COMPARISON OF MICROBIAL DIVERSITY AND GROUNDWATER GEOCHEMISTRY IN TWO BACKFILLED PHOSPHATE MINE PANELS, S.E. IDAHO

Microbial abundance and diversity were compared with groundwater quality data (including O₂, total dissolved selenium (Se), iron (Fe), manganese (Mn), selenate (SeO₄^2‑), selenite (SeO₃^2‑), nitrate (NO₃^‑), sulfate (SO₄^2-), pH, and Eh) to evaluate biogeochemical factors influencing dissolved Se concentrations in saturated mine panels backfilled with phosphate waste rock at two S.E. Idaho mines. At Agrium’s Dry Valley, low Se concentrations were measured in a monitoring well that was placed in randomly-backfilled mine panels. The mine waste backfill deposit had been reclaimed with a vegetated cover, and had been intermittently saturated with nitrate (NO₃^‑)‑ and SeO₄^2‑‑bearing water that was pumped out of an active mine pit over a few years. Nitrate and SeO₄^2‑ concentrations monitored in the well increased initially, but dropped quickly following each application of pit water, leaving rock unsaturated above the water table. In spite of unsaturated conditions in the upper backfill, conditions have remained sub-oxic and groundwater Se concentrations in monitoring wells GW7D and GW7D2a/2b have remained at or below the Idaho groundwater standard of 50 µg/L. Data from the Dry Valley wells show low concentrations of dissolved O₂, dissolved Se, NO₃^‑, and total dissolved Fe at consistent pH, with elevated concentrations of SO₄^2‑ and total dissolved Mn. In contrast, groundwater samples collected from well GW11, completed in comparable mixed (run-of-mine) waste rock at J.R. Simplot’s Smoky Canyon Mine, had a higher dissolved Se concentration with measurable dissolved O₂. Shifts in the relative abundance of bacteria, based on analysis of 16s rRNA sequence data, with known capacity to reduce SeO₄^2‑-, including Pseudomonas, Shewanella, Cupriavidus, Geobacter, and Dechloromonas, appears to reflect the differences in oxygen concentration in water between the two sites. A greater abundance and diversity of SeO₄^2‑-reducing organisms were detected in sediment concentrated from groundwater collected from Dry Valley, where Se concentrations are naturally attenuated to lower concentrations under previously saturated and presently sub-oxic conditions. This supports the potential for in situ control of Se through management of water to control oxygen concentrations within backfill.