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ENVIRONMENTAL HEALTH DEPARTMENT . au � SAN JOAQUIN COUNTY e :,y Donna K, Heran, R.E.H.S. Unit Su Director pervisors 304 East Weber Avenue,Third Floor Carl Borgman,R.E.H.S. Laurie A. Cotuila, R.E.H.S. Stockton,California 95202-2708 Mike Huggins, R.E.H.S., R.D.I. Program Manager Douglas W.Wilson,R.E.H.S. tJ0Ra Telephone:(209)458-3420 Margaret I.agorio,R.E.H.S. Fax: (209) 454-0138 Robert McClellon,RB.H.S. Website:rvww.sjgov.org/chd/ Jeff Carruesco,R.E.H.S. PAUL SUPPLE MAR 01.2006 ATLANTIC RICHFIELD COMPANY M HAMZEH Y HAJIAN PPT PO BOX 6549 111 QUINTAS LANE MORAGA CA 94570 MORAGA CA 94556 RE: Former ARCO Station #548 SITE CODE: 1127 1612 Hammer Lane Stockton CA 95209 San Joaquin County Environmental Health Department(SJCIEHD) has reviewed Site Conceptual Model Update Report(SCM)dated November 23, 2005 submitted on your behalf by URS Corporation (URS)and has the following comments. Two additional groundwater monitoring wells, (MW-7 and MW-8), were installed in December 2004 for use at this site. These wells were surveyed to mean sea level and it appears that elevation measurements from MW-7 have been incorporated into the calculation of the groundwater gradient during the routine quarterly monitoring events. However, monitoring wells MW-1 thru MW-6 are not screened across the same interval, as MW-7, and since the elevations from MW-7 have been used in the gradient calculations a shift in the flow direction has been noted,from predominately east to predominately west/southwest. SJCIEHD does not consider it valid to be calculating the gradient and flow direction in this manner. MW-7 is screened similar to the vapor wells (VW-1 thru VW-5), which are the only other wells on site whose screen intervals intercept first groundwater. However, it does not appear that the vapor wells were ever surveyed to mean sea level, SJCIEHD directs that all site wells be resurveyed to the same benchmark, and a recalculation of the gradient and flow direction from past quarterly events, beginning with January 2004, be completed. Unless you can show that MW-7 is hydraulically equivalent to MWA thru MW-6, two calculations and maps should be completed, one using measurements from MW-7 and VW-1 thru VW-513, and one using measurements from MW-1 thru MW-6. The SCM concludes that the vertical and lateral extent of the groundwater contamination 7` has been defined. The vertical extent of the groundwater contamination is defined in the former tank area by MW-8, which has been non-detect for petroleum hydrocarbon constituents analyzed over the last three reported groundwater monitoring and sampling events. During the first three monitoring and sampling events there were reportable concentrations of contaminants in this well. Petroleum hydrocarbon contamination was detected in soil samples collected from,the MW-8 boring from 45 feet below surface grade (bsg)to total depth of 98 feet bsg. The contaminant concentration trends and ratios suggest to SJC/EHD that these results may be due to cross contamination of the soil samples. The lateral extent of the groundwater contamination has not been defined towards the east. Concentrations of methyl tertiary butyl ether(MtBE) above the primary 1a ,s 4 Uti • r-