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n Date: July 27,2004 Quarter 2nd Quarter,2004 • ARCO QUARTERLY GROUNDWATER MONITORING REPORT Facility No 6080 Address 85 E Louse Avenue, Lathrop, CA ARCO Environmental Engineer Paul Supple Consulting Co/Contact Person URS/Jeffrey Brooks Consultant Project No 38466542 OL058 Primary Agency/Regulatory ID No San Joaquin County Environmental Health Department Assembly Bill 681 Status Property Owned by Atlantic Richfield Company WORK PERFORMED THIS QUARTER (Second 2004): 1 Performed quarterly groundwater monitoring and sampling 2 Prepared and submitted quarterly groundwater monitoring report. WORK PROPOSED FOR NEXT QUARTER (Third 2004): 1 Perform quarterly groundwater monitoring and sampling 2 Prepare and submit quarterly groundwater monitoring report QUARTERLY RESULTS SUMMARY (Second 2004): Current Phase of Project Monitoring Frequency of Groundwater Sampling MW-1 through MW-3 sampled quarterly Frequency of Groundwater Monitoring Quarterly Is Free Product (FP) Present Onsite No Current Remediation Techniques Natural attenuation Approximate Depth to Groundwater 9 42 to 10 19 feet below ground surface Groundwater Flow and Gradient East @ 0 002 feet/foot (ft/ft) ANALYTICAL DATA REVIEW: Please note that as of April 1,2004,the laboratory has changed the carbon range for gasoline range organics(GRO) from C6 — C10 to C4 — C12 (to include the oxygenates) The resulting data may be impacted by the potential inclusion of additional total petroleum hydrocarbons as gasoline (TPHg) and non-TPHg analytes within the revised carbon range, which may result in a higher reported concentration Where the resulting GRO concentration is based on a single analyte(i e,a non-TPHg analyte or a single TPHg analyte),the analyte will be identified if possible Based on the review of the analytical and hardcopy data (cham-of-custody records, blanks, dilutions, holding times, LCS/LCSD results, MS/MSD results, and surrogate recoveries), the data are acceptable as reported by the laboratory DISCUSSION: Methyl tert-butyl ether(MtBE) was the only analyte reported above method detection limits in groundwater samples collected from site monitoring wells MW-1 and MW-2 this quarter Concentrations are generally consistent with the previous monitoring event K SWpmccssVS SP ARC05 WSOQualerlyl2 QTR 0456080 GWMR 2Q04 dot Page 1