EFFECTS OF FLOODPLAIN RESTORATION ON COPPER, LEAD, URANIUM, AND ZINC GEOCHEMISTRY, SILVER BOW CREEK, MONTANA

Numerous flood events have washed tailings and mine waste into the floodplain of Silver Bow Creek near Butte, Montana, resulting in degradation of water quality and impairment of the riparian zone. Restoration activities along a 35-km section of Silver Bow Creek were initiated in 1999 and are currently ongoing. These activities include the removal of streamside tailings, channel reconfiguration, and revegetation. In August 2012, a synoptic water quality study was conducted on a 5-km study reach consisting of three subreaches: a previously restored subreach, a subreach undergoing active restoration, and an un-restored subreach. These contrasting subreaches provided an opportunity to quantify the effects of floodplain restoration on metal concentrations and loads. Water quality samples were collected at 20 stream sites in the afternoon of August 5, with sample collection occurring over a short time period to minimize the effects of diel metal cycling. A preliminary analysis of the data set indicates that: (1) copper concentrations exceed State of Montana chronic aquatic life standards throughout the entire study reach, including restored and un-restored subreaches; concentrations of other metals are generally below the chronic standards, (2) concentrations and loads of most metals increase within the un-restored subreach, but the observed increases are attributable to a dewatering operation that discharges groundwater to the stream at the end of the reach undergoing active remediation, and (3) concentration and loads of uranium increase within the un-restored reach and these increases may be attributable to the floodplain tailings that remain in place. These findings suggest that metal concentrations within the study reach are largely controlled by upstream sources under the low-flow conditions observed in August 2012, rather than the floodplain tailings in the lower part of the study reach (uranium concentrations, in contrast, may be affected by the tailings that remain).