“The Two Moons well is above the Big Sky golf course and at this location groundwater flows from the southwest to the northeast,” said Mike Richter, a research specialist with the Montana Bureau of Mines and Geology and Task Force board member. “This flow direction suggests a source of nutrients entering the Meadow Village aquifer up gradient from the golf course and southwest of the Town Center.”

Groundwater from the Two Moons well eventually flows to the West Fork of the Gallatin River, a stream already impacted by nitrogen pollution. Elevated nitrogen affects river health when it triggers algae blooms, such as the one observed this summer, which alter aquatic habitat and reduce the high oxygen levels required by aquatic insects and trout.

Regular groundwater sampling by the collaborative group will continue in order to pinpoint this source, and other potential sources, of nitrate to safeguard the community’s drinking water supply and protect local fisheries.

“Long-term water level and water chemistry monitoring will let us know if there are changes over time,” said James Rose, a hydrogeologist with Montana Bureau of Mines and Geology and an associate Professor at Montana Tech. “Concerns about climate change, development, and drought can be evaluated and any impacts can be determined.”

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