ScholarWorks at University of Montana

Remediation of legacy metal mining waste and ecosystem recovery: the Clark Fork Basin

Monitoring data is used to track clean up and restoration of river systems contaminated by metal mining. However, effects of metal exposures and natural processes are frequently confounded spatially and temporally, thereby complicating evaluations of the efficacy of remedial actions on ecosystem recovery. The purpose of this study was to identify environmental factors that best predicted temporal patterns in macroinvertebrate composition within the Clark Fork Basin during a 9-year period (1993-2001) of variable discharge. The study encompassed a 220 km reach extending from lower Silver Bow Creek to the Clark Fork River at Turah. The entire study reach was subdivided into upper and lower segments based on coarse geomorphic features, stream flow, and species distributions. Hierarchical partitioning identified Cu exposure and peak annual velocity (a measure of flood disturbance) as the strongest predictors of temporal changes in total, EPT, and mayfly richness. The relative effects of each variable could not be generalized, however. Within the upstream segment, shifts in richness were best predicted by a gradual recession in dissolved Cu concentration, while within the lower segment, richness was most affected by a combination of flood disturbance and accompanying pulses of Cu exposure, identified by sediment Cu concentration and by Cu body burdens in a bioindicator. The analysis suggests that the ecosystem is recovering in response to remedial actions in the upper basin. As Cu is further abated, other environmental factors and biotic interactions are expected to exert more influence on, and ultimately define species distributions.