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ENVIRONMENTAL HEALTH DEPARTMENT SAN JOAQUIN COUNTY Unit Supervisors Donna R.Heran,R.E.H.S. 304 East Weber Avenue Third Floor Carl Borgman,R.E.H.S. Director Mike Huggins,R.E.H.S.,R.D.I. Al Olsen,R.E.H.S. Stockton, California 95202-2708 Douglas W.Wilson,R.E.H.S. • cq., NSP • Program,Manager Telephone: (209) 468-3420 Margaret Lagorio,R.E.H.S. �rFOR Laurie A.Cotulla,R.E.H.S. Fax: (209) 464-0138 Robert McClellon,R.E.H.S. Program.Manager Mark Barcellos,R.E.H.S. JOE ALDRIDGE JUN 0 3 2002 ULTRAMAR INC 685 WEST THIRD ST HANFORD CA 93230 RE: Former Beacon #12474 SITE CODE: 1173 3440 E. Main St. RO#: 00066 Stockton, CA., 95205 San Joaquin County Environmental Health Department (EHD) has reviewed the "Problem Assessment Report / Corrective Action Plan" dated March 18, 2002, as submitted by Horizon Environmental, Inc. on April 4, 2002, and has the following comments. The Problem Assessment Report (PAR) part of this combination report discusses the investigative phase for this leaking underground storage tank (UST) site by describing the distribution of petroleum impacted groundwater, the mass of petroleum left in soil and groundwater, and the results of a sensitive receptor search. The Corrective Action Plan (CAP) part of this report discussed the feasibility of multiple remedial alternatives. Conclusions included within the PAR section of the report address the lateral limits of petroleum contamination but did not include a discussion of the vertical extent of the soil impact. EHD has reviewed the data and has concluded that soil contamination is known to extend fo 112 feet below ground surface (' bgs), with an abrupt concentration.decline between 85' and 95' bgs. Low levels of petroleum were present in soil samples collected at the maximum depth investigated. The lateral extent of impacted soil appears to be virtually defined but with a significant concentration of benzene (5,900 ug/1) in MW-12, the deepest screened well for this site, the vertical extent of groundwater contamination is not defined. There does not appear to be significant barriers to vertical migration with few clay units drawn in cross-sections included in the report. The clay units depicted in the cross-sections are shown with limited lateral extent and the potential for known petroleum contaminants to migrate downward exists. Pumping domestic wells are located at 400 and 600 feet from the site. Perforated sections are completed from 72' to 125' bgs in one well and 168' to 188' bgs in the other. These wells are not currently monitored for fuel contaminants and the actual status of these wells is not clear to EHD. Without onsite vertical extent determined, a potential threat to these receptors may exist.