The Gallatin County Planning and Zoning Commission postponed approval of 265 housing units south of Big Sky on Thursday over concerns about how the development would impact the Gallatin River.

The proposed development is off U.S. Highway 191 between Lone Mountain Trail and Michener Creek Road.

It would have 130 apartments above first-floor retail spaces and 135 single-family homes clustered in groups with shared open space, garages and parking areas. The development would also include a hotel and trails connecting to the larger Big Sky trail system. The gravel pit now on the site would be redeveloped and about 110 acres of the 175-acre property would remain open space.

The developer, Big Sky Rock LLC, said the units, which would be built in stages over the next 10 to 15 years, would be affordable and provide much-needed workforce housing for year-round residents of Big Sky. Big Sky Rock LLC didn’t specify how it would ensure the units serve the target population.

The main sticking point for the project was the developer’s proposal to serve the entire development with several shared on-site septic systems.

The developer said each septic system would have a flow of less than 5,000 gallons per day, which wouldn’t require a Montana Department of Environmental Quality discharge permit. However, if all wastewater infrastructure were centralized, there would be a large enough daily flow that a discharge permit would be required. To obtain a discharge permit, developers must comply with requirements, including regular water monitoring.

Members of the Planning and Zoning Commission expressed skepticism that septic systems in such a dense development would best protect the nearby waterways. They postponed consideration of the project until they have more information about alternative ways to manage wastewater in the area.

Big Sky Rock LLC said it would provide the information the commission needs to make a decision and is working with the DEQ on the septic system design. The developer also said it would join a centralized system if one were created in the area and noted that it can’t move forward without DEQ approval.

The Gallatin Local Water Quality District submitted concerns about overloading and maintaining the septic systems and said they could pose a threat to the nearby Gallatin River and Michener Creek.

Steve Johnson, a member of the Gallatin Canyon/Big Sky Zoning Advisory Committee, which green-lighted the development before it reached the Gallatin County Planning and Zoning Commission, said he too is hesitant about the proposed septic system.

He said Big Sky residents are now looking at forming a water and sewer district that would create a centralized system with a wastewater treatment plant in the Gallatin Canyon area. As development continues, a district would be a more viable and environmentally friendly way to serve residents than the method of having septic systems for each property, he said.

“Our interest is in seeing something done the right way,” he said. “As has often been the case in a rapidly developing area like Big Sky, development is starting to get ahead of the infrastructure to support it.”

Guy Alsentzer, executive director of the Upper Missouri Waterkeeper, voiced concerns that the proposed septic system could pollute the Gallatin River. He urged the Planning and Zoning Commission to think long-term about the decisions necessary to protect waterways as development continues in Big Sky.

Perrin Stein can be reached at 406-582-2648 or at pstein@dailychronicle.com. Follow her on Twitter @PerrinStein.

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