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Multi-scale evaluation of groundwater flow and metal sourcing to streams in the Bonita Peak mining district
Newman, Connor P.
Newman, Connor P.
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2025
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Abstract
Hydrologic and geochemical processes associated with acid mine drainage that result in the weathering of sulfide minerals and fluxes of metals to streams vary through space and time. Remediation of former mine sites relies on appropriate recognition and quantification of these changing processes. The need for both conceptual and quantitative understanding is particularly acute in headwater streams because these watersheds generate much of the water essential for ecosystem function and downstream human use, and headwater regions may be particularly sensitive to climatic changes. This research focuses on the Upper Animas River watershed in southwestern Colorado, a region substantially affected by historical mining over the past nearly 150 years to identify processes governing groundwater recharge and movement to streams, and associated metal weathering and fluxes, that operate over orders of magnitude in spatial and temporal extent. The three primary conclusions of this research are 1) metals are sourced from weathering of mineral deposits at depth (>500 meters below ground surface) and are transported long distances (multiple kilometers) by deep groundwater flow paths that terminate at springs and draining mines; 2) groundwater flow paths were modified by emplacement of hydraulic bulkheads resulting in new mineral equilibrium controls within the subsurface and a net decrease in iron and zinc export from the watershed; and 3) the solute budget of streams is controlled by discharge and metal fluxes from multiple distinctive groundwater systems with unique geochemical compositions and timescales of response to climatic perturbations. These results illustrate that groundwater flow, groundwater/surface-water interactions, and metal fluxes vary over spatial extents ranging from meters to multiple kilometers and on timescales ranging from days to decades. Data collection and modeling approaches that account for dynamic systems over these ranges of spatial and temporal extent are necessary when studying mountain headwater regions.
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