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Nomellini Construction Company - 2 - 29 June 2018 1045 West Charter Way Stockton, San Joaquin County During the fourth quarter 2017, AGE measured depth to water between 13.22 feet and 15.02 feet below the tops of the well casings, and calculated a flow direction radially outward towards the southwest, southeast, and northeast, AGE detected up to 32,200 micrograms per liter (ug/L) total petroleum hydrocarbons as gasoline (TPH-G), 3,590 ug/L benzene, and 21.9 ug/L MTBE in groundwater samples collected during the fourth quarter 2017. AGE operated a dual phase extraction (DPE) system from 26 October 2009 to 23 September 2013. The DPE system removed 4,535 pounds of vapor phase TPH-G, and extracted 4,042,230 gallons of groundwater, removing 78.6 pounds of dissolved phase TPH-G. Central Valley Water Board staff have the following comments: 1. Based on the soil gas sample results, AGE concluded that the Site meets vapor intrusion media specific criteria outlined in the State Water Resources Control Board's Underground Storage Tank Low Threat Closure Policy (LTCP). AGE recommended that no additional soil gas sampling be performed. Based on available data, Central Valley Water Board staff concur with AGE'S conclusion. 2. In the Work Plan, AGE stated that in a letter dated 5 May 2017, Central Valley Water Board staff directed that groundwater monitoring wells should be installed rather than collection of grab samples in the proposed areas of B-23 and B-24. This is incorrect in the 5 May 2017 letter, Central Valley Water Board staff stated that wells should be installed at the locations of proposed borings B-23 and B-25, and that based on available data additional delineation northwest of MW-7, the location of proposed boring B-24/well MW-17, did not appear warranted at that time. However, based on increased petroleum hydrocarbon concentrations in well MW-7 since issuance of the 5 May 2017 letter, Central Valley Water Board staff now concur with AGE that additional delineation northwest of MW-7 is appropriate. Central Valley Water Board staff concur with installation of well MW-16 and MW-17, as proposed by AGE in the Work Plan. Please submit a Well Installation Report by 15 October 2018. 3. Central Valley Water Board staff request that you install a well at the location of proposed boring B-25 to laterally delineate petroleum impacts to groundwater in the intermediate zone. Therefore, please add to the scope of work installation of a third groundwater monitoring well, constructed similarly to proposed wells MW-16 and MW-17 at the location of formerly proposed boring B-25. 4. The benzene concentration at the Site exceeds LTCP closure criteria, and increasing post-remedial concentrations are observed in Site wells. Please include an assessment of remedial approaches in the next monitoring report if benzene concentrations continue to increase or fail to drop below the LTCP thresholds.