2006 Annual Report 4d.Progress report. National Program 201, Problem area: 2.3 Improved Irrigation and Cropping of Reuse of Degraded Waters. This report serves to document research conducted under a Specific Cooperative Agreement between ARS and the University of California, Riverside. Additional details can be found in the report of the parent project 5310-61000-012-00D. Saline irrigation discharge waters containing elevated levels of potentially toxic trace elements, such as arsenic, selenium, molybdenum, and boron, continue to negatively impact agriculture in many regions of the western U.S. Dissolution and mobilization of these trace elements from native soils are highly affected by oxidation-reduction and pH changes in the soil-water system. Study of these two master variables in natural settings, and their effects on trace element speciation and reactivity, is highly difficult. Subsequently, the focus of our research has been in two key areas:. 1)Develop new ways to buffer or constrain pH and redox at well-defined levels in controlled, soil-water microcosms in the laboratory. This research is needed in order to examine selenate reduction as related to pH and redox.. 2)Characterize speciation changes for these trace elements, in particular selenium, using the newly developed buffering techniques. Researchers at the ARS Salinity Laboratory and U.C. Riverside conducted an initial pH and redox buffering studies using closed, anaerobic soil-water suspensions amended with soluble or solid phase suspensions of redox active chemicals. Using these initial results, selenate reduction rates were evaluated over moderately oxidizing to moderately reducing conditions, which encompass many agricultural soils. Higher rates of selenate reduction were observed under moderate, iron-reducing conditions, although substantial variability within replicates of the same treatment was typical. To further expand the potential range of redox zones for study, additional buffering experiments were conducted with a more diverse assemblage of redox active compounds, including many reduced sulfur complexes. Based on the results of the bufferings studies we determined selenate reduction rates using variable concentrations of dithiothreitol, an organic sulfide complex, as a redox controlling compound. Selenate reduction rates were enhanced at low redox levels, but at the highest concentrations of dithiothreitol (and lowest redox levels), reduction rates were suppressed, possibly due to complexation between the reduced sulfides and the different soluble forms of selenium. These results suggest that under natural conditions where iron reduction is prevalent, selenate reduction rates may be the most enhanced. Under strongly sulfidic conditions, such as in permanent wetlands or estuaries, selenate reduction may be impeded due to complex interactions of reduced sulfur compounds with soluble selenium, or from suppression of microbial selenate reduction processes. These results are useful to the design of Se reduction treatment processes and control of Se release in agricultural drainage waters.