SITE INFORMATION AND CORRESPONDENCE

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GEOMATRIX APPENDIX A ORC Replacement Calculations GROUNDWATER FLUX ESTIMATE Hydraulic parameters of the saturated zone at the site were estimated based on the drawdown data observed in well MW-4a during the City of Tracy's dewatering activities for the expan- sion of Grant Line Road in 1994 (Geomatrix, 1995).b The hydraulic conductivity (K) of the medium-grained sand layer was estimated to range from 10-5 to 10-4 feet per second (fps). This estimated K is within the published range of K values for silty sand and clean sand (Freeze and Cherry, 1979).° Using an average K of 5 x 10-5 fps, an estimated porosity of 0.35, and hydraulic gradient of 0.005 (estimated hydraulic gradients from site data range from ap- proximately 0.003 to 0.006), an average groundwater flow velocity at the site is estimated to be 0.06 feet per day (ft/d). Using an assumed cross-section of about 10 feet long (length of screened interval of monitoring well) by 10 feet wide (typical radius of influence for ORC in wells), the average groundwater flux through the monitoring wells at the site is estimated to be about 6 cubic feet of water per day(ft3/d) for wells MW-7, MW-8, and MW-12. The soil encountered within the depth of the screened interval for MW-6 was mostly clay and silty sand, which would result in a lower K. Assuming a K of 5 x 10-6 fps, the average groundwater flux through MW-6 is estimated to be about 0.6 ft3/d. OXYGEN DEMAND ESTIMATE The total oxygen demand for Alternative 3 was estimated based on the groundwater flux and the concentrations of total petroleum hydrocarbons quantified as gasoline and diesel (sum of TPHg and TPHd) in groundwater(Geomatrix, 2000). The flux of dissolved petroleum hydro- carbons passing through the monitoring wells was estimated by multiplying the groundwater flux by the concentration of TPH at each well (i.e., the sum of TPHg and TPHd). The oxygen demand was then estimated using an approximate rate of 3.5 pounds of oxygen required for the biodegradation of each pound of petroleum hydrocarbons and an assumed oxygen utiliza- tion rate of about 50 percent. These estimates are summarized in the following table: Estimated TPHg+TPHd Estimated Estimated Groundwater Flux Concentrations TPH Flux Oxygen Demand Well (ft'/d) (µg/1) (pounds per year) (pounds per year) MW-6 0.6 39,500 0.54 3.8 MW-7 6.0 3900 0.53 3.7 MW-8 1 6.0 1300 1 0.18 1 1.3 MW-12 1 6.0 2340 10.32 12.2 a Well MW-4 was located south of the site;the well was subsequently decommissioned for the City's expansion of Grant Line Road. b Geomatrix Consultants, Inc., 1995,Results of Groundwater Protective Action and Monitoring, Corral Hollow and Grant Line Roads Site, Tracy, California, January 19. c Freeze,R.A. and Cherry,J.A., 1979, Groundwater,Prentice-Hall. \\oakI\deptdata\Doc_Safe\2000s\C2463\2463 resp RWQCB App A.doc A-1