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based on the lithology of the borings. Two of the wells DMW-10 and DMW-11, would be located <br /> approximately 150 and 300 feet downgradient of DMW-9, espectively. DMW-9 is located approximately <br /> 150 feet downgradient of the former waste oil UST, and is screened between 60 to 70 feet bgs. <br /> Concentrations of 1,2-DCA in DMW-9 have ranged from approximately 20`to 30 µg/L during the past <br /> several quarters,the highest concentrations currently reported in site wells. <br /> I <br /> i <br /> The work plan also proposed the installation of DMW-12 and DMW-13 to address data gaps in the lateral <br /> distribution of 1,2-DCA to the northeast and northwest ofe source area, respectively. f <br /> I <br /> SJCEHD Comments on U S Work Plan <br /> Comments contained in the December 3, 2007 work plan approval letter provided by SJCEMD approved <br /> of the installation of DMW-10, DMW-12, and DMW 3, but recommended that DMW-11 not be <br /> installed at this time, pending results from DMW-10. <br /> SJCEHD found that 1,2-DCA had not been adequately delineated in the shallow groundwater zone <br /> between approximately 32 to 52 feet bgs. SJCEHD found that only two wells were screened within this <br /> depth zone, DMW-6, screened 32 to 52 feet bgs, and DMW-7, screened 30 to 65 feet bgs. SJCEHD <br /> recommended additional wells within this depth zone. <br /> SJCEHD found that vertical delineation of 1,2-DCA in the depth range of 80 to 90 feet bgs is not <br /> convincingly demonstrated by the non-detect data from DMW-8, screened from 80- to 90-feet bgs, <br /> because DMW-8 is not located in source area. SJCEHD recommended collection of Hydropunch samples <br /> from a direct push boring located within the source area. <br /> URS Responses to SJCEL Comments <br /> URS agrees with the recommendation to postpone installation of a monitoring well at the previously <br /> proposed location of DMW-11 pending results from DMV-10. URS plans to proceed with permitting, <br /> installation, and sampling of the wells DMW-10, DMW-12, and DMW-13 which have been approved by <br /> you. <br /> With regard to delineation of 1,2-DCA within the depth range of 32- to 52-feet bgs, URS agrees that only <br /> two wells, DMW-6 and DMW-7, are screened in this interval. Data from in-situ samples collected from <br /> direct-push borings show a decrease in 1,2-DCA concentration within this interval, reaching non-detect <br /> approximately 270 feet downgradient from the source area (HP-10, 52 feet bgs). URS proposes that this <br /> result and the downgradient extent of 1,2-DCA in this depth range be confirmed by installing an <br /> additional monitoring well, DMW-11, to be located adjacent to DMW-10. .Monitoring wellW-11 <br /> would be completed with a 10=foot sc`�-�reen'1~laced wikhin"the 32to1F52=footbepth range basedon <br /> lithology encountered in the �orin forJDMW:10! The revised location of DMW-11, together with the <br /> proposed locations for DMW-10, DMW-12, and DMW-13 are shown on Figure 1. <br /> Regarding the recommendation for a direct-push boring to the 80- 90-foot depth range in the source area, <br /> experience has shown that the depth of penetration of dir ct push at this site is problematic. Only two <br /> URS Corporation y <br /> 2870 Gateway Oaks Drive,Suite 300 J <br /> Sacramento,CA 95833 <br /> Tel:916.679.2200 <br /> Fax:916.679.2900 <br />