Geophysical Mapping of Groundwater Seeps Impacted by Mine Influenced Water from Abandoned Hardrock Mines in the Bonita Peak Mining District, CO.
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It is estimated there are more than 500,000 abandoned mines in the United States, and many of the highly toxic and dangerous mines are listed on the Superfund National Priority List (NPL). In EPA Region 8; mines, smelters, and mills make up more than 50% of the NPL sites, and EPA Regional Science Priorities include a focus on groundwater – surface water interactions to aid defining groundwater loading to surface water systems at mine and mineral processing sites where this loading can impact human health or ecological receptors. Current practices and available tools to characterize and monitor stream impacts from acid mine drainage often rely on limited water-sampling events that provide a snapshot of conditions during high and base flow conditions from point sample locations. Typically, the resulting conceptual site models and remediation and / or mitigation strategies are therefore designed to address these static conditions even though research indicates significant temporal and spatial variability in these systems.
To address these needs and broaden the spatial and temporal dimensions of such investigations, we present the application of electromagnetic induction (EMI), magnetometry, and thermal imaging methods to assist project teams in isolating high loading stream sections and likely interactions between groundwater and surface water at three locations within the Bonita Peak Mining District near Silverton, CO. Understanding the groundwater-surface interactions at large mine sites and mining districts can pose significant resource challenges such as high elevation, extreme weather, property access, and other uncertainties that limit the ability of project teams to obtain high spatial and temporal resolution data. Surface geophysical methods such as EMI, magnetometry, and fiber optic distributed temperature system (FODTS) can provide reconnaissance large scale (km scale) investigations to rapidly survey and monitor these areas. From this large-scale investigation, the identification of likely inflow zones or seep inputs allow project teams to consider a watershed or river-reach approach to mine impact mitigation and remediation. The identification of groundwater connections allows the incorporation into sampling design over the hydrograph.
Surveys included EMI and FODTS in the Upper Cement Creek and California Gulch of the Animas River and included magnetometry in the Mayflower Mine Section of the Animas River. The combined interpretation of the electrical, magnetic, and thermal properties identified and classified seep magnitude and stream bank inputs such that project teams and stakeholders can target protective remedies or perform detailed investigations focused on likely high loading inputs. This resource effective ground and water- based geophysical approach offers a watershed scale investigation to improve the characterization of mine impacted surface waters toward achieving restoration and risk management goals.