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soils that are weakly to strongly cemented. The soil survey indicates that for septic tank absorption fields, the slow permeability can be overcome by increasing the size of the absorption field, backfilling trenches with coarse material, and installing long absorption lines. Additional information provided by the USDA-Soil Conservation Service (Appendix B) indicates that Stockton clay from this area has a very high cation exchange capacity (CEC) varying from 45 to 60b meq/100g. The significance of the CEC to adsorb ammonia-nitrogen will be discussed later in Section 6.0. 3.2 REGIONAL HYDROGEOLOGY Information on the regional hydrology was obtained from the Department of Water Resources (DWR) and from local well drillers. Maps maintained by the DWR Central District Office indicate that the depth to water in wells in the area varies seasonally and has been historically increasing (i.e., the water table has been dropping). The water level in the fall is approximately 10 feet lower than the level in the spring. The reported depth to water in the Spring of 1993 was 75 feet, and the reported direction of flow was to the southeast. First groundwater in the vicinity of the property is encountered at approximately 90 feet bgs. Standing water in completed wells is approximately 80 feet bgs. A second aquifer is encountered at approximately 120 to 130 feet bgs. The DWR considers both these aquifers to be in hydraulic communication. Older residential wells in the area may be completed in either the shallow or intermediate aquifer (less than 130 feet bgs). Newer residential wells must have the borehole sealed to 100 feet bgs and so are generally completed to deeper aquifers. Water in the upper aquifer is of good quality, but the water has higher salt levels than are found in deeper aquifers. The agricultural chemical dibromochloropropane (DBCP) is often present in the upper aquifer in areas that have been utilized for growing grapes. a 3.3 GROUNDWATER CHARACTERISTICS i The DWR periodically samples wells in San Joaquin County for nitrates and other parameters. Data from 1969 to 1993 for the three wells closest to the property were reviewed at the DWR central District. None of the wells had nitrates above the drinking SAC 125.12 3 DAMES &MOORE