Draining mine tunnels contribute metals and acidity to streams throughout the western United States. One remediation strategy that is increasing in usage is the emplacement of hydraulic bulkheads to impound water within the subsurface. Potential secondary effects of bulkhead usage are difficult to quantify however because of the inherent complexity of most mountain hydrologic systems.
In cooperation with the U.S. Environmental Protection Agency (EPA), the U.S. Geological Survey (USGS) is working to investigate processes controlling groundwater flow and acid mine drainage (AMD) geochemistry in several locations with hydraulic bulkheading. This research assists in understanding the potential efficacy of the bulkhead remediation strategy at other locations within the western United States.
In order to gain understanding of the hydrologic and geochemical processes occurring in the subsurface, the USGS collects water-quality data from draining mines, groundwater wells, and springs. A comprehensive suite of water-quality parameters is utilized including trace metals, rare earth elements (REE), stable isotopes, tritium, carbon-14, and noble gases. These water-quality data are paired with hydrologic evaluation of the groundwater-flow regime to quantify hydrological and geochemical processes affecting water quality in flooded mine workings, which can inform remedial decisions.
Results
Results from the Captain Jack mine site, located west of Boulder, Colorado, indicated that the groundwater system is disconnected and that the water within the mine workings has very little connection to the groundwater in the adjacent bedrock. Extremely high hydraulic gradients are maintained between the bedrock and mine groundwaters, even when water is impounded within the mine workings (see figure below).
Based on stable isotopes and groundwater age indicators it appears that old groundwater sustains high metal concentrations flowing into the impounded mine water and that sulfide oxidation (the process that generates AMD) can still occur when the workings are flooded (see figure below).
Related Links
EPA Superfund Site: Captain Jack Mill, Ward, CO - https://cumulis.epa.gov/supercpad/cursites/csitinfo.cfm?id=0800892
Geochemistry, mineralogy, and acid-generating behaviour of efflorescent sulphate salts in underground mines in Nevada, USA - https://doi.org/10.1144/geochem2018-074
- Overview
Draining mine tunnels contribute metals and acidity to streams throughout the western United States. One remediation strategy that is increasing in usage is the emplacement of hydraulic bulkheads to impound water within the subsurface. Potential secondary effects of bulkhead usage are difficult to quantify however because of the inherent complexity of most mountain hydrologic systems.
In cooperation with the U.S. Environmental Protection Agency (EPA), the U.S. Geological Survey (USGS) is working to investigate processes controlling groundwater flow and acid mine drainage (AMD) geochemistry in several locations with hydraulic bulkheading. This research assists in understanding the potential efficacy of the bulkhead remediation strategy at other locations within the western United States.
In order to gain understanding of the hydrologic and geochemical processes occurring in the subsurface, the USGS collects water-quality data from draining mines, groundwater wells, and springs. A comprehensive suite of water-quality parameters is utilized including trace metals, rare earth elements (REE), stable isotopes, tritium, carbon-14, and noble gases. These water-quality data are paired with hydrologic evaluation of the groundwater-flow regime to quantify hydrological and geochemical processes affecting water quality in flooded mine workings, which can inform remedial decisions.
Results
Results from the Captain Jack mine site, located west of Boulder, Colorado, indicated that the groundwater system is disconnected and that the water within the mine workings has very little connection to the groundwater in the adjacent bedrock. Extremely high hydraulic gradients are maintained between the bedrock and mine groundwaters, even when water is impounded within the mine workings (see figure below).
Sources/Usage: Public Domain.Groundwater level elevations within the mine workings (indicated by the red dotted lines) and within crystalline bedrock outside the mine workings (indicated by the blue dotted lines) are substantially different from one another. These vertical and horizontal gradients in groundwater level elevations indicates that the hydrologic system is highly compartmentalized and that there is little connectivity between the mine water and nearby groundwater. Although the water levels increased substantially following closure of the bulkhead (in May 2018) there was still little communication between the adjacent groundwater and the mine water (Newman, 2023). Based on stable isotopes and groundwater age indicators it appears that old groundwater sustains high metal concentrations flowing into the impounded mine water and that sulfide oxidation (the process that generates AMD) can still occur when the workings are flooded (see figure below).
Sources/Usage: Public Domain.Insights gained from stable isotopes and environmental tracers in this study allow a conceptual model to be built that provides information pertinent to groundwater mixing and transport, sulfide oxidation mechanisms, and timeframes of remediation. This conceptual model of the site (not to scale) is based on hydrologic and geochemical data. Piston flow and dispersion flow are denoted using different symbolized arrows. Groundwater residence time (τGW) and residence time in the workings (τW) are differentiated. A conceptual cross-sectional plot of relative intensity of sulfide oxidation, physical groundwater connectivity, and groundwater age mixing is shown on the bottom (Newman and others, 2023). Related Links
EPA Superfund Site: Captain Jack Mill, Ward, CO - https://cumulis.epa.gov/supercpad/cursites/csitinfo.cfm?id=0800892
Geochemistry, mineralogy, and acid-generating behaviour of efflorescent sulphate salts in underground mines in Nevada, USA - https://doi.org/10.1144/geochem2018-074
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