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.� r � ,wd"4 '1C'ory„7s .e '�d� '1 +'4 '{�:'.W s». 1^•.� 'q � x 'm�:»"� <br /> F i <br /> CHAPTER 5 <br /> CONCLUSIONS AND RECOMMENDATIONS <br /> a. Based on the results of previous investigations by Gettler-Ryan, <br /> Inc. (GR) and Emcon Associates (EMCON) , and current investigations by <br /> Brown and Caldwell 'BC) , the following conclusions have been made in <br /> +s regards to KxCO Petroleum Products Company (ARCO) Station 2133, <br /> Stockton. <br /> FA Groundwater <br /> Shallow groundwater beneath the site has significant levels of <br /> �•j dissolved hydrocarbons, There is no floating hydrocarbon product <br /> remaining on the groundwater. The Clay 11 and 12, 1987, groundwater <br /> samples collected from the existing monitoring wells show high <br /> concentrations of benzene, toluene, xylene, and total petroleum fuel <br /> hydocarbons. A comparison of BC's May 11 and 12, 1987, groundwater <br /> • sampling results with EMCO[rs October 10, 1985, results (EMCGN report <br /> included as Appendix A) shows significant changes in the hydrocarbon <br /> rconcentrations in the groundwater monitoring wells. There is a <br /> general patten indicating that the recovery system has been <br /> effective in drawing the hydrocarbons toward the recovery well. The <br /> hydrocarbon ,concentrations are highest in Well E-3, which is closrest <br /> to the recovery well. Hydrocarbon concentrations gradually decrease <br /> -• with greater distance from the recovery well. This is true for all <br /> of the welly except Wells E-9 and E-10, located at the northern-most <br /> +' part of the site. Well �,-9 has higher hydrocarbon concentrations on <br /> ey May 11 and 12, 1987, than it did on October 10, 1985. Well E-10 had <br /> floating product in October 1985, but still had very high hydrocarbon <br /> +a concentrations in May 1987. The recovery system seems to have had <br /> a� little effect on these wells, ',hick indicates that they may be <br /> influenced by a possible product source to the north of <br /> tt the site. -1 <br /> Municipal Well Water ` <br /> The water samples collected from the municipal water supply <br /> wells show no detectable hydroLarbons. However, 1.3 micrograms per <br /> !s li-ter (ug/1) of toluene was found in the sample from Well 20L1. <br /> Although toluene was ietected in Well 20LI, it is thought to be a <br /> sampling error. The sam}:)_e is thought to have been contaminated <br /> during its collection. Because of the depth of screened intervals <br /> and the location of sanitary surface seals of these municipal wells, <br /> there is little threat of contamination due to shallow groundwater <br /> containing high levels of hydrocarbons entering these wells. <br /> tau •. <br /> '1F <br /> �llti` w <br />