4 Arsenic, mercury, and metals, sampled in sediment. 5 Total mercury in unfiltered water samples. 6 Organochlorine compounds including DDT and PCBs, sampled in fish tissue. 7 Miscellaneous industrial chemicals and combustion by-products, sampled in sediment. Ground-Water Highlights Ground water of the Sacramento Valley accumulated in aquifers from precipitation in low hills surrounding the valley and from infiltration of rain, rivers, and irrigation on the valley floor.
Ground water is affected by agricultural and urban land uses.
Bentazon, a herbicide applied to rice fields, was detected in 71 percent of shallow wells sampled in the rice-growing area, despite having been suspended from use since 1989. Bentazon concentrations measured in this study did not exceed any existing drinking-water standard. To protect rivers from pesticide contamination, the rice-field water is required, by means of mechanical controls, to remain on the fields for about 1 month. During that time, pesticide levels decrease by various processes, but evaporation of the water may increase the salinity of the shallow ground water by leaving salts behind. Urban growth of the Sacramento metropolitan area has affected ground-water quality.