Laserfiche WebLink
Possibly less), the data are sufficient to compare the relative ability of each lithologic unit to <br /> transmit groundwater. <br /> One sample of the upper unit was submitted to the laboratory to determine its hydraulic <br /> conductivity and porosity. This sample, collected from boring GP-4 at a depth of 16 feet, had a <br /> hydraulic conductivity of 3.7 x 10-6 cm/sec. This is a typical value for fine-grained sediment that is <br /> generally non-water-bearing, and implies that the unit is a moderate aquitard. <br /> Two samples of the middle unit were tested for hydraulic conductivity. A sample from the middle <br /> d <br /> Of the unit had a hydraulic conductivity of 1.5 x 10-3 cm/sec, and a sample at the base had a <br /> hydraulic conductivity of 5.1 x 10-4 cm/sec. Both are within the range of reasonably good aquifers. <br /> This explains why the middle unit is the first water-bearing unit beneath the site. <br /> One sample of the lower unit had a hydraulic conductivity of 1.1 x 10-6 cm/sec, implying that it is a <br /> slightly better aquitard than the upper unit. A second sample, collected from a sandy lens within this <br /> unit at a depth'of 36 feet in GP-3, had a hydraulic conductivity of 1,1 x 10-4 cm/sec, similar to that <br /> of the sand in the middle unit. <br /> The hydraulic conductivity data suggest that the sandy middle unit is semi-confined by the upper <br /> clay unit. This suggestion is supported by the fact that when wells KF-4 and KF-5 were drilled in <br /> July of 2000, saturated soil conditions were not encountered until the middle unit was drilled, at a <br /> depth of 17 to 18 feet, yet groundwater was present in both wells at a depth of approximately 8 feet <br /> only four days later. Hence, the piezometric surface was at about 8 feet, but the static water level <br /> was almost 10 feet deeper due to confinement by the low-permeability upper clay unit. This clayey <br /> unit is thus a significant barrier, both to upward flow of groundwater and downward percolation of <br /> any petroleum hydrocarbons that may have leaked from the underground storage tank facility. <br /> 2.2 Regional Groundwater Flow <br /> The prevailing direction in which groundwater flows and the hydraulic gradient are important <br /> aspects of the conceptual model, because these factors exert a strong influence on the mobility and <br /> migration patterns of dissolved petroleum hydrocarbons. Before examining the local groundwater <br /> flow pattern in detail, however, it is instructive to consider the flow pattern on a larger scale, <br /> because this helps to reveal more general trends and overall patterns. At this scale, the schematic <br /> model for the site involves generally northeastward groundwater flow. <br /> Figure 6 is simplified from a groundwater elevation map for 1993, produced by the San Joaquin <br /> County Flood Control and Water Conservation District. The map depicts a linear groundwater <br /> basin (depression) extending southeast from about the intersection of Hammer Lane and Pacific <br /> Avenue in northwest Stockton to Littlejohns Creek southeast of the city, with a slight interruption <br /> between Waterloo Road and Fremont Street in east Stockton. in 1993, groundwater was depressed <br /> to over 60 feet below sea level in this basin. Southwest of the depression, where the Kwikee Foods <br /> site is located, the general direction of groundwater flow was therefore to the northeast, into the <br /> • 7 <br />