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BOISE, Idaho (AP) — A federal judge has ordered the U.S. Forest Service to revisit some of the decisions made when officials signed off on a mining company's plans to broaden its exploratory drilling in a historic mining district in the mountains near Idaho City.

U.S. District Judge Edward Lodge on Thursday vacated sections of the environmental assessment the agency produced on the CuMo Mine project, saying the agency acted arbitrarily a year ago in concluding the expanded drilling does not significantly impact water quality.

Lodge also ordered the Forest Service to step back and do additional research and analysis on the impacts drilling could have on groundwater; develop a strategy for monitoring water quality before, during and after drilling; and craft a plan to treat any waters contaminated during drilling.

In August 2011, the Forest Service gave Mosquito Consolidated Gold Mines permission for exploratory drilling that would develop a more precise picture of the area's molybdenum, copper, tungsten and silver deposits.

"The very nature of drilling holes 1,500 to 3,000 feet into the ground seems likely to impact the underlying surface including the groundwater," Lodge wrote in his 45-page opinion. "The appropriate course would be for the Forest Service to have conducted some baseline study and analysis of the groundwater in the area in order to reach the finding of no significant impact."

While it's unclear how the Forest Service will respond, environmentalists say the decision deals a temporary setback to the Canadian company's immediate exploration plan. Boise National Forest officials did not immediately respond to telephone messages left Thursday by The Associated Press.

Mosquito Gold said in a statement that it will cooperate with the Forest Service to ensure the missing information is provided in a timely manner.

"The court agreed that the project's drill-and-seal practices protect groundwater quality but concluded there simply was not enough information about effects of the drilling itself on the geologic structure that may contain groundwater," President and CEO Brian McClay said in the statement.

In his ruling, Lodge leaves it up to agency officials to determine if an amended environmental assessment is needed or a more exhaustive environmental impact statement is appropriate.

Vancouver-based Mosquito Gold wants to construct more than 10 miles of temporary roads, build 137 drill pads and drill 259 holes in the exploratory phase of the project 35 miles north of Boise. Company officials believe the area holds the world's largest deposits of molybdenum, used in the manufacturing of steel products, and on its website, the CuMo project is described as the company's flagship project.

A coalition of environmental groups led by the Idaho Conservation League sued the Forest Service last year, claiming the agency and the company were overlooking the impact drilling could have on water quality in a drainage where streams and creeks ultimately flow into the Boise River system.

John Robison, the conservation league's public lands director, said state studies found soils in the historic mining district to be contaminated with arsenic and lead. Although the state report concludes the current risk to the public is low based on current land use, Robison argued a drilling process that uses pressurized water and other materials could introduce toxins and heavy metals into the groundwater and the area's highly interconnected underground hydrological network.

"The big winner here is the Boise River and a water system that is vital to Idahoans, irrigators, recreationists and fish and wildlife," Robison said. "The judge essentially found that the mining company and the Forest Service couldn't just cross its fingers and hope that nothing bad happens."