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i quarterly reports have been received and evaluated,PHS-EHD will make a decision as to the future need of this well" A copy of the May 6, 1996,letter is attached as Appendix A • A groundwater monitoring well(MW-1)was installed at the site on July 30, 1996 The lithologic log of well MW-1 is attached as Appendix B The geology at well MW-1 includes approxunately 16 feet of interbedded silty sands Thin layers of moderately dense silty clays are present at 0 5 to 1 foot in depth and 14 5-16 feet in depth At approximately 16 feet below ground surface,a saturated clayey-silty sand is present The borehole was terminated in this clayey silty sand Depth to groundwater in well MW-1 has ranged from 8 89 to 12 37 feet below ground surface(as measured from the top of the casing) MW-1 was sampled on July 30, 1996, November 15, 1996, and March 19, 1997 Table 2 presents historical groundwater data Samples collected from well MW-1 during the most recent quarterly sampling events (November, 1996 and March, 1997), indicate that petroleum hydrocarbon constituents are below analytical detection limits, with the exception of methyl t-butyl ether (MTBE) MTBE has been identified in well MW-1 in minor / concentrations The values of MTBE identified in well MW-I are less Lan or equal to the California Dnnking Water d Action Level of 35 pg/L set by the California State Department of Health Services The March 1997 groundwater sampling data(field data sheets,laboratory reports,and chain of custody documentation)are presented in Appendix C In order to meet closure requirements of PHS-EHD, BC has calculated the mass of benzene remaining in the soil Figure 2 presents a graphical representation of this mass The mass of benzene remaining in the soil was determined using the methodology and assumptions presented in Appendix D The estimated mass of soil impacted with bengene- 0 008 pounds BC has also prepared a Case Closure Checklist, required by the Central Valley Regional Water Quality Control Board(Regional Board) The Case Closure Checklist is included as Appendix EJ In addition, BC conducted a'/z mile well search to identify potential receptor(Four domestic/irrigation wells exist within '/x mile of the site The closest,yell is an tiTigation well approximately 1,400 feet northwest of ARCO Facility 5450 Appendix F contains copies of the Departmenf of Water Resources Water Well Drillers Report for the four domestichrrigation wells Figure 3 presents the approximate locations of the four wells A number of monitoring wells exist in association with the McCormick & Baxter superfund site located approximately %< mile • south of the ARCO facility Based on the results of two consecutive quarters of groundwater monitoring, the mass balance calculations, and the case closure checklist, ARCO has met the requirements for closure at this site Therefore, ARCO requests that PHS-EHD issue a"No Further Action" letter upon concurrence from the Regional Board ATTACHMENTS Figure I Groundwater Constituent Concentration Map Figure 2 Mass of Residual Benzene in Soil Figure 3 Wells Within ''/2 Mile Radius Table I Summary of Soil Analytical Results Table 2 Summary of Groundwater Monitoring Data Water Elevations and Analyses Appendix A PHS-EHD Letter Dated May 6, 1996 Appendix B Lithologic Log for MW-1 Appendix C Field Data Sheets, Laboratory Reports,and Chain-of-Custody Documentation Appendix D Methodology and Assumptions for Soil Mass Balance Calculations Appendix E Case Closure Checklist Appendix F Department of Water Resources, Water Well Drillers Report Use or disclosure of data continued on this sheet is subject to the restriction spectfied at the beginning of this document 48701REPORTS11ST97QTR DOC I