Integration of Surface Water Management with Mitigation of Ground Water Impacts at a Proposed Phosphate Mine Overburden Facility

Environmental impact evaluation of proposed phosphate mine overburden fills at the J.R. Simplot Smoky Canyon Mine, in Southeastern Idaho indicated that leaching of the overburden by infiltrating precipitation could potentially contaminate local ground water with selenium. The Idaho Department of Environmental Quality (IDEQ) required that ground water impacts be reasonably localized to the immediate mine vicinity. A number of best management practices (BMPs) were incorporated into the design of the overburden fills to reduce potential surface water and ground water impacts, but ground water quality impacts were still predicted to extend off site in the long term (100 years). A number of alternatives were then evaluated to reduce the infiltration of precipitation into the overburden through use of low permeability covers. These were rejected for cost and engineering feasibility reasons. Use of modified infiltration trench technology for storm water management at the periphery of the proposed overburden fills was then evaluated for recharge of runoff into the local ground water. Both the State and the EPA already approve this technology as a storm water management BMP. Ground water impact modeling showed that incorporation of runoff recharge areas into the margins of the overburden fills would be effective in containing the area of ground water impacts to the immediate mine vicinity. This design was eventually approved by the IDEQ, EPA, and the Federal land management agencies and has potential applicability in similar hydrogeologic situations at other mining operations