Silver Bow Creek and the Technical Impracticability Decision for the Alluvial Aquifer in Butte, Montana

This paper will describe successful surface water remedial actions and technical impracticability of groundwater remediation for a creek that has long been contaminated by mining wastes. Historic mining-related discharges into Silver Bow Creek from the Butte, Montana mining district (the Butte Priority Soils Operable Unit Superfund site) degraded surface water quality so that the creek has not been suitable for aquatic life since mining began more than 120 years ago. Milling and smelting plants were constructed along the creek in the late 1800s and large mine waste impoundments were constructed within the floodplain and low-lying wetlands. Because remedial investigations found that metals contamination was so ubiquitous and water quality exceedances so large, it was widely believed that water quality standards could never be met.  Remedial actions over the last 15 years, however, have been highly successful in improving surface water have quality. Base flow water quality in Silver Bow Creek has improved to where metal concentrations are starting to meet Montana water quality standards.

In addition to massive mine waste removals and stream reconstruction, effective control, capture, and treatment of contaminated alluvial groundwater has helped improve water quality in Silver Bow Creek.  Although the surface water component of the remedy has been successful, the aquifer is technically impracticable to remediate, regardless of the extent of waste removed. Estimates for aquifer cleanup range from centuries to thousands of years.  Even under a total removal scenario, heavy metal contaminants could not be flushed from the aquifer to the point where the aquifer would be suitable for domestic use. It is also unlikely that untreated groundwater could ever discharge to surface water without causing water quality exceedances. The determination of technical impracticability led to the controversial decision to leave wastes buried in the floodplain, thus requiring groundwater capture and treatment in perpetuity.