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Californi IV egional Water Quality Cool Board <br /> 4 IIIIZ; Central Valley Region , <br /> Linda S.Adams <br /> SecEnvietaryforronmental <br /> �ij;t/ � Arnold <br /> E Noiecsemol Sacramento Main Office J t ar.,negger <br /> 11020 Sun Center Drive M200,Rancho Cordova,California 9567PbI 14 Governor <br /> Protection phone(916)464-3291 • FAX(916)464-4645 DEChltp:/h %,v,oaterboards ca_goo/centralvellel' C 2 7 2006 <br /> 21 December 2006 P'JV1PGiN10EN_I HEALTH <br /> oE.r a,+rr10`LEE /!CES <br /> Mr. Don Pratt, Environmental Business Manager Mr. Gurpreet and Ms. Kuldeep Dhat <br /> Atlantic Richfield Company 501 S. Cherokee Avenue <br /> P.O. Box 1257 Lodi 35240 <br /> San Ramon 94583 <br /> WORK PLAN APPROVAL, ARCO #0760, 225 S. CHEROKEE LANE, LODI, <br /> SAN JOAQUIN COUNTY (REGIONAL BOARD CASE # 390330) <br /> In a letter (Letter) dated 31 October 2006, California Regional Water Quality Control Board — <br /> Central Valley Region (Regional Board) staff requested a workplan by 15 December 2006 to <br /> continue the Site characterization for all petroleum hydrocarbons, including 1,2-Dichloroethane <br /> (1,2-DCA). The Letter also required an investigation of potential soil vapor intrusion to Site <br /> and neighboring structures, to determine if additional measures are needed to mitigate this <br /> potential risk. On 18 December 2006, Regional Board staff received the Work Plan for Soil <br /> and Groundwater Investigation and Soil Vapor Survey (Workplan) from your consultant Secor <br /> International Incorporated. The Workplan proposes the installation and sampling of three <br /> shallow groundwater monitoring wells (MW-6 through MW-8) and three shallow soil vapor <br /> probes (VP-1 through VP-3) for the investigation (enclosed Figure 2). The depth to water <br /> ranged from 59 to 60 feet below ground surface in July 2006. Groundwater appeared to be <br /> moving towards the southeast. The most recent quarterly groundwater monitoring data (June <br /> 2006, enclosed Table 2 and Figures 3 and 4) show that petroleum hydrocarbons <br /> concentrations in MW-2 increased up to an order of magnitude to a three-year high for: Total <br /> Petroleum Hydrocarbons as gasoline (TPHg, 3,500 micrograms per Liter, or ug/L), benzene <br /> (8.8 ug/L), toluene (3.4 ug/L), ethylbenzene (19 ug/L), xylenes (230 ug/L), MtBE (86 ug/L), and <br /> 1,2-dichloroethane (21 ug/L). <br /> Comments- <br /> 1 . <br /> omments:1 . The Workplan proposes the installation of new offsite monitoring wells to the south, to <br /> the southwest, and to the northwest of the former USTs and dispensers. An additional <br /> monitoring well to the southeast is necessary to delineate the downgradient extent of <br /> the pollution. Please submit an addendum to the Workplan by 12 January 2006 that <br /> includes the installation of this additional groundwater monitoring well. <br /> 2. The purpose of the vapor survey is to address threats from vapor migration to onsite <br /> and neighboring buildings. A residence is located less than 100 feet west of the former <br /> USTs. At a minimum, an additional vapor well is needed at the western property line <br /> adjacent to the residence and south of vapor well VP-3. Please include the installation <br /> of the additional soil vapor well in the addendum required in Comment 1. <br /> California Environmental Protection Agency <br /> zd Recycled Paper <br />