ARCHIVED REPORTS_1999 - Laserfiche WebLink

Home Browse Search

Austin Road Landfill Section 3 Contaminant Plume Characterization Report Field Investigation Results variable thickness,only extending down to 11 feet msl in the eastern half of the site. The western half(as represented by boring logs for MW-9 and EW-2)appears to have more clay and clayey sand—most of the interval logged from the surface to-88 feet msl(125 feet below ground surface [bgs])contained clay and clayey sand. Below the clay-bearing interval is mostly interlayered sands and silts with occasional beds of clay. The only major occurrence of clay is an interval from-80 to-118 feet msl noted in the MW-6 boring log on Figure 3-1. No other borehole logs show such a thick interval of clay;its extent near the landfill is unknown. North of the landfill,as seen in the ARL-boring logs,this clay layer is not present or may be related to the silt layer encountered at approximately-85 feet msl. " The major water-bearing unit beneath the landfill appears to be an interval of sand between about -40 and-80 feet msl. North of the landfill,the sand unit appears to thicken and is interbedded with silt beds (refer to borehole logs ARL-1A and ARLAC on Figure 3-1). The silt layers become thicker and more abundant north of ARL-1A where there appears to be three distinct sand units beneath -40 feet msl. The shallowest unit(encountered between-45 and-55 feet msl in ARL-1B)does not appear to be in hydraulic connection with the underlying intermediate sand unit(encountered in ARL-1B between-65 and-85 feet msl)because it pinches out between ARL-1C and MW-14. This interpretation is supported by the fact that no trichloroethene(TCE),tetrachloroethene(PCE)or other compounds were detected in the sample from this unit(ARL-IA-GW-85). The only water quality data from the intermediate sand unit was collected from ARL-3A(sample ARL-3A-105). No organic compounds were detected in this sample,though it is unclear if this is due to hydraulic isolation or distance from the source(Figure 2-1). The lower unit was encountered between-110 to -138 feet bgs in boring ARL-1Band ARL-3 ,respectively. The vertical extent of this sand layer is unknown because no borings were drilled deeper than-138 feet msl(180 feet bgs). This deeper sand is separated from the intermediate sand unit by a silt layer. The lower sand unit appears to be in hydraulic connection with the upper sand unit in the vicinity of ARL-1C as depicted in Figure 3- 1,as detectable concentrations of TCE and PCE have been found in samples collected from this unit. 3.2 Hydrologic Setting/Groundwater Movement The following discussion incorporates water level data and conclusions from previous investigations and describes the direction and rate of groundwater flow during the field investigations. The depths to water measured in the borings installed by CDM ranged from 61 to 84.6 feet bgs or -24 to-42 feet msl. During the same period,water levels measured in the monitoring wells ranged from-20.11 feet msl(April 1998)to-39.60 feet msl (December 1997). Since 1996,water levels have been measured between-20 and-43 feet msl. Most measurements were between-28 and 43 feet msl with the exception of several high measurements recorded at MW-2 and MW-5. Water elevations are illustrated on the cross sections(Figures 3-1 through 3-3). However,only comparable measurements are shown on the same cross section. On cross section A-A' (Figure 3-1), measurements taken in the borings and monitoring wells between November 1997 and January 1998 are shown. On cross section B-B' (Figure 3-2),measurements taken during boring installations in December 1997 and January 1998 are shown. On cross section C-C' (Figure 3-3),measurements taken during boring installations in May and June 1998 are shown. Water elevations are shown in CDM Cramp Dresser&McKee 3-2 W.\REPORTS\.STOCKTOMARLPLUME.98\SEC3.WPO CS 010768 City of Stockton 1909554M