Temporal and spatial monitoring of soil selenium at Kesterson Reservoir, California

The selenium and salt content of the top 15 cm of the soil profile at Kesterson Reservoir (Merced County, California) have been monitored annually to develop a data set that provides a foundation for: (1) evaluating the status of the selenium inventory and biological hazards at Kesterson Reservoir; (2) understanding selenium fluxes near the soil surface; and (3) making long-term predictions of the selenium concentrations available for plant uptake and dissolution into rainwater ephemeral pools. Results of this monitoring program indicate that the soil selenium inventory at Kesterson may be categorized in terms of three main patterns: (1) spatial trends associated with historic Reservoir operations; (2) temporal trends due to the oxidation and remobilization of the selenium inventory and; (3) temporal trends due to seasonal cycles. It is evident that the selenium inventory and distribution within the soil profile will evolve slowly whereby the fraction of the total inventory that is now immobile (est. at 93%) will oxidize to more mobile and bioavailable forms. Two major issues with broad importance were raised concerning sampling the surface soil selenium environment at Kesterson which may be helpful to others conducting investigations of similar nature. These issues include: (1) the recognition that variations in surface soil contaminant concentrations due to seasonal redistribution may obscure long term trends and; (2) large spatial variability in soil contaminant concentrations make it difficult to obtain large enough data sets to detect statistically significant changes in the contaminant inventory until large changes have already taken place. A combination of both process-oriented and synoptic type sampling are recommended to better define time trends.