Environmental selenium speciation and biogeochemistry in the hyporheic zone and the rhizosphere /by Libbie L. Oram.

Processes occurring in interface environments, such as the hyporheic zone and the rhizosphere, can exhibit far-reaching ecosystem effects as they control the exposure and distribution of environmental selenium (Se). The influence of hyporheic exchange and rhizosphere processes on Se biogeochemistry and mobility is unknown. Research that addresses Se cycling in these complex interface environments is required to more accurately predict Se bioavailability and environmental impact. Furthermore, soils and plants on reclaimed mine sites and streams downstream of reclaimed and operating mine sites in the Western Phosphate Resource Area (WPRA) have been impacted by high Se concentrations; however the mechanistic processes occurring at these sites remain unclear. In the presented research, we are integrating data from both molecular and macroscopic experiments on natural environmental samples from the hyporheic zone and the rhizosphere.;880-01Using the tracer and geochemical measurements, the active hyporheic zone of EMC was determined to be 12 {plusmn}3 cm. Pore water redox profiles indicated that a transition to suboxic conditions begins at approximately 6 cm. Modeling of nonreactive pore water Se suggested some Se uptake is occurring, and pore water Se was inversely related to sediment Se concentrations. Micro-SXRF analysis of EMC sediments showed reduced elemental Se or selenides throughout the profile and selenite in surface sediments. Field geochemical measurements and microscale analysis both indicated that reduction in the EMC hyporheic zone promotes sequestration of surface water Se.;Percent extractable Se in aster rhizosphere soil was greater than extractable Se in corresponding bulk soils in all samples. X-ray spectroscopic results indicated that the rhizosphere and bulk soil Se species were predominantly reduced Se( -II,0), and the aster roots accumulated high concentrations of Se(VI). Results showed that plant-enhanced Se bioavailability and root surface oxidation of reduced soil Se is occurring in the rhizosphere.;The research presented describes Se speciation and geochemical processes that occur in the plant-root and ground water-surface water interfaces and can be used to develop management practices that minimize Se bioavailability, ecosystem risks, and unacceptable exposure routes, for both the WPRA and other Se-impacted sites.Thesis (Ph. D., Environmental Science)--University of Idaho, May 2010