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To address the data gaps in lateral distribution of 1,2-DC , the work plan proposed installation of four <br /> groundwater monitoring wells to be completed with Il f o scre��within the 45 to 70 foot bgs interval, <br /> 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, respectively. 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 pg/L during the past <br /> several quarters, the highest concentrations currently reporte in site wells. <br /> The work plan also proposed the installation of DMW-12and DMW-13 to address data gaps in the lateral <br /> distribution of 1,2-DCA to the northeast and northwest of the source area, respectively. <br /> SJCEHD Comments on US Work Plan <br /> Comments contained in the December 3, 2007 work plan a:proval letter provided by SJCEMD approved <br /> of the installation of DMW-10, DMW-12, and DMW-13, 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, DM - n b s, and R_-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 he depth range of 80 to 90 feet bgs is not <br /> convincingly demonstrated by the non-detect data from DMW78 screened from 80- to 90-feet bgs, <br /> because DMW-8 is not located in source area. SJCEHD re ommended collection of Hydropunch samples <br /> from a direct push boring located within the source area. In a telephone conversation on February 9, 2008, <br /> SCEMD indicated that vertical delineation provided by la non-detect result would be most strongly <br /> supportive of eventual site closure. <br /> URS Responses to SJCE Comments <br /> URS agrees with the recommendation to postpone installation of a monitoring well at the previously <br /> proposed location of DMW-1l pending results from DM -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 concentr,ition within this interval, reaching non-detect <br /> approximately 270 feet downgradient from the source area I(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, DL M11. to be located adjacent to DMW-10. Monitoring well DMW-11 <br /> would be completed with a ,�...+. Pri, placed within Ithe 32-to_.5.2-f� oot bgs depth range based on <br /> URS Corporation �QY^ ^' t� f V_IV `P *' l� 5 ��,T1i� <br /> 2870 Gateway Oaks Drive,Suite 300 r *� <br /> Sacramento,CA 95833 L� <br /> Tel:916.679.2200 <br /> Fax:916.679.2900 <br />