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__ ____ ----- WESTERN _WESTERN GEO- ENGINEERS SECTION 6.0 SITE I-IYDROGEOL.OGY The three exploratory borings and the tank excavations explored the subsurface conditions to a maximum depth of 29'. The subsurface consists of clay, clayey silt, silty sand and sandy gravel. Cross sections A-A' and B-B' (prior to tank placement) and cross seoilons C-C' and D-D' (post tank and fill placement) show the general 1 lithology (see figures 7, 8, 9, and 10). A medium brown clayey silt is found at the surface grading into a silty sand at approximately the 15 foot depth (37 feet above mean sea level). This silty sand has clay and sandy gravel lenses throughout as seen In boring 2. A sandy gravel is present in all the borings at approximately the 25 foot depth (24 feet above mean sea level), and overlays sand in borings 1 and a clay in borings 2 and 3. All borings were terminated at t3 feet in silty clay. Moderate to strong hydrocarbon odor was encountered while drilling and sampling boring, MW-1. Soil samples collected from 15 and 29 feet had no odor but water collected during the boring had a strong hydrocarbon odor with a trace of yellow fluorescent skill on top. The vapor readings obtained from the soil and water samples indicate that the hydrocarbon odor is derived from the formation water found in the pore spaces of the soil. No hydrocarbon odor was observed in scads and water samples collected from borings MW-2 and MW-3. Ground water was first encountered at approximately the 12 foot depth (39' above mean sea level) in the borings for the monitor wells. This shallow aquifer seems to be static (unconfined conditions) as no piezometric rise was observed during the boring or in the monitor wells. Stabilized ground water levels in the monitor wells were measured to the nearest 0.01 feet on May 7, 10, 13, 20, 27 and June 1, 1988. No floating product was encountered, in any of the wells except a trace of yellow fluorescent skim observed in MW 1 on May 7, 1988. The ground water flow direction has been influenced by the open excavation, which was filled with new tanks and clean pea gravel on May 28. 1988. The intial ground water flow direction, measured for this assessment on May 7, 1988, appeared to be towards the northeast or from monitor well 2 to monitor well 3. Subsequent water measurements (May 10, 13, 20 and 27) indicate a shift in the ground water flow direction to the northwest or from monitor well 2 to monitor well 1. The ,lune 1, 1988 water measurements show a shift of ground water flow direction trending back to the north, but still in a northwest direction (see figures 0A - 11G). Prior to monitor well placement and duping the ground water measurements taken May 7, 1988 the fuel tank excavation was open and abundant precipitation had occurred due to thunderstorms in the area. For the weeks that followed the weather had changed from cool and wet to hot and dry, The excavation acted as a recharge trench during the cool and wet period up and including May 7 and as an area of discharge due to evaporation during the following weeks of hot and dry weather. Once new tanks and clean fill were placed within the excavation, the evaporation, slowed or even ceased and flow direction shifted back toward the north as indicated by the June 1, 1988 measurements. Also evident from water measurments taken from the monitor wells is the drop of the water level which ranges from 0.29' in monitor well 1 (closest to the excavation and had the slowest recovery rate) to 0.17' in monitor well 3 (recovery slightly faster than MW 1). Monitor well 2 (fastest recovery rate) showed a drop in water level of 0.27' during the period of May 7, to June 1, 1988(see figure 12). The overall gradient appears to be relatively flat. page 18 . I