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Site Assessment Work Plan Broadbent&Associates,Inc. Former ARCO Station No.443 Vacaville,California November 8,2013 Page No.ii Between June 2007 and May 2010, nine onsite and off-site borings were advanced and monitoring wells MW-1D, MW-1S through MW-4S,and MW-5 through MW-8 were installed in these borings. The purpose of the investigation was to determine the extent of soil and groundwater plumes. Elevated petroleum hydrocarbon concentrations in soils were not detected with the exception of shallow monitoring well MW-1S which is located adjacent to SB-26. Petroleum hydrocarbons at lesser concentrations were detected in soil samples collected from MW-6(offsite)and MW-8(near former dispenser islands). Historical groundwater samples have indicated presence of petroleum constituents in all groundwater monitoring wells associated with the Site with the exception of wells MW-5 and MW-6. Only wells MW-1S and MW-1D currently contain significant concentrations of petroleum constituents. A bulk groundwater extraction pilot test and six bulk groundwater extraction events have historically been conducted at the Site using MW-1S as an extraction point. These extraction events have apparently reduced concentrations of GRO and benzene in MW-1D but their impact on shallow wells MW-1S through MW-4S was inconclusive(Stantec, 2013). 2.3 MW-8 Data Review In December 2008,a four-inch diameter monitoring well, MW-8,was installed using a ten-foot length of 0.020-inch machine-slotted well screen at an approximate depth of between 103 to 113 feet below ground surface (bgs). MW-8 was installed with a ten-inch conductor casing to an approximate depth of 95 feet bgs. During the grouting of the conductor casing,the driller kept turning the 18-inch auger while the grout was being pumped. Due to concern raised by SICEHD official present onsite,the augers were stopped from being rotated until it was time to extract a section of auger. Soon after this practice began,the drill rig broke down and therefore the 80 feet of 18-inch hollow stem auger had to be abandoned as the grout solidified in the annulus between the augers and the conductor casing. During a reported telephone conversation on January 29, 2009,a variance to leave the augers in the ground indefinitely was granted by SICEHD(Stantec, 2009). During the March 18, 2013 meeting that was attended by representatives from ARC, Broadbent,and SJCEHD,the integrity of MW-8 was discussed and it was recommended that MW-8 be considered for abandonment and a new replacement well be installed near it. Meeting minutes that were submitted incorrectly refer MW-8 as MW-1D; however, historical reports and boring logs clearly indicate that the flight of auger was grouted in well MW-8 and not in MW-1D. The MW-8 well installation report and boring log are included as an attachment. To follow up with SJCEHD's recommendations and determine an appropriate solution, Broadbent reviewed the historical field and analytical data for MW-8. With the exception of consistently observed low concentration of 1,2 DCA and one benzene detection of 0.57 pg/L in June 2012, MW-8 has been non-detect for other analytes since November 2010 sampling event(Broadbent,2013). The depth to water is consistent with the surrounding wells and the well has not been reported to become"dry" during purging activities. The historical sampling activities at MW-8 have been successful with the necessary amount of water being able to be purged from the well without needing an aquifer recharge. Moreover,the field parameters such as dissolved oxygen,turbidity,and conductivity that are recorded prior to sample collection are consistent with the nearby monitoring wells. The recorded field parameters do not show any abnormal pattern or data.