SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

David Stavarek, SLIC Unit Central Valley - California Regional Water Quality Control Board April 17, 1998 Page 3 A thin bed of"ferrous green" smectite clay was intercepted in deeper borings at about 12.0 to 12.5 feet below surface grade. This uniform,laterally continuous, clay bed is six inches thick, and underlain by six inches of green silty-clay. The bed of green clay is underlain by two feet of uniform, olive-green,fine-grained sand and silt with minor clay. Field evidence indicates that the Washington Street site is underlain largely by greenish- brown, oxygen-deficient, deltaic sands and silts. Ground-Water Quality in the Delta Region Ground-water samples from monitoring wells located in the west part of the Washington Street facility contain higher concentrations of Total Dissolved Solids(TDS),including sodium and chloride ions,than water samples taken from monitoring wells located east of the facility. On March 9, 1998, TDS concentrations ranged from 1,400 to 3,300 mg/l,with a mean concentration of 2,320 mg/l. The corresponding Specific Conductivity (Sp Cond) readings ranged from 2,010 to 4,250 ,umhos/cm, with a mean of 2,996 Azmhos/cm. High TDS concentrations are largely the result of mineral dispersion from brackish sediments. The U.S. Geological Survey(USGS),in explaining the origin of chlorides present in natural waters, states the following, "....When porous rocks are submerged by the sea at any time after their formation, they become impregnated with soluble salts, in which chloride plays a major role. Fine-grained marine shale might retain some of this chloride for very long periods. In all these rock types,the chloride is mostly present as sodium chloride crystals or as a solution of sodium and chloride ions (USGS, 1985, p.118). Given favorable hydrogeologic conditions, connate waters can mix with relatively pure surface and ground waters. According to the California Department of Water Resources (DWR), "The one limiting factor to the use of ground water in San Joaquin County is its tendency to dissolve mineral salts. Unlike surface water,which does not appreciably increase in salt content except with use,ground-water picks up minerals while in transit. From the time water enters the ground it continues to add to its mineral load while at the same time changing its mineral proportions..... As ground-water percolates from areas of recharge to areas of withdrawal,mineral salts are dissolved from the formations through which the water moves. The chemical and physical nature of the formation determines in part the makeup of mineral salts in solution and the total amount(DWR, 1967,p.86)."