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_ _.� -�_". � �r fiver-. ;.,:A,• y; ; _ d I � 2.0 SITE CONCEPTUAL MODEL The SCM is an evolving compilation of the status of understanding of existing conditions. It is presented as plan view maps of groundwater hydraulic conditions, known extent of 1,2-DCA contamination, and time versus concentration plots showing 1,2-DCA concentration trends in groundwater. Geologic cross sections showing well construction, litbology, and contaminant sources are also included. Altogether, the SCM components provide a framework for assessing i data gaps and planning further site assessment activities. 2.1 Existing Groundwater Monitoring Wells Three groundwater-monitoririg wells,MW-1, MW-2, and MW-3, were installed by Kleinfelder in �b'k June 1987. As a result of falling water levels, all but one of the wells was dry by January 1989, ,v� AXON and in July 1990, the wells were abandoned and replaced with DMW-1, DMW-2, and DMW-3 by 0 V11 Dames &Moore. With the installation of DMW-8 and DMW-9 in March 2004, nine, 1 5 groundwater-monitoring wells have been completed at the Site. C�.Y1/ Following the installation of DMW-8 and DMW-9, all of the wells were re-surveyed by CSS Environmental Services on June 18, 2004. The survey purpose was to provide control for the new wells and to bring the site survey data into compliance with State regulations. The survey provided horizontal and vertical 1 control data in accordance with GeoTracker requirements:The survey results indicated that prcvioUj elevations hadbeer�i ._error on e�a��sli¢htly ore than 2�fh_e_t.. Corrected well elevations are shown in Table 1, together with other well construction details. Site water level elevations have been calculated on the basis of the corrected well elevations beginning with the June 14, 2004, groundwater monitoring round and are reflected in Table 2. Monitoring wells DMW-1 through DMW-4 are screened from approximately 65 to _feetaael�&. ground surface fb s). To accommodate rising water levels DMW-5 through DMW-7 were installed at shallower depths and screened from 51 to 71 feet bgs,32 to 52 feet bgs, and 30 to 65 feet bgs respectively. Monitoring well DMW-8 was screened from 80 to 90 feet bgs and DMW-9 from 60 to 70 feet bgs to investigate possible 1,2-DCA contamination at those depths. A.IS-foot screened interval was used for monitoring wells DMW-1 through DMW-5x Monitoring wells DMW-5 and DMW-6 are screened over 20-foot intervals. DMW-7 is screened over 30 feet, L and 10-foot screens were used in DMW-8 and DMW-9. t"P 2.2 Groundwater Elevations and Gradient Historical groundwater elevation data for the Site are presented in Table 2, and a groundwater elevation contour map for the first quarter 2007 monitoring round is provided as Figure 3. A plot of groundwater elevations versus time for DMW-1 is provided on Figure 4. 2.2.1 Groundwater Gradient ' The groundwater gradient on March 28, 2007 was toward the north-northeast at a gradient magnitude of 0.001 feet per foot. An examination of quarterly groundwater monitoring datacol . nning 4 shows at the ent direction has ly to helected since thenorth-northeast orlthe past three years, at magnitude ranging from 0.011 to 0.O�l�et per foot. 3