Transport, Transformation, and Retention of Arsenic in a Headwater Stream: Hydrologic, Biological, and Geochemical Controls

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0207784SchreiberIt is well-known that arsenic, a toxic and carcinogenic element, is readily transported in natural waters and that its forms, speciation, and concentrations are affected by biogeochemical reactions. In this proposal, we address the transport, transformation and retention of arsenic in a stream-aquifer system impacted by past mining activities. Given our current understanding of the structure of stream-aquifer system in close contact with the mine, we developed a number of questions regarding the role of hydrologic, geochemical and biological processes controlling the speciation and concentration of arsenic. The proposed research will center on addressing two central questions: (1) what factors control spatial and temporal variations in arsenic fluxes to the stream? (2) How do hyporheic processes influence retention of arsenic in the stream?To address these questions, we will utilize a combination of field, laboratory, and modeling techniques to evaluate the controls on arsenic mobility within the stream, with specific emphasis on characterizing the role of the hyporheic zone in acting as a source or a sink for arsenic. The proposed research elements include employing hydrologic and geochemical monitoring to characterize groundwater and surface water end-members as well as to describe biogeochemical conditions within the hyporheic zone. In addition, transient storage models will be used to evaluate the hydrologic characteristics of the hyporheic zone and, through incorporation of reactive uptake terms, to address the role of the hyporheic zone in retarding or promoting arsenic transport to the stream.Results of this proposed research will have implications for our general understanding of the biogeochemical cycling of redox-sensitive trace elements such as arsenic. The recent tightening of the arsenic drinking water standard from 50 to 10 ug/L will drive federal and state agencies to closely examine sources of both natural and human-introduced arsenic contamination. Improving the knowledge of the processes that control arsenic mobility will greatly aid these agencies in developing plans for protecting and treating drinking water supplies.