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Section 3 Field Investigation Results The following section discusses the results of the soil and groundwater investigations detailed in the previous section. It includes a presentation of the aquifer parameters obtained from the pump test and a description of contaminant plume migration at the site. 3.1 Lithologic Interpretation - The geology of the site and surrounding area has been compiled from lithologic logs of borings drilled during this investigation and previous investigations(EMCON 1991,1995;Twinning Lab 1996). Three cross sections showing the subsurface layering were drawn such that most of the new borings(denoted ARL-XX)are included. The locations of these cross sections are shown on Figure 2-1. The sediments underlying the Austin Road Landfill and the surrounding area consist of interlayered sand,silt,and clay with occasional beds of coarse sand and gravel. The observations in this investigation are consistent with those of previous investigations which characterize the sediments as fluvial(river-borne)deposits. In the geologic cross sections(Figures 3-1 to 3-3), sediments are grouped into three units according to relative permeability,meaning the degree to which the sediments can conduct and transmit groundwater: High permeability: gravel,well-graded and poorly-graded sand Medium permeability: silt and silty sand Low permeability: high plasticity and low plasticity clay and clayey sand The clay and clayey sands,considered to be of low permeability,essentially act as barriers to groundwater flow while the silt,sand,and gravel allow increasingly greater groundwater flow. - Because the soil layers are highly interbedded over the scale of the site,and since information collected by different investigators and over different times have been combined in this analysis, there is some uncertainty in correlating individual layers between boreholes. In general,layers of similar permeability(as defined by the lithologic groups above)are considered continuous across several boreholes when materials of similar lithology and composition were encountered at similar depths. Additional evidence for continuity of layers is water quality data. Assuming that the compounds detected in all existing wells and borings have a common point of origin(i.e.the landfill),these compounds can be used as secondary or supporting evidence that the hydrologic units are in hydraulic connection with each other. This premise was useful in constructing cross section A-A'(Figure 3-1)which is oriented north-south,and provides a detailed interpretation of the hydrogeologic conditions at ARL. In areas where geologic information was insufficient, groundwater quality results were used to interpret subsurface conditions. In general,clay and clayey sands seem to be abundant in the vadose(unsaturated)zone,down to about-40 feet mean sea level(msl). Near the landfill,the interval including clay demonstrates CDM Camp Dresser&McKee 3-1 W:IREPORTSiSTOCKTOMARLPLUME.98\SEC3.WPO CS 010766 City of Stockton 1908554006