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Project 113-26, Page 6 The rate of horizontal flow through the soil is calculated by multiplying the hydraulic conductivity of the soil by the groundwater gradient. The groundwater gradients found at the study property would cause a flow rate of 0.02 inches/year in a silty clay with a hydraulic conductivity of 10-8 m/s, a rate of 20 inches/year for a fine sand with a hydraulic conductivity of 10-5 m/s, and about 650 feet/year for a medium-grained sand with a hydraulic conductivity of 10-3 m/s. The rate of groundwater flow can vary greatly depending on arrangement of high and low conductivity layers Ancient rivers deposit channels of sandy or gravelly soil that serve as groundwater conduits compared to surrounding silts and clays. If a sand layer, such as the blue sand layer found below 6 feet in borehole 7, extended to the southeast, it could carry contamination a surprisingly great distance. However, the actual rates of groundwater movement can only be determined with pump tests in the field. San Joaquin County Public Health Services (Letter, February 24, 1993) was concerned about soil on the study property to the south and southeast of the former excavation, but not to the west and southwest. They have no information about the area to the north This work shows that the contamination plume does not extend to the area southeast and east of the former excavation. The extent of contamination to the north and to the southwest is still unknown. It is still unclear whether this contamination plume orginated on the study property or at the abandoned gas stations on the northwest and southwest corners of Grant Line Road and Holly. The Count also asked for tests for TPH-d and ethylene dibromide (February V, Y Y 1993). However this correspondence was not received on time by Parker Environmental Services to be included in this sample group. These tests will be made in the next round of quarterly monitoring. ' IV. Recommendations. After careful consideration and analysis of the data collected for this report, it is the opinion of Parker Environmental Services that The Customer Company should: ' 1 Continue quarterly monitoring, adding tests for EDB and TPH-d. 2. To further characterize the extent of contamination, either. A Supervise and dnll five boreholes along the west end of the property- B Install a monitoring well west of the tanks. A map showing locations of proposed boreholes and the proposed monitoring well is in the Appendix 24 Parker Environmental Services 4185 Rialto Court Pittsburg CA 94565-6116 ' Phone (510) 439-1024 Fax (510) 439-2566