Development and application of field methods for determination of the extent of acid mine drainage contamination and geochemical characteristics of stream sediment recovery

Abstract

Metal-mining processes can negatively affect surface and ground waters through the generation of acidic mine drainage (AMD). Introduced metals typically are deposited from the water column onto the stream bed downstream of the AMD input(s). The drivers of metal deposition onto stream sediments are through either the formation of insoluble colloids, adsorption to particles, or complexation with sediment surfaces. Metals can be removed from stream sediments through physical (scouring) or chemical processes (dissolution and desorption). These processes have been examined in published literature but those studies largely used ex-situ analysis techniques because experimental conditions are easier to control in the laboratory. In the studies presented herein, I developed a novel method to examine AMD metal deposition in-situ. The study site was the North Fork of Clear Creek (NFCC), located in Gilpin County in central Colorado, which receives 2 point sources of AMD as it flows through the city of Black Hawk, Colorado. I modified an existing field method to monitor benthic invertebrate populations in-situ, to examine the flux of metals on stream sediment in both deposition and recovery scenarios. Also using my developed methods, I was able to examine the effects of scouring (physical removal), stream flow, and sediment surfaces on metal sediment deposition. I found that significant metal loss can occur on sediments after 8 weeks of reference water exposure, and that biofilm growth on sediment surfaces can accumulate significant concentrations of metals from the water column. A combination of in-situ and ex-situ analysis can allow for greater realism and applicability in the data while not sacrificing rigor, replicates and reproducibility. Examination of the processes of metal deposition and removal from sediment can lead to a better understanding of the fate of AMD-associated metals and better prediction of the time course and extent of post-remediation recovery from AMD contamination.