The process and conditions by which arsenic (As) goes from being in sediment loads, in solids, and into the drinking water may effect public safety and environmental health. Dr. Nell Hoagland has been awarded a postdoctoral fellowship to work at the Colorado School of Mines to determine the conditions driving the sequestration or release of arsenic (As) from the hyporheic zone in rivers impacted by acid mine drainage. Using a combination of microbial and hydrological methods, the researcher will investigate the spatial and temporal scaling of the hyporheic zone, where groundwater-surface water interactions modulate the timing, magnitude, and speciation of solutes. This work will build upon previous U.S. Geological Survey (USGS) investigations of the hyporheic zone in the Animas River of Colorado, where an accidental breach of mine pond wastewaters in August 2015 led to the release of 11 million liters of tailings directly into the river. By constraining the location and timing of total As and As(III) fluxes to the stream channel, this study will provide the geochemical and hydrological context needed to develop engineering solutions for river recovery in mine-impacted catchments. New insights related to the timing of As release to the Animas River will be shared in a publically accessible technical report with public health officials and municipal water authorities in the affected region. This project will engage undergraduate and graduate students at Colorado School of Mines and local community colleges by offering a summer short-course, where students will learn about the field of environmental hydrology and the impacts of acid mine drainage. <br/><br/>The biogeochemical processing of metalloids such as arsenic is unpredictable in redox transition zones. Redox conditions in the hyporheic zone will change on seasonal and storm-event scales, impacting net As release or storage. The working hypotheses for this study include: (a) the export of total As and the ratio of As(III)/As(V) from the hyporheic zone in an acid mine drainage stream increases over the course of a storm event in response to increasingly reduced sediment conditions, and (b) the hyporheic zone acts as a source of As to the stream channel during the summer (e.g. wet, high discharge) and a sink of As in the winter. The proposed work will evaluate the hyporheic zone at high temporal and spatial resolution in order to provide new information on microbial and mineralogical mechanisms controlling As speciation across redox gradients. Deliverables will include a conceptual model that relates climatic controls on hyporheic zone area and exchange rates to the mobilization of contaminants from hyporheic sediments at the reach-scale. The conceptual framework in combination with reactive-transport modeling will guide interpretations of contaminant transport at the watershed-scale.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.