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GROUND WATER MONITORING WORK PLAN DREDGE MATERIAL DISPOSAL SITE 1,ROBERTS ISLAND,PORT OF STOCKTON 2.3.3 Local Hydrogeologic Setting Water was encountered in borings advanced through the dredge materials and into the underlying peat. The ground water elevations recorded in the existing monitor wells present an inconsistent picture of the ground water gradient. This may be due to the wells being screened in the dredge materials and not in the underlying aquifer materials. Consequently, the water levels in the existing monitor wells may reflect a mound of water perched above the local native aquifer system. Instead, the ground water gradient within the shallow aquifer may be towards the interior of the island and away from the surrounding surface waters. This inward gradient reflects the recharge of shallow ground water from surface waters surrounding the delta islands. The ground water recharge is due to the island's land surface lying below sea level, and ground water drains that keep the island from flooding due to rising ground water levels. 2.3.4 Beneficial Uses of Water Roberts Island is located within the boundaries of the Sacramento-San Joaquin Delta, as defined in the Basin Plan (Figure 1; RWQCB, 1998). In general, the surface waters and Deltaic Deposits may not be consistent sources of potable ground water. The Basin Plan acknowledges this general interpretation of surface water quality when it states "[b]eneficial uses vary throughout the Delta and will be evaluated on a case-by-case basis." In addition, the Basin Plan allows exceptions to the "beneficial use designation of municipal and domestic supply (MUN) [where] the total dissolved solids (TDS) exceed 3,000 mg/l (5,000 µmhos/cm, electrical conductivity)... [or] there is contamination, either by natural processes or by human activity (unrelated to the specific pollution incident) that cannot reasonably be treated for domestic use... [or] the water source does not provide sufficient water to supply a single well capable of producing an average, sustained yield of 200 gallons per day." Water samples have been collected from the dredge sediments and shallow peat layer. However, because the dredge materials are elevated as much as 20 feet above natural ground surface, water within the sediments is considered to represent a local isolated perched layer. Water within the dredge sediments is interpreted to not represent ground water within an aquifer. In addition, water derived from peat layers in the Delta is considered to be an undesirable source of ground water because the naturally elevated organic compounds generate elevated concentrations of disinfection byproducts (DBP, such as trihalomethanes). DBPs, which are potential carcinogens and are very expensive to remove, form when organic compounds in water react with chlorine during the normal disinfection process required for public water supplies (USGS, 1997). Therefore, as possible, water derived from peat is avoided in the water supply wells and it is common practice to construct water supply wells below the peat layers in the delta. January 2005 Page 5 Environmental Risk Services,Corp.