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S E C ® R . Conceptual Site Model z March 24, 2004 Page 20 pounds of TPHg, 720 pounds of TPHd, 8W&pounds of BTEX, and 26 pounds MtBE remained in soil beneath the Site on September 1996 Information obtained from a file review conducted by SECOR indicated that SVE was the most appropriate remediation technology in the vicinity of the Site due to a relatively low mass remaining in soil and an approximate 10-foot increase in groundwater elevations since monitoring and sampling was initiated Based on soil mass balance calculations monitored natural attenuation may be the most appropriate course of action a The extent of impacted sod beneath the site should be explored through additional site assessment activities Based on data obtained from future site assessment work a more accurate estimate of the mass of petroleum hydrocarbons remaining in soil can be calculated Proposed site assessment h activities are presented in section 9 0 !b T-P 08 16 Wrp r .. 1+(A � 8 3 2 Groundwater Condition Summary 0 2 I b TP�kp Z 16 b„T,*t Groundwater samples collected from monitoring wells located onsite indicate a general decline in petroleum hydrocarbon concentrations over time (Figures 10 through 12) Mass balance estimates milli t� tl� t appraximately, 7 pounds of TPHgr036 5 pounds of 8.7 pounds of benzene, anc�1 pLnds of MtBE remains in groundwater beneath the site Information obtained from a file review conducted by SECOR indicated that groundwater extraction and treatment was not an 4 effective remediation technology in the vicinity of the Site Groundwater mass balance calculations and data reported by adjacent sites indicate that augmented monitored natural attenuation may be the most appropriate course of action for the remediation of groundwater beneath the Site Onsite wells do not define TPHg to the north, east, and west Onsite rlls do not define benzene to the north, east, and west and onsite wells do not define MtBE to the east Existing data suggests that groundwater is laterally and vertically defined offsite to the north, northeast, and east, with the exception of former Texaco monitoring well MW-18-80 and MW-18-120 Data to be collected from the proposed monitoring wells should provide a more accurate estimate of the mass of petroleum hydrocarbons remaining in groundwater Proposed site assessment activities are presented in } section 9 0 8.4 Groundwater Receptors 1 Groundwater analytical data reported from monitoring wells located north, northeast and east of the Site indicates that groundwater immediately down gradient of the Site does not appear to be significantly impacted by petroleum hydrocarbons (Figure 9) Groundwater concentrations reported from monitoring wells located down and cross gradient of the Site indicate that natural attenuation is sufficient to reduce hydrocarbon concentrations to low levels A Mass flux calculation was completed to assess hydrocarbon impact to water supply wells identified in section 7 0 The mass flux calculations were based on the following assumptions • To conservatively estimate the mass of MtBE In groundwater a site-wide concentration of 590 pg/L was assumed, • Concentrations of constituents in the dissolved phase are assumed to vary linearly with 40 distance This assumption was made to facilitate a calculation by removing the I %Chevron\920331REPORTSICSMICSM 92033(REV B)doc