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r Second Quarter 1998 Groundwater Monitoring Report and Closure Request for Former Diesel Tank Site • CoIberg Boat Works,401 North Stockton Street,Stockton,California Page 3 Groundwater Samlin and Anal sis On April 0, 1998, Apex personnel collected groundwater samples from monitoring wells MW-I through MW-6 in accordance to the Apex SOP included as Appendix A The samples were subnutted, under chain-of-custody documentation, to California Laboratory Services of Rancho Cordova, California, a state-certified analytical laboratory The samples were analyzed for concentrations of total petroleum hydrocarbons, as gasoline (TPHg) and diesel (TPHd), by Modified EPA Method 8015, benzene, toluene, ethylbenzene, total xylenes (BTEX) and methyl t- butyl ether (MTBE) by EPA Method 8020 The analytical results are summarized in Table 2 The laboratory analytical reports and chain-of-custody form are included in Appendix B Figure 4 shows the distributions of TPHg, TPHd, benzene and MTBE in the shallow groundwater on April 30, 1998 at the subject site MASS BALANCE CALCULATION FOR FORMER DIESEL TANK SITE Estimated Area of Diesel Impact During the moiutonng well installations, soil samples were collected from MW-1 through MW-6 . at 15 feet and 20 feet below grade Diesel was not detected in the soil samples collected from any of the soil borings The results of the soil boring and well installation were documented by Apex in Results of Subsurface Investigation, dated August 21, 1997 Based on the past quarterly groundwater monitoring results, the groundwater at the subject site flows in an east and north-east direction Diesel has not been detected in the groundwater samples collected from the down-gradient well (MW-5) or the up-gradient well (MW-6) near the former diesel tank area Therefore, diesel impact to the groundwater at the former diesel tank area is confined to the immediate proximity of the former diesel tank basin and the monitoring well MW-4, which is adjacent to the former diesel tank basin The vertical extent of groundwater diesel impact is above 15 feet below grade since no soil samples collected from the mon itonng well borings contained diesel at 15 and 20 feet below grade Figure 5 shows the estimated extent of diesel impact to groundwater on April 30, 1998 at the former diesel tank location Residual Diesel Mass Balance Calculation Since diesel in the groundwater is defined and not migrating, a mass balance calculation was performed to estimate the residual diesel mass remained in the shallow groundwater and subsurface soil at the former diesel tank area The results of the mass balance estimation indicates that the residual diesel mass in the groundwater and soil is approximately S poun s The detailed mass calculations and assumptions are included in Appendix C •1 �1.2��.Qr rv�-ss �ti ck JY C 1_PROJECTICOL01 001-COLBERG BOAT WORKSIQMR04981QMR0498 DOC