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Work Plan for Additional Assessment ARCO Service Station 2130 Stockton, California Page 3 o 15 SENSITIVE RECEPTORS Two active water supply wells are located within 2,000 feet of ARCO Station 2130. The wells are operated by the City of Stockton Municipal Utilities Department (CSMUD) . City Well #4 is located approximately 1 ,200 feet south southeast of the site on Segovia Lane. It is 600 feet deep and has a production capacity of 1 .5 million gallons per day (MGD) . City Well #12 is located approximately 1 ,600 feet south of the site at 341 Leslie Avenue. It is 410 feet deep and has a production capacity of 0.82 MGD. No other wells were located within 2,000 feet of the site (URS, 2004) . URS performed a sensitive receptor survey of the site in 2004. No surface water bodies, schools or wildlife habitats were identified within 500 feet of the site. The nearest school identified is Pulliam Elementary located approximately 1 ,300 feet southeast of the site. CHARACTERIZATION STATUS Hydrocarbons in the unsaturated zone appear to be confined primarily to the former UST area . The lateral extent of impacted soil generally increases with depth , particularly in the area beneath the pump islands. However, because the water table has risen to approximately 40 feet bgs, the impacted soil locations are now below the water table. The mass of TPHg in soil prior to remedial action at the site was calculated to be 10,782 pounds of TPHg . The operation of the SVE system removed approximately 9, 109 pounds of TPHg . Therefore , a net of 1 ,673 pounds of TPHg remain at the site (URS, 2004) . The remaining contamination may be present in either the soil or groundwater. The historical direction of groundwater flow beneath the site has been variable (reported toward the southeast, northeast, and east) . Review of historical groundwater monitoring data (since mid-2001 ) indicates that flow has predominantly been toward the southeast. TPHg impacted groundwater is generally confined to the vicinity of the dispenser islands and the UST complex. The historic maximum dissolved concentration of GRO was 340,000 micrograms per liter (Ng/L) in vapor well VW- 1 in May 1997. During the third quarter 2006, the maximum dissolved GRO concentration was 21 , 000 pg/L in VW- 10. Historically, downgradient well MW-6 has contained relatively low concentrations of GRO. However, the highest concentration detected was in second quarter 2006 at 2,800 pg/L. Fourth quarter 2006 GRO concentrations in MW-6 were 98 pg/L. Benzene concentrations in groundwater are primarily located in the former UST and dispenser island areas. The maximum benzene concentrations reported in groundwater have been in vapor extraction wells VW-8 and VW- 10, which are located approximately 20 feet south-southeast of the dispenser islands. The maximum benzene concentration detected during the fourth quarter 2006 was 1 ,300 pg/L in VW- 10. The lateral extent of methyl tert butyl ether (MtBE) in the shallow water bearing zone is delineated by the existing monitoring well network. Concentrations up to 14. 1 pg/L (9/2/99) have been reported in off- site well MW-6, which is located downgradient from the pump islands and UST complex. However, MtBE has not been detected in MW-6 since December 2003, suggesting that MW-6 currently delineates the downgradient extent of the dissolved MtBE. 1 ,2- DCA concentrations in groundwater are primarily located in the dispenser island areas and east of the USTs. The maximum 1 ,2-DCA concentration reported in groundwater was in vapor extraction well VW-5 (660 pg/L) in March 2003. The maximum 1 ,2- DCA concentration detected during the fourth quarter 2006 was 270 pg/L in VW-5.