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FORMER AMERICAN FOREST PRODUCTS FACILITY Section No.4.0 Additional 1,2-DCA Investigation and Site Conceptual Model February 2005 • And Second Quarter 2004 Quarterly Monitoring Report Page 4-1 4.0 SITE CONCEPTUAL MODEL air This section presents a site conceptual model based on historical groundwater data with an emphasis on the most recent four quarters ending with the second quarter 2004. The site conceptual model includes a discussion of lithology, groundwater gradients, and vertical and horizontal extent of groundwater contamination. These discussions are supplemented with groundwater elevation contour maps and plots, and isoconcentration maps R 4.1 SITE GEOLOGY Lithologic data for the site has been collected from exploratory soil borings and borings for monitoring well installation. Boring logs for monitoring wells DMW-8 and DMW-9 are included in Appendix A; and a lithologic cross-section, interpreted from well installation boring logs and soil boring information, is presented in Figure 3. fi 4 The upper 80 feet of soils underlying the site are composed of interbedded clays, silts, and fine sands. In general, the upper 15 to 25 feet of sediments consist predominantly of dense clayey silts and silty clays with locally discontinuous fine sand layers grading into fine sandy silts and silty sands. 4.2 GROUNDWATER ELEVATION AND GRADIENT w Y .. All of the site's monitoring wells were resurveyed on June 18, 2004,by CSS environmental Services,Inc. The purpose of the survey was to provide control for the new wells and to bring the site survey data into compliance with State regulations. California has adopted regulations (California Code of Regulations „ [CCR] Title 23, Division 3, Chapter 16, Article 12) that require all responsible parties to submit compliance data, including analytical results, location and elevation data electronically, through the State Geographical Information system known as GeoTrackerrm. The survey provided horizontal and vertical control data in accordance with GeoTrackerT7”requirements. The new survey yielded elevation data that were slightly over 2 feet greater than the previous survey data (Table 2). This may be attributable to referencing the previous survey to an inaccurate local benchmark. As shown in Table 2, the new survey data are used to calculate water-level elevations for wells DMW-I through DMW-7 beginning with the June 2004 monitoring round and for wells DMW-8 and DMW-9 beginning with the March monitoring round. Figure 4 presents groundwater elevation contours for the third and fourth quarters of 2003, and the first and second quarters of 2004. Graphs of groundwater elevations versus time are provided in Appendix D. ., Water level elevations calculated from the new survey data were used for Figure 3 and the graphs in Appendix D. The groundwater elevation contour maps for September 2003 and June 2004 show a northerly ground- water gradient of approximately 0.010, which is consistent with the historical record for the site since 1 1990. The contours based on water-level measurements collected in December 2003 and March 2004, which represent the rainy season,are more irregular,possibly because of local recharge. 4.. KAWprocess1227361American ForestlWell installation Report revOl.doc llnr