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,k S E C ® R t F Conceptual Site Model March 24, 2004 Page 21 logarithmic diffusion rate Because concentrations typically decrease exponentially with respect to distance, a linear relationship is likely conservative, To conservatively estimate the cross sectional area a saturated thickness of 20 feet and cross sectional width of 60 feet was assumed, • Based on historical records a hydraulic conductivity of 0 0411 cm/s was assumed, • A historical high groundwater gradient of 0 02 ft/ft was assumed (Figure 6), a • A conservative water supply well pumping rate of 35 gallons per minute was assumed Based on mass balance calculations and observed hydrocarbon concentrations in down gradient wells constituent concentrations migrating offsite would be reduced below secondary maximum contaminant levels (MCLs) for drinking water and are not likely to impact the five offsite water supply wells discussed in section 7 0 .x 9 0 SITE ASSESSMENT WORK PLAN The objective of this site assessment work plan is to determine the lateral and vertical extent of 4 impacts as stated during a regulatory meeting with SJEHD g� This work plan describes the field work and data collection process for these site assessment activities 9.1 Scope of Assessment This site assessment is framed by the existing facility property boundaries, bounded by commercial properties identified to have had a potential release to the environment to the west, north, and east A city of Stockton municipal yard is located to the southwest West Charter Way and South Lincoln Street are located immediately to the north and east, respectively b Based on historical data and current groundwater monitoring data the existing monitoring wells have been identified as having submerged screens Therefore, the existing three groundwater monitoring wells will be replaced and removed from the monitoring and sampling plan The locations of the s proposed monitoring wells are presented on Figure 2 An alternative use for MW-1, MW-2, and MW- 3 will be evaluated and presented in a future report -N^ 1Hwti -S a& 3�'fssI Sc Y-f.56, r^ a 5,.>e ' rw� s e k Results of previous assessment activities indicate petroleum hydrocarbons have not been vertically defined in groundwater beneath the Site and soil appears to be defined along the northwestern property boundary Determining the extent of impacted sod and groundwater laterally beneath the Site is problematic due to the following factors k • Additional primary sources of impacted sod and groundwater located immediately down and cross gradient, -u,l, F{,5 w ?JuS\t -1 r� 11Chevron%920331REPORTSICSMICSM 92033(REV B)doc