Multiscale Approaches to Evaluating Regional Chemical Processes and Trace-Element Distribution

Neil M. Dubrovsky and John M. Neil

In:
Sorenson, S.K., ed., Proceedings abstracts of the American Water Resources Association's symposium on the National Water-Quality Assessment (NAWQA) Program -- November 7-9, 1994, Chicago, Illinois: U.S. Geological Survey Open-File Report 94-397, p. 14.

Abstract:
From 1985 through 1989, a three-tiered study design was used to determine the distribution and geochemical processes affecting selenium concentrations in the regional aquifer of the San Joaquin Valley, California. First, 273 production wells were sampled valley-wide to describe the selenium distribution in the regional aquifers. Second, monitoring wells were installed along ground-water flow lines from the Coast Range to the valley trough in critical subregions in the western San Joaquin Valley. Third, a specific site was investigated to evaluate selenium mobility in chemically reduced parts of a regional aquifer. Selenium concentrations in the production wells rarely exceeded 10 micrograms per liter; in contrast, data from the monitoring wells showed that leaching by irrigation water produced a zone of recently recharged ground water with selenium concentrations greater than 100 micrograms per liter in the upper 30 to 60 meters of the aquifer. The site-specific data showed that selenium is removed from downward-moving shallow ground water by microbially-mediated reduction to insoluble species. Selenium therefore is not mobile in the reducing conditions that predominate in the most productive zone of the regional aquifer in the western San Joaquin Valley.