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L'Y <br /> Page.4 <br /> July 12, 1991 <br /> Job No.400511-EI <br /> A <br /> 3.0 BACKGROUND <br /> f The initial drilling program for site evaluation was conducted in February of 1985. Since that time, <br /> ater monitoring wells and three(3) recovery wells 1}avu been installed. Al <br /> twenty-Ewa(22) groundw <br /> l of these wells have gone dry aver the past five years. Ten of the wells have been converted to vapor <br /> extraction wells as part of the currently operating soil remediation system. <br /> h drocarbon contamination in the soil appears <br /> Based on previous consultants reports and field data, y <br /> to be localized in the vicinity of recovery well RW-25(see Figure 2). Field data shows that the highest <br /> concentrations of Tata! Petroleum Hydrocarbons 7 gg�s�E}neing <br /> 11, 16, 18} IZW-24in tile Sand 26)showed lovent gas are w or <br /> drawn from RW-25. The surrounding wells(M <br /> non-detectable concentrations of TPHg in tite soil vent gas. For these reasons, tite additional vapor <br /> wells have been arranged to encompass the area surrounding RW-25. <br /> 4.0 PROPOSED SCOPE OF WORK <br /> g In order to address the concerns of the Public Health Services,San Joaquin County and to expedite site <br /> remediation, DERS proposes the following scope of work: <br /> kr a Drill,log,sample,and install four(4) on-site monitoring wells at the locations shown on <br /> the Proposed Well Location Map, Figure 3 in order to further delineate the limits of the <br /> € hydrocarbon plume and determine the ground-water flow direction and gradient in the <br /> !* site vicinity. The wells will be completed to an approximate depth of 100 feet. Sell <br /> t.c• <br /> samples will 6e collected at 5-foot depth intervals. At the conclusion of the drilling <br /> il activities, selected soil samples will be submitted for laboratory analyses to aid in <br /> determining the extent of hydrocarbon contamination. <br /> i <br /> The ground-water monitoring wells will be drilled and advanced to intersect the <br /> po[entiometric surface and will be screened to accommodate anticipated seasonal <br /> variations throughout the year (i.e., about 10 feet below and 10 feet above the current <br /> groundwater levels). if an aquitard(i.e.,stiff clay layer) is encountered b loa vaw the f er <br /> table, and is found not to be contaminated using field methods e. ., g p <br /> analyzer), then the vertical continuity of this clay layer will be verified by sampling and <br /> i <br /> advancing the borehole 5 feet. If the clay layer is found to be less than 5 feet thick, she <br /> t , clay will be considered a localized clay lens and the borehole will be advanced to the <br /> i' designed depth. If the clay layer is found to be greater than 5 feet thick,the clay will be <br /> considered a continuous confining layer and the borehole will be plugged with bentonite <br /> t : <br /> f <br />