SU0012793 - Laserfiche WebLink

Home Browse Search

i _;. Page 102 landfill do not detect contamination. Historically,MW=S,which is screened in the shallow sand unit, but is located on the upgradient edge of the landfill,has also detected PCE and TCE. The MR-6 and MW-7 screen intervals are below the thick layer of clay observed in well MW-6 from 117 to 115 feet bgs. This clay layer likely serves as an aquiclude(water flow barrier),preventing contamination from moving vertically into the deeper sand layers in the area bcneath the landfill. The absence of this clay unit in areas north of the landfill may allow downward migration of constituents. North of the MW-14 location,the plume appears to be moving downward within the aquifer system, as contamination has been measured in deeper samples but not in shallower ones. VOCs were not detected in any of the samples collected by CDM between 65 and 90 feet bgs,suggesting the absence of contamination at these shallower depths. However,VOCs were detected in some of the samples collected at depths from 140 to 180 feet bgs by CDM(1999). , Groundwater.flows in'the project area are generally to the north-northwest. The groundwater gradient is also reflective of the weather cycles and pumping,with a steeper,north trending gradient in dry years and a more gentle,northeast gradient in wet years(Kleinfelder, 1988; CDM, 1999). The dry-year gradient is apparently affected by greater drawdown in the area northeast of Stockton. Perched water may occur intermittently during the wet season,above relatively impermeable silts or clays in the sediment sequence. Hydrogeological data obtained from the adjacent Forward,Inc. Landfill site indicate that vertical and horizontal transmissivities are highly varied,and there is substantial interconnection between upper and lower aquifers. The development of a regional cone of depression east of Stockton,in response to pumping of agricultural and municipal wells,indicates the connectedness of the regional aquifers. The flow directions and gradients are consistent with the groundwater quality results collected during i CDM's 1999 investigation. The plume of PCE and TCE trends in a northeasterly direction from the landfill. In addition,water contamination has been found at shallower depths near the landfill,but at deeper depths further from the landfill,indicating that the plume is moving downward as well. The downward migration VOCs emanating from the landfill may be influenced by the location of sand layers s described and also by vertical flow conditions. No widely correlatable shallow aquifer sands,or silt and clay a uicludes(barriers) reported, which .,, reduces the ability to predict lateral groundwater flow or vertical interconnections.. Groundwater tests associated with the adjacent Forward Inc. Landfill site determined that the rate of groundwater flow in the uppermost regional aquifer was in the range of 0.08 to 0.8 feet per day,which is a relatively high flow ! (City of Stockton 1994). Due to the heterogeneous nature of the Victor formation sediment units, aquifer properties,including conductivity and rate of flow are anticipated to vary by a factor of 10 or greater, depending on location. In general,the available information suggests that the primary aquifer under the Rd site is an unconfined aquifer within the Victor formation. _. The VOC contaminant plume extends from the northern edge of the landfill perimeter to approximately 4,000 feet to the northeast of the site,based on 1999 data(CDM 1999A). This is only nominally more than the 3,750 foot distance reported in the 1994 EIR ,The primary difference in the plume movement since the 1994 EIR summarized data is the indication that the plume has migrated deeper at the leading edge of the plume than appeared in the past. The primary contaminants of concern are still PCE and j TCE,with concentration levels up to 69 and 48 pg/L,respectively,reported in CDM 1999A. The highest VOC concentrations are present within a sand layer which extends over an interval of approximately 80 to 104 feet bgs(CDM 1999A). The investigation results indicate that contaminant concentrations decrease at distances from the'source,with the plume migrating in deeper.lithologic units at distance from the site. The plume shape reflects the north to northeast groundwater flow direction,with the leading edge of the GRASSETTI EAWMONME NTAL CONSULTING 1.