SOURCE TERM DEVELOPMENT FOR CONTAMINANT FATE AND TRANSPORT MODELING OF SELENIUM AND OTHER CONSTITUENTS OF CONCERN AT THE PROPOSED BLACKFOOT BRIDGE PHOSPHATE MINE, CARIBOU COUNTY, IDAHO

Phosphate mine permitting under National Environmental Policy Act (NEPA) in southeast Idaho has required increasingly complex source term analyses to support numerical models of groundwater flow and contaminant fate and transport. The analysis for the proposed Blackfoot Bridge Mine illustrates this trend and considers chemical loading from overburden and stockpiled ore stored in a variety of hydrologic and geochemical environments ranging from external dumps to pit backfills intersecting the water table. Constituents of concern (COCs) for the model were determined from column leaching tests and from regional studies of contaminant mobility in phosphate mine waste. The source term analysis for the Blackfoot Bridge model considers material balances for the ore stockpile and overburden disposal facilities and incorporates data from geochemical testing, numerical modeling of infiltration through cap and cover systems, and equilibrium-based modeling of geochemical reactions. Results from the source term analysis are used to compare the expected environmental impacts from each of the proposed project alternatives as part of the Environmental Impact Statement (EIS).