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Mechanisms of water-rock interaction and implications for remediating flooded mine workings elucidated from environmental tracers, stable isotopes, and rare earth elements
Citation:
Newman, C., K. Walton-Day, R. Runkel, AND R. Wilkin. Mechanisms of water-rock interaction and implications for remediating flooded mine workings elucidated from environmental tracers, stable isotopes, and rare earth elements. APPLIED GEOCHEMISTRY. Elsevier Science Ltd, New York, NY, 157:105769, (2023). https://doi.org/10.1016/j.apgeochem.2023.105769
Impact/Purpose:
Acid mine drainage (AMD) results from the exposure and oxidation of sulfide minerals during and after mining and may cause contamination of surface water and groundwater. Although acidity alone may cause ecological damage, much of the concern related to AMD is attributed to increased solubility of base metals under acidic conditions. Geochemical effects of AMD may last for thousands of years and require complex mitigation measures to reduce contaminant transport in surface-water and groundwater systems. This research focused on mechanisms of water-rock interaction and hydrologic limitations on remediation strategies within flooded mine workings at the Captain Jack Superfund Site located near Ward, Colorado, USA. To understand water-quality variability a diverse dataset was collected that included stable isotopes (δ2H, δ18OH2O, δ18OSO4, δ34S, δ13CDIC), rare earth elements (REE) and other environmental tracers (tritium [3H], noble gases). The work is of interest to the international environmental community, including EPA’s regional, program, and local partners.
Description:
Contamination from acid mine drainage affects ecosystems and usability of groundwater for domestic and municipal purposes. Colorado has numerous abandoned mines which may drain acidic and metal-rich water, necessitating remediation. The Captain Jack Superfund Site outside of Ward, Boulder County, Colorado, USA, is an example of a site with mining-related contamination. The site hosts a draining mine adit that was remediated through emplacement of a hydraulic bulkhead to preclude acid mine drainage from entering nearby Lefthand Creek. During impoundment of water within the mine workings in 2020, a diverse and novel dataset of stable isotopes of water, sulfate, and carbon (δ2H, δ18OH2O, δ18OSO4, δ34S, δ13CDIC), rare earth elements, and environmental tracers (noble gases and tritium) were collected to understand groundwater recharge and mixing, mechanisms of sulfide oxidation and water-rock interaction, and the influence of remediation on the hydrologic and geochemical system. Stable isotopes of water indicate that groundwater distal from the mine workings has seasonally variable recharge sources whereas water within the workings has a distinctive composition with minimal temporal variability. Stable isotopes of sulfate indicate that sulfide oxidation occurs both within the mine workings and in adjacent igneous dikes, and that sulfide oxidation may occur under sub-oxic conditions with ferric iron as the oxidant. Stable isotopes of carbon track the neutralization of acidic waters and the carbon mass budget of the system. Rare earth elements corroborate stable isotopes in indicating groundwater compartmentalization, and additionally illustrate enhanced mineral weathering in the mine workings. Environmental tracers indicate mixing of modern and pre-modern groundwater and inform timelines that active remediation will likely be required. Together these datasets provide a useful template for similar investigations of abandoned mine sites.
URLs/Downloads:
DOI: Mechanisms of water-rock interaction and implications for remediating flooded mine workings elucidated from environmental tracers, stable isotopes, and rare earth elements
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