HYDROGEOLOGY OF A MINE PLAN PROPOSAL AT SMOKY CANYON MINE, SE IDAHO

Before making decisions or taking actions related to management of federal phosphate leases, the National Environmental Policy Act (NEPA) requires the Bureau of Land Management (BLM) to disclose and consider potential impacts to the human environment that may result from those actions.

In 2003 the J.R. Simplot Company (Simplot) proposed an expansion of their existing Smoky Canyon phosphate mine, Caribou County Idaho. An understanding of the hydrogeologic character of the mine site played an important role in the impact analysis. Historically, the mining of phosphate in SE Idaho has led to the leaching and release of contaminants into surface and groundwater adjacent to mine sites. Selenium is the key contaminant.

The mine area is located on the east flank of the Webster Range, and on the east limb of Webster Syncline. The stratigraphy generally dips moderately to the west. Majority of recharge takes place west of the mine in the higher topographic areas. The groundwater flow in the mine vicinity is generally from west to east. Structurally, the mine is located in the Idaho-Wyoming Fold and Thrust Belt and is underlain by regional thrust faults. Generally, the Phosphoria Fm acts as an aquaclude. The Dinwoody Fm., above the Phosphoria Fm., contains smaller areas of isolated flow. The massive carbonates and sandstones of the Wells Fm., below the Phosphoria Fm., host the local and regional aquifer. Groundwater flow is fracture and bedding plane controlled. Portions of the groundwater flow are intercepted by thrust faults and directed northward. Groundwater flow from beneath the mine is discharged from several springs and creeks. Much of the discharge is less than 50 years old.

Initial fate and transport analysis of Simplot’s proposed mine plan indicated exceedence of the selenium standards in both groundwater and surface water. In response, Simplot submitted an alternative reclamation design. This alternative design included an engineered “Store and Release” cover system designed to limit percolation of meteoric water into the seleniferous waste rock and subsequently limit the amount of leachate released. Analysis with the cover system in place indicated compliance with the applicable water quality standards. Based in part on this analysis and analysis of other impacts, a fully mitigated mine plan was approved by BLM in 2008.