Distribution of reduction-oxidation conditions and relation to trends in nitrate in groundwater, Central-Eastside San Joaquin Valley, California
A 2,700 km2 study area in the eastern San Joaquin Valley near Modesto, California, was investigated to: describe three-dimensional reduction-oxidation (redox) conditions by using water chemistry data compiled from multiple sources; analyze relations of redox conditions to explanatory factors; and determine relations between nitrate (NO3) trends and redox conditions and other factors. Redox conditions are primarily oxic, but increasingly anoxic conditions were correlated with proximity to the valley center, decreased depth to water, and areas of historical groundwater discharge. Anoxic conditions in areas with shallow depths to water occur in both modern recharge, which has abundant organic carbon as an electron donor due to interactions of the shallow water table with soils, and pre-modern groundwater in discharge areas. Although NO3 concentrations were significantly less in wells with anoxic than oxic or mixed redox conditions, dissolved N2 gas data from 39 wells indicate that denitrification caused NO3 concentrations to decrease from above to below the drinking water threshold in only 1 well (maximum excess N2-N of 11 mg/L), indicating that denitrification does not currently protect many wells from NO3 contamination. Trends in NO3 concentrations showed only minor relations to redox conditions. Although NO3 concentrations and trends in the study area are primarily controlled by loading from agricultural land use rather than by redox conditions, greater understanding of the factors controlling the distribution of redox conditions contributes to efforts to simulate NO3 transport at a regional scale.
Additional publication details
|Publication Subtype||Journal Article|
|Title||Distribution of reduction-oxidation conditions and relation to trends in nitrate in groundwater, Central-Eastside San Joaquin Valley, California|
|Series title||Hydrogeology Journal|
|Contributing office(s)||California Water Science Center|
|Other Geospatial||San Joaquin Valley|