ARCHIVED REPORTS_XR0011912 - Laserfiche WebLink

Home Browse Search

i • Based on lithology, moisture content, and hydraulic conductivity, we have recognize d several hydrogeologic units beneath the site (Figure 3) The Riverbank Formation is the pnncifal aquifer beneath the site A sample of this sand at a depth of 81 fee in GT-1 l had a hydraulic conductivity of 8 8 x 10-5 cm/sec (Table 1), indicating that this aquifer is a fair source of groundwater However, as the cross sections show, the aquifer is not uniform, due to channeling, interbedding with finer- grained silt beds, and soil formation For example, a relatively thick clayey to silty interv2€ between 90 and 110 feet was damp to dry when drilled in GT-11, and this interval separated twe saturated sand beds above and below it (shown as "first and "second"Riverbalik aquifers in Figu-e 5) The hydraulic conductivity of this fine-grained interval is—4 1 x 10,E cm/sec indicating that it is a moderate aquitard and probably restricts communication between,t ee aquifer beds to some degree The three main lithologic units of the Modesto Formation form separate hydrogeolc gic units, although groundwater communication between them is possible The upper silt bed does not contain water, and its fine-grained nature inhibits surface infiltration The underlying sand bed (Modesto Channel in Figure 3) has suitable aquifer charactenstics, but did not contain eater until fairly recently It has higher hydraulic conductivity than the Riverbank Formation (Table 1), but is more limited in extent both laterally and vertically, and therefore has lower water-producing potential The lower part of the Modesto Formation has the lowest hydraulic conductivity of any unit that has been tested(10-6 to 10-8 cm/sec), and therefore sigruficantly retards fluid flow between them Along the axis of the "Modesto Channel", where the silt bed has been strongly eroded and is only a few feet thick, however, or in some localities where the hydraulic conductivity may be greater(e g west of GT-10),the two formations are apparently in hydrologic communicai ion 3.3 Field Evidence of Soil Contamination PID readings and visual examination of soil samples indicated that both new borings penetrated the plume of contaminated soil As shown in the boring log of GT-10 (Appendix A), slight to moderate hydrocarbon odors were first detected at a depth of about 23 feet, but low readings were measured by the PID as shallow as 15 feet As anticipated, the strongest odors and highest PID rear Ings were noted in the depth interval from 45 to 55 feet, but hydrocarbons were still present in soil samples at the bottom of the boring(60 feet) Stronger odors and higher PID readings were recorded in GT-11, and evidence of contamination was noted from 40 to 95 feet Below this depth, no o cors were detected, but upon return to the office, PID readings of 2 ppm were measured in the samf les at 100 and 105 feet 3.4 Laboratory Evidence of Soil Contamination Based on discussions with Mr Jeffrey Wong of PHS/EHD, 16 soil samples were selected for laboratory analysis The samples were analyzed for TPH-gasoline, TPH-diesel, MTBE, and BTEX The laboratory report is included as Appendix C, and the results are summanzed along with previous laboratory data in Table 2 The laboratory results for TPH-d diesel were plotted and contoured on the cross sections Figures 4 and 5) and were also used to revise and update the isocontour maps that were prepar.d for the 6