COLLABORATIVE RESEARCH: Selenium Stable Isotopes as Indicators of Selenium Transport and Cycling

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9725799 Johnson The proposed work develops measurements of selenium (Se) stable isotope ratios in surface water and ground water systems for use in studies of Se transport and chemical cycling. Previous work is very limited because of technical limitations that we have recently overcome. Parallels between selemium and sulfur isotope systematics combined with previous Se isotope work, suggest that the dominant mechanism for Se isotope fractionation is reduction of soluable oxyanions (Se6 or Se4) to insoluble SeO. Because this transformation can determine the mobility and bioavailablity of Se, Se isotope measurements could be a valuable tool in Se transport and cycling studies. For example, isotope ratio shifts in groundwater over time or flow distance could provide evidence for active reduction of selenium. In contrast, Se concentration decreases could be caused by reduction, reversible sorption or changing input concentration. Se isotope studies thus provide complementary information to standard techniques. Also, if sources of selenium in a given system are isotopically distinct, Se isotope measurements may serve to determine the dominant source or delineate areas impacted by point source inputs. Experiments in the first half of the study will measure isotope fractionation caused by various processes, build on previous work, and fill in major gaps in the basic understanding of Se isotope variation. The experiments focus on chemical reduction (bacterial and abiotic) as the dominant isotope fractionation mechanism, but alos investigate plant growth, volatilization of alkyl selenides, sorption and precipitation. After the isotpic fractionation "rules" have been ascertained, field experiments in groundwater, surface water, and estuarine settings are planned to demonstrate three distinct applications of the new technique. The group assembled includes a unique combination of expertise: 1) It is currently the only group measuring selenium isotope ratios on environmental sample s. 2) One collaborator has extensive expertise in bacterial reduction of selenium. 3) The group has experience with Se-impacted sites in the San Joaquin Valley and elsewhere, and has good connections with many other researchers in the field.