SITE INFORMATION AND CORRESPONDENCE_CASE 2

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ABLE 1 CHECKLIST OF REQUIRED DATA FOR NO FURTHER ACTION REQUESTS AT UNDERGROUND TANK SITES r Site Name and Location: Former Mobil Service Station 99ACA,25355 N. Hwy 99,Acampo, San Joaquin County(RB#399049) Y 1. Distance to production wells for municipal, domestic, agriculture, A 2010 sensitive re' San "unurvey reported 102 industry and other uses within 2000 feet of the site. domestic supply wells(includinrveg onsite) within 2,000' re ease. of the site. None of the wells are threatened by the yr 2. Site maps, to scale, of area impacted showing locationreleas s of any According to the County Assessor, four unknown use former and existing tank systems, excavation contours and sample USTs were removed in 1978 or 1979. In 1988 a new locations, boring and monitoring well elevation contours, gradients, owner excavated soil to confirm USTs removal. and nearby surface waters, buildings, streets, and subsurface During a Phase 2 investigation in 1999,groundwater utilities; monitoring revealed a release of petroleum hydrocarbons had occurred. Y 3. Figures depicting lithology(cross section), treatment system Site lithology consists of clay,silt, and sand to 168, diagrams; the total depth investigated.: N 4. Stockpiled soil remaining on-site or off-site disposal There were no SJCEHD records of the USTs removal and no uanii evidence of stockpiled soil from a 1988 USTs pit excavation. Y 5. Monitoring welts remaining on-site,.fate; Ten(1 0)monitoring wells(MW-1 through MW-10) will be properly destroyed. _ 6.Tabulated results of all roundwater Depth to groundwater varied from 72'bgs to 93'bgs. Groundwater flow = � _ - — - `direction-varied-from�east to-southeast'Groundwater-gradient varied,- elevations and depths to wafer, from 0.0032 ft/ft io,0.007 ft/ft. k 7. Tabulated results of all sampling All data adequately tabularized in various reports, including closure report and analyses: k i Y❑ Detection limits for confirmation sampling Lead analyses 8. Concentration contours of contaminants found and those remaining in soil and The extent of the identified groundwater, and both on-site and off-site: contamination is described in the available reports. FY1Lateral and 0 Vertical extent of soil contamination Lateral and F1 Vertical extent of groundwater contamination 9. Zone of influence calculated and assumptions used for subsurface remediation An engineered remediation was not system and the zone of capture attained for the soil and groundwater remediation required by the regulatory agency- system; gency.s stem; 71 10.Reports/information Unauthorized Release Form FY QMRs(38)3-99 to 12-10 0 Well and boring logs 0 PAR FRP Other Site Conceptual Model& Closure Report 3-11 Mass Calculations, Trend Analysis,and Request for Closure 10-11 Y 11.Best Available Technology(BAT)used or an explanation for not using UST removal and natural attenuation. BAT; Y: s 12. Reasons why background wasps unattainable Residual soil and groundwater contamination remains on-site. BA T,• Y 13.Mass balance calculation of substance treated Initial soil mass was not estimated. Consultant estimated versus that remaining; approximately 70 lbs. TPH remain in soil and 2.2 lbs of TPH remain in roundwater. Y 14. Assumptions, parameters, calculations and A 2010 soil vapor survey passed the Region 2 ESLs for commercial 7 model used in risk assessments, and fate and use.Soil results also passed the ESLs(no USTs removal results). transport modeling; Consultant states site does not represent a significant risk. Y 15. Rationale why conditions remaining at site will Soil and groundwater contamination reportedly are limited in extent. not adversely impact water quality, health, or other Land use(commercial)is not expected to change in the foreseeable benef&pial uses;and - r I future. Groundwater was estimated to reach WQGs by 2023. By: JLB Comments:According to'the County Assessor, four unknown use USTs were removed in 1978 or 1979.An excavation of the former tank pit in 1988 showed the USTs were removed. During a Phase 2 investigation in Date: 1999,groundwater monitoring revealed a release of petroleum hydrocarbons had occurred at the subject 4/23/2012 site. Based upon the limited extent of contamination reported in soil and groundwater, a stable groundwater plume with declining concentrations,groundwater estimated to reach WQGs in 7023, no foreseeable changes in future land use(commercial),and minimal risks from soil,soil vapor,and groundwater,Regional Board staff concur with San Joaquin County's Closure Recommendation- - S . i f