HYDROGEOLOGIC OVERVIEW OF PHOSPHATE MINING IN IDAHO INCLUDING THE SELENIUM PROBLEM

This talk provides an overview of ground-water flow systems at phosphate mines in Idaho and problems of dissolved selenium found in ground water and area streams. The structural setting of the marine sedimentary rock that include the phosphate ore is complex. Thrusting and folding have produced long, northwest trending folds with numerous tear faults. Open pit mines have been constructed along outcrops of the Phosphoria Formation. This formation has signficantly lower hydraulic conductivity than overlying and underlying formations and tends to separate upper ground-water flow systems from lower, more regional flow systems. Each mine site has a unique structural setting and thus unique ground-water flow systems. Isolated livestock deaths were discovered in 1996 associated with excessive selenium uptake near historic phosphate mining areas. The primary selenium source appears to be infiltration of runoff through external waste rock piles. The highest metal concentrations are observed during spring runoff when piles are initially flushed. Concentrations generally decrease in flowing streams through the summer months. Questions have been raised relative to the role that intermediate and regional ground-water flow systems play in the stream selenium problem since ground-water contributions to the streams from these systems would have the greatest impacts during low flow.