Anomalous concentrations of silica in ground water of the eastern San Joaquin Valley, California.

Abstract

Anomalous concentrations of silica in ground water of the eastern San Joaquin Valley originate from several diverse sources and are subsequently modified by recharge or diagenesis. Statistics, geochemical models, and column studies identified potential sources of silica in ground water and those parameters most important in influencing its distribution. Principal components analysis indicated inverse time and potassium as the parameters which best relate to silica. Inverse time relates to ground-water recharge while potassium relates to either the rhyolitic Friant Pumice, the hardpans of the eastern San Joaquin Valley, or diagenetic losses. A lumped parameter model suggests that recharge is responsible for the drop in silica concentrations beneath Fresno since 1971. The Madera County model indicates higher silica concentrations associated with the Friant Pumice, older geologic units with hardpans, finer grained sediments, and areas of ground-water discharge. Lower silica concentrations are associated with unweathered sediments, areas receiving recharge, and areas underlain by the Corcoran Clay. Column leaching studies produced silica concentrations from the Friant Pumice of up to 90 mg/1, Turlock Lake and Riverbank Formation hardpans from 40-50 mg/1, and younger Modesto Formation sediments from 10-40 mg/l. Spatially, volcanogenic sediments impart the highest silica concentrations to the ground water, 70-95 mg/1, and occur along the eastern margin of the valley, north of the San Joaquin River, or in San Joaquin River fluvial deposits. Iron-silica hardpans, found at various depths along the east side of the valley, are associated with silica concentrations of 40-70 mg/l. The typical arkosic sediments of the eastern valley have silica concentrations of 20-40 mg/l. Silica concentrations in ground water are modified by recharge and diagenetic processes. Either natural or artificial recharge having 4-16 mg/1 silica may lower ambient silica concentrations in ground water. Recharge occurs beneath some rivers and in transmissive paleochannel deposits; artificial sources include agricultural overirrigation and basin recharge. The down-gradient loss of silica to the sedimentary column, more prevalent at depth, apparently is by the sorption of silica on clays, the formation of clays, or deposition of amorphous silica.