Hydrologic assessment and numerical simulation of groundwater flow, San Juan Mine, San Juan County, New Mexico, 2010–13

Coal combustion byproducts (CCBs), which are composed of fly ash, bottom ash, and flue gas desulfurization material, produced at the coal-fired San Juan Generating Station (SJGS), located in San Juan County, New Mexico, have been buried in former surface-mine pits at the San Juan Mine, also referred to as the San Juan Coal Mine, since operations began in the early 1970s. This report, prepared by the U.S. Geological Survey in cooperation with the Mining and Minerals Division of the New Mexico Energy, Minerals and Natural Resources Department, describes results of a hydrogeologic assessment, including numerical groundwater modeling, to identify the timing of groundwater recovery and potential pathways for groundwater transport of metals that may be leached from stored CCBs and reach hydrologic receptors after operations cease. Data collected for the hydrologic assessment indicate that groundwater in at least one centrally located reclaimed surface-mining pit has already begun to recover. The U.S. Geological Survey numerical modeling package MODFLOW–NWT was used with MODPATH particle-tracking software to identify advective flow paths from CCB storage areas toward potential hydrologic receptors. Results indicate that groundwater at CCB storage areas will recover to the former steady state, or in some locations, groundwater may recover to a new steady state in 6,600 to 10,600 years at variable rates depending on the proximity to a residual cone-of-groundwater depression caused by mine dewatering and regional oil and gas pumping as well as on actual, rather than estimated, groundwater recharge and evapotranspirational losses. Advective particle-track modeling indicates that the number of particles and rates of advective transport will vary depending on hydraulic properties of the mine spoil, particularly hydraulic conductivity and porosity. Modeling results from the most conservative scenario indicate that particles can migrate from CCB repositories to either the Shumway Arroyo alluvium after 1,320 years and from there to the San Juan River alluvium after 1,520 years or from southernmost CCB repositories directly to the San Juan River alluvium after 2,400 years after the cessation of mining. 1Retired. Introduction Coal combustion byproducts (CCBs), which are composed of fly ash, bottom ash, and flue gas desulfurization material, are produced by operation of the coal-fired San Juan Generating Station (SJGS), located in San Juan County, New Mexico. CCBs have been buried nearby in former surfacemine pits at the San Juan Mine (SJM), also referred to as the San Juan Coal Mine, since the SJGS power plant began operation in the early 1970s (Mining and Minerals Division [MMD], 2017a). Disposal of CCBs is overseen by the MMD of the State of New Mexico Energy, Minerals and Natural Resources Department (NMEMNRD). Regional dewatering for oil and gas production has been ongoing in the area since the 1960s (GO-TECH, 2014, 2016). Local dewatering for surface and underground mining has been ongoing since the early 1970s. It is anticipated that power generation, and hence, local dewatering will cease at some time in the future. (For this project, it was assumed that regional oil and gas dewatering will cease at the same time). Upon cessation of dewatering activities, it is expected that groundwater in dewatered areas will recover, potentially allowing groundwater to transport metals associated with CCBs from repositories to hydrologic receptors along unknown pathways. To address these concerns, the U.S. Geological Survey (USGS), in cooperation with the MMD of the NMEMNRD, prepared a hydrogeologic assessment at and in the vicinity of the SJM, including numerical groundwater modeling, focused on identifying the timing of groundwater recovery and potential pathways for groundwater transport of metals that may be leached from CCBs after the cessation of mining at the SJM.