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Page 99 Current Setting Conditions The following discussion updates the 1994 EIR description of groundwater incorporating information from the more recent studies listed above. All well sampling locations described herein are indicated on Figure K-1. Regional Groundwater Since 1994,above average rainfall and better groundwater discharge management have resulted in more stable groundwater levels. Site Hydrologic Setting and Groundwater Movement The sediments underlying the Austin Road Landfill are consistent with the regional geology consisting of interlayered sand,silt, and clay with occasional beds of coarse sand and gravel. The 'interpretation of the hydrogeology in the various investigations completed since the 1989 SWAT investigation are consistent, and characterize the sediments as fluvial (river-borne)deposits(Kleinfelder, 1988;CDM, 1999A). Figures K-1,K-2,and K-3 show the landfill,monitoring well locations and downgradient receptors. Hydrogeologically the site is underlain mainly by clay and clayey sands in the area above the water table, down to about-40 feet sea level(MSL). The thickness of clay also varies near the landfill with more clay on the western side. Below the clay-rich interval is mostly interlayered sands and silts with occasional beds of clay. This is referred to as the intermediate zone. The only major occurrence of clay is an interval from-80 to-118 feet MSL noted in the MW-6 boring log. No other borehole logs show such a thick interval of clay;its extent near the landfill is unknown. The major water-bearing unit beneath the landfill appears to be the intermediate 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(CDM, 1999A). The silt layers become thicker and more abundant north of ARL-IA where there appears to be three distinct sand units beneath-40 feet MSL. The shallowest unit does not appear to be in hydraulic connection with the underlying intermediate sand unit because it pinches out between data points ARL-1C and MW-14 reported by CDM(1999A). This interpretation is supported by their being no trichloroethene(TCE),tetrachloroethene(PCE),or other compounds being 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 (CDM, 1999A). The lowest unit has shown detectable WE and PCE,suggesting it is in hydraulic connection with the upper unit. The depths to water measured in the borings installed by CDM in 1998 ranged from 61 to 84.6 feet bgs or -24 to-42 feet MSL. Since 19%,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. Maximum PCE were reported in the monitoring wells north of the landfill,which are screened within the sand layer which extends over the 80 to 104 feet below ground surface(bgs)interval. The wells screened in this interval are MS-10,MW-11,MW-13, and MW-14. Most of the wells on the perimeter of the GRASSETTI ENVIRONMENTAL CONSULTING