PASSIVE TREATMENT OF SELENIUM IMPACTED GROUNDWATER WITH A BIOWALL

Selenium is a significant environmental problem in the Phosphate Region of southeastern Idaho. Elevated selenium has been identified in shallow groundwater and surface seeps at numerous mines in the region. We have been pilot testing the use of permeable reactive barriers (PRBs) as a passive treatment technology for impacted shallow groundwater at a mine in the Phosphate Region since 2010.

Selenium can exist in numerous oxidations states, making the chemistry of selenium complicated. Speciation plays an important role in determining the mobility of selenium in soil and groundwater systems. The microbial reduction of selenate and selenite to elemental selenium coupled with the precipitation of elemental selenium has been shown to be effective at removing selenium from groundwater.

The initial pilot testing efforts indicated that a thorough understanding of the local hydrogeology is required to determine flow directions, ensure effective capture of impacted groundwater, and calculate residence times. As a result, a comprehensive investigation was completed to fully characterize groundwater quality and flow in the subsurface prior to the construction of the second PRB.

The hydrogeologic investigation included excavation of several test pits to delineate the geometry of alluvial channel deposits. Several shallow monitoring wells were installed upgradient and downgradient of the PRB, including two sets of nested wells, which helped characterize horizontal and vertical gradients and flow directions within the zone to be treated.

Subsequent pilot testing based on lessons learned in initial pilot test work and a robust hydrogeological investigation has demonstrated that an appropriately designed PRB can reduce selenium concentrations in shallow groundwater from approximately 600 mg/L to less than 5 mg/L with a residence time of approximately 24 hours.