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• P&s Engineering Science, Inc. former hazardous materials/waste storage area source and downgradient areas have been characterized sufficiently for determining suitable remedial alternatives. Four quarterly rounds of groundwater monitoring have been performed in 1997 at the seven on-site monitoring wells. Monitoring consisted of water level measurements, sampling, and chemical analyses. The distribution and magnitude of groundwater contamination with chlorinated hydrocarbons has been consistent throughout the year. Groundwater elevations have gradually declined from spring to summer and risen in the winter, consistent with intense local agricultural pumping; the gradient has remained relatively flat and oriented toward the east-southeast. DATA GAPS: NORTHEAST CORNER OF THE AASC PROPERTY Groundwater contamination at the northeast corner of the site may have resulted from past chlorinated solvent releases from the sanitary sewer line. The sewer line extends eastward ' from Building 3 beneath the parking lot and past MW-4 at the northeast corner of the property. The status of subsurface knowledge at the northeast corner of the property and immediately off-site to the east is discussed below. A site water table map based on water levels measured on November 12, 1997 is shown on Figure 2. Detections of the principal volatile organic groundwater contaminants at three depth horizons are shown on Figures 3, 4, and 5. Lateral Extent of Contamination. The groundwater gradient and inferred flow directions have consistently been oriented to the east-southeast in all investigations since September 1996. TCE and 1,1-DCE have been consistently detected at the northeast corner of the property in MW-4. Therefore, it is likely that the eastern limit of groundwater contamination extends beneath South Airport Way. There is only one off-site, downgradient data point at P- 8 located about 500 feet southeast of MW-4. Groundwater Contaminant Concentrations, Shallow Depth. One previous CPT probe location, P-6, and MW-4 are located close to the sewer line alignment. In the southeastern portion of the property, groundwater analyses from MW-5 have normally been "non-detect" but have had sporadic detections of TCE and chloroform at low concentrations. Other than MW-5, there are no sample analysis results from the shallow groundwater farther than about 40 feet downgradient of the sewer line. Groundwater Contaminant Concentrations, Intermediate Depth. In the intermediate depth range (70 to 76 ft), there are two analytical results from the downgradient side of the sewer ' line (P-6 and P-8). The location close to the line, P-6, had a detectable concentration of TCE and the P-8 result was non-detect. Groundwater Contaminant Concentrations, Deeper Lam. From the deeper water bearing zone (>100 ft) there are no downgradient analytical results. Soil Stratigraphy. The soil boring logs from Phase I and CPT logs from Phase II investigations indicate that much of the site is underlain by silty clay and clayey silt. This stratigraphy would be expected to act to retard migration of residual contaminants that are 0997\09072\wks2add 12-18-97 2-2