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• r 4 GEOPHYSICAL INVESTIGATION REPORT -2- MUSCO FAMILY OLIVE COMPANY TRACY, CALIFORNIA map. We also proposed that a tracer be added to the 84-MGAL Pond to track fluid migration in the subsurface from either the pond or spray fields. REVIEW OF GEOPHYSICAL INVESTIGATION REPORT Resistivity surveys were performed along nine lines as shown in Figure 1. In addition, an EM-31 terrain conductivity survey was performed in the vicinity of Well MW-2. The investigation was conducted from 8 October 2002 through 16 October 2002. A detailed review of the resistivity results appear to confirm that the site is underlain by a series of cut-off stream channels and overbank deposits trending in a generally northeasterly direction. Shallow and deep saturated intervals of variable water quality appear to be present along with some dry channels. The resistivity surveys confirm the heterogeneous nature of the subsurface stratigraphy. The surveys also appear to confirm the absence of a continuous water table aquifer underlying the facility. Groundwater, when present, appears to be perched in isolated and discontinuous cut-off channels; hence, the variability in water quality. The terrain conductivity survey shows an area of very high conductivity around and in the vicinity of Well MW-2. This coincides with the poorest groundwater quality found thus far at the facility (TDS=10,000 mg/L). This groundwater may be perched in a discontinuous, isolated alluvial cut-off channel, as no other similar groundwater type has been observed at the facility. RECOMMENDATIONS The surface geophysical surveys provided new data such that eight borings originally proposed near or between the nine resistivity lines are no longer necessary (see Figure 1). One of our concerns at this phase of the investigation is the lack of data near the northeastern boundary of the property. We recommend that a resistivity survey be performed in this area, if feasible, to determine if there are any saturated subsurface alluvial channels that may be potential targets for further investigation. The locations of the five borings originally proposed can be changed based upon the results of the survey. Another concern is the potential of wastewater percolating from the 84-MGAL Pond to cause groundwater impacts. We recommend that a resistivity survey be performed, if feasible, downslope of the pond in a NW-SE direction through Well MW-3. The purpose of the survey would be to determine if there are any saturated permeable zones not already penetrated by a boring that need to be investigated. If any permeable zones are found, consideration should be given to adding a tracer, such as Rhodamine WT, to the 84-MGAL Pond wastewater. Wells installed in any alluvial channels found can be monitored for the tracer to determine if they are pathways for wastewater migration. .II{D:c:\ny documentAMUSCOVesist