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_ yk NvIR01�;1ENVAij <br /> J AN JOAQUIN COUNTY Unit Supervisors <br /> puiry <br /> Third Floor Carl Borgman>R.E.H.S. <br /> Donna K. Herau,R.E.H.S. 304 East Weber Avenue, ins,R.E.H.S.,R.D.I. <br /> Mike Huggins,California 95202-2.708 las W.Wilson,R.E.H.S. <br /> Q :{ Director <br /> Stockt011; Doug <br /> �'. Laurie A. Cotulla, R.E.H.S. 209 468-3424 Margaret Lagorio,R.E.H.S. <br /> Program Manager Telephone'. Robert McClellon, R.E.H.S. <br /> • �4 = a•p Fax: (249) 464-0138 <br /> �rFOR Jeff Carruesco,R.E.II.S. <br /> Website:www.sjgov.org/elid/ <br /> MAR 01200 <br /> PAUL SUPPLE <br /> ATLANTIC RICHFIELD COMPANY M HAMZEH Y HAJIAN PPT <br /> PO BOX 6549 111° QUINTAS LANE <br /> MORAGA CA 94570 MORAGA CA 94556 <br /> RE: Former ARCO Station #548 SITE CODE: 1127 <br /> i 1612 Hammer Lane <br /> Stockton CA 95209 <br /> j San Joaquin County Environmental Health Department'(SJC/EHD) has reviewed Site. <br /> Conceptual Model Update.Report(SCM) dated November 23;�2005 submitted on your <br /> behalf by URS Corporation (URS) and has the following'comments. <br /> Two additional groundwater monitoring wells, (MW-7 and W18), were installed in <br /> December 2004 for use at this site. These wells were sbrveyed to mean sea level and it <br /> appears that elevation measurements from MW-7 have been incorporated into the <br /> calculation of the groundwater gradient during the routine quarterly monitoring events. <br /> However, monitoring wells MW-1 thru MW-6 are not screened`across the same interval <br /> as MW-7, and since the elevations from MW-7 have been used in the gradient <br /> �. calculations a shift in the flow direction has been noted,.from predominately east to <br /> predominately west/southwest. SJCIEHD does not consider it'valid to be calculating the <br /> gradient and flow direction in this manner. MW-7 is screened similar to the vapor wells <br /> (VW-1 thru VW-5), which are the only other wells on site whose screen intervals <br /> intercept first groundwater. However, it does not appear,that the vapor wells were ever <br /> surveyed to mean sea level. SJCIEHD directs that all site wells be resurveyed to the <br /> same benchmark, and a recalculation of the gradient and flow direction from past <br /> quarterly events, beginning with January 2004, be completed. ;Unless you can show that <br /> MW-7 is hydraulically equivalent to MW-1 thru MW-6, two calculations and maps should <br /> f be completed, one using measurements from MW-7 and VW-1,thru VW-5D, and one <br /> { using measurements from MW-1 thru MW-6. <br /> The SCM concludes that the vertical and lateral extent of the groundwater contamination <br /> # has been defined. The vertical extent of the groundwater contamination is defined in <br /> the former tank area by MW-8, which has been non-detect for petroleum hydrocarbon <br /> constituents analyzed over the last three reported groundwater:monitoring and sampling <br /> events. During the first three monitoring and sampling eV"ents there were reportable <br /> concentrations of contaminants in this well. Petroleum hydrocarbon contamination was <br /> detected in soil samples collected from the MW-8 boring from 45 feet below surface <br /> grade (bsg) to total depth of 98 feet bsg. The contaminant concentration trends and <br /> ratios suggest to SJCIEHD that these results may be due to cross contamination of the <br /> soil samples. The lateral extent of the groundwater contamination has not been defined <br /> towards the east. Concentrations of methyl tertiary butyl ether (MtBE) above the primary <br /> i <br />