HYDROLOGIC AND CHEMICAL INTERACTION BETWEEN WASTE DUMPS AND A FLOODED MINE POOL AT THE COMBINATION MINE, PHILIPSBURG DISTRICT, MONTANA

The Combination Mine, located approximately 12 miles northwest of Philipsburg Montana, was intermittently mined for silver, gold, and copper from 1885 until the 1990s. After the Combination Mine was formally abandoned groundwater seeps downgradient of the mine and waste rock pile were found to be above the regulatory water quality standards. Since the groundwater seeps were downgradient of both the mine pool and waste rock pile the source of the groundwater seeps could have been from the mine pool or shallow flow paths through the waste rock pile. In order to properly plan and execute a remedial action on the groundwater seeps it was important to determine the source of their recharge. This study investigated the source of recharge to several groundwater seeps in the Combination Mine vicinity. Stable water isotopes (d2H, d18O) were used to identify potential pathways of recharge to the groundwater seeps. Temporal changes in geochemistry were monitored to identify dilution and differences in hydraulic residence time. Sulfate isotope (d34S and d18O ) samples were collected and analyzed to determine if different recharge sources could be identifed. Benchtop experiments were done in which 2 kg samples of mine waste were interacted with water for a week, with the leachate samples analyzed for pH, metal concentration, and d18O and d34S of dissolved sulfate. Through the use of these methods it was determined that for the largest groundwater discharge is likely influenced by the mine pool. However other small seeps appeared to be less likely to be influenced by the mine pool water