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Quik Stop Market No. 132 January 6, 1993 Project No. F7132.34 Page 4 SITE HYDROGEOLOGY During the drilling operations, each boring was logged noting soil type, soil moisture, and depths at which groundwater was encountered. Previous investigations conducted at the site (Exceltech, September 1990; November 1990 and July 1991) have indicated ' the presence of two aquifers beneath the site. This information is included in cross- sections A - A' and A' - A" (Figures 2, 3, and 4) created from the subsurface information obtained from exploratory boring logs and observations made during ' previous excavation operations at the site. ' Boring MW-6 was drilled to a depth of 21.5 feet and boring MW-7 was drilled to a depth of 23 feet into the water-bearing unit of what appears to be an upper aquifer. The soil encountered in both borings initially consisted of a moist silty clay which extended to a depth of approximately 4 feet. Beneath this clay and extending to a depth of approximately 10.5 feet were beds of moist silty to clayey sands and sandy clays grading into a bed consisting of moist sand. This sand overlay a bed of moist silty clay ' which extended to a depth of approximately 18 feet where a saturated silty to clayey sand was encountered. This water bearing unit is interpreted as representing an upper aquifer. Underlying this aquifer at depths of approximately 21 to 22 feet was a moist silty to sandy clay which has been interpreted as representing the aquitard. Drilling was stopped when the aquitard was encountered. As shown in Figure 3 and discussed in our previous report, the depth, soil type and stratigraphic location of the water bearing zones encountered in MW-1R and MW-5 indicated the presence of two aquifers in the vicinity of these wells. This interpretation was supported by analytical data which detected the presence of petroleum hydrocarbon related compounds in water samples collected from the upper aquifer but not in ' groundwater samples collected from the underlying lower aquifer (Exceltech, July 1991). As shown in Figure 4, the depth, soil type, and stratigraphic location of the saturated zones encountered in MW-6 and MW-7 would suggest that these two water- bearing zones represent a continuation of the upper aquifer encountered in MW-1R and MW-5. ' Groundwater was encountered at approximately 18 feet below the surface in MW-6 and 20 feet below the surface in MW-7. On December 10, 1992 after well development, the groundwater levels in MW-6 and MW-7 was measured and found to be 16.26 feet and 16.27 feet below the top of the casing in MW-6 and MW-7, respectively. This rise in groundwater levels from the depths at which it was first encountered would suggest the aquifer is semi-confined to confined at these locations. These water levels resulted in a groundwater elevation of 12.45 feet below mean sea level feet for MW-6, and 12.80 feet below mean sea level for MW-7. A comparison of the December 1992 water levels in existing wells with those of the previous quarter show a wide range of variations ranging from a rise of 2.35 feet in MW-4, to a drop of 2.20 feet in MW-5. The groundwater elevation contour maps developed from these elevations are significantly different from previous groundwater elevation contour maps (see Plates 5 and 6).