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76 Service Station No. 11190Ri �� 5 October 2016 <br /> 1206 East March Lane {,f <br /> Stockton, San Joaquin County 06 2016 <br /> approved by SJCEHD staff in a let related 25 June'2012. The original scope of work proposed <br /> installation of two wells, which haX&'�J iodifled to a single well due to the presence of <br /> multiple underground utilities in the City of Stockton right-of-way, and difficulties obtaining <br /> access agreements from private property owners. Petroleum hydrocarbons were not detected <br /> in soil samples or the initial groundwater sample collected from MW-10. <br /> Central Valley Water Board staff have the following comments on the investigation of pollutant <br /> impacts from this Site: <br /> 1. In November 2012, a CPT boring was advanced at the Site. The grab groundwater <br /> sample from CPT-1 at 76 to 80 feet below ground surface (ft bgs), contained total <br /> petroleum hydrocarbons (TPH) as gasoline (TPH-G) at 1,100 ug/L, MTBE at 4,800 ug/L, <br /> and TBA at 1,000 ug/L. Site wells are screened from 40 and 64.5 ft bgs. Based on <br /> available data, deeper groundwater has been impacted by petroleum hydrocarbons. <br /> Installation of groundwater monitoring wells is needed to monitor deeper groundwater <br /> and to define the lateral and vertical extent of petroleum impacts. Therefore, by <br /> 15 December 2016, submit a Site Assessment Work Plan presenting a scope of work <br /> for installation of a sufficient number of wells to define the lateral and vertical extent of <br /> groundwater pollution. At a minimum, three groundwater monitoring wells need to be <br /> installed and screened across the 76 to 80 ft bgs depth, to allow for calculation of <br /> groundwater gradient and flow direction in deeper groundwater. One of these deeper <br /> wells should be installed in the same approximate location as CPT-1. <br /> 2. The lateral extent of petroleum hydrocarbon impacts to groundwater has not been <br /> defined in shallow groundwater associated with the wells screened from 40 to 64.5 ft <br /> bgs. During the second quarter 2016, TBA was detected in well MW-5, located along <br /> the eastern side of the station property, at 3,200 ug/L. The WQO for TBA is 12 ug/L. <br /> During the fourth quarter 2015, the groundwater flow direction was calculated to be to <br /> the east-northeast; a groundwater flow direction was not calculated for the second <br /> quarter 2016. There are no groundwater monitoring wells east, north, or south of MW-5. <br /> Well MW-3 is located to the southeast of MW-5, but is screened across a shallower <br /> interval. <br /> MTBE was detected in well MW-7 at 1,600 ug/L during the fourth quarter 2015, and at <br /> 620 ug/L during the second quarter 2016. Groundwater pollution is not defined to the <br /> north or northeast (down-gradient) of well MW-7. Well MW-10, located northwest of <br /> MW-7 was installed in June 2016. Soil and initial groundwater samples, collected in <br /> June 2016, did not contain petroleum hydrocarbons. Based on the results from well <br /> MW-10, the groundwater pollution plume appears to be defined to the northwest of <br /> MW-7. Additional delineation is needed to the north and northeast (down-gradient ) of <br /> MW-7, based on the fourth quarter 2015 groundwater flow direction calculation. <br /> Base on available Site data, installation of additional shallow monitoring wells (from 40 to <br /> 64.5 ft bgs) are needed to south and east of MW-5, and to the north and northeast of <br /> MW-7. Therefore, please include a scope of work in the Work Plan due <br /> 15 December 2016, to complete lateral delineation of shallow groundwater. <br /> 3. In the 4Q2015 Report, ATC included results of a sensitive receptor survey performed in <br /> August 2011. The closest identified well was located approximately 1,250 feet west of <br />