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a Mr. Kenneth Earnest May 18, 1994 :IL Page 3 i f f 1r Possible Installation of Vadose Zone Well(s) Soil borings of sufficient diameter to allow the possible installation of nested 2-inch-diameter vadose wells will be advanced to a depth of approximately 65 feet below grade (or approximately to the water table). Borings will be terminated ifkwo consecutive soil samples record a concentration of less than 5 ppm organic vapors with the PID and/or wet soil is encountered. The soil borings will be advanced at the locationsr indicated on Figure 6. Soil borings will be advancer/ and sampled in general accordancewith protocols described in Enclosure A. Up to 3 vadose wells will be constructed in each soil boring. The number of vadose wells at each nest location will be determined by observations madelin theh field. Each vadose well will be constructed with no more than 15 feet of flush-threaded, Schedule 40 polyvinyl chloride (PVC) well screen. The screen will be factory-slotted well, casing with 0.020-inch wide (#20) slots. Blank, Schedule 40 PVC will extend from the top of each screen to just below grade. The annular space opposite the well screen will be backfilled with No. 3 Monterey sand pack, or equivalent. The sand pack will extend.approximately 1 foot above the top of the well screen. A bentonite seal at least 2 feet thick will be placed in the annular space above the sand pack. Above the bentonite seal, either another vadose well constructed in a similar manner as previously discussed will be installed, or, if no additional vadose,wells will be installed in the boring, a cement grout with approximately 5 percent bentonite will be emplaced and will serve to inhibit movement of surface water (or vapors) downward through the well annulus. A water- tight, locking cap will be installed on the top of the casing riser of i ach vadose well. A diagram 1 f: of typical vadose well nest construction is illustrated on Figure 7! Replacement of Monitoring Well MW-7 11 Monitoring well MW-7 (Figure 2) was damaged during the excavation and removal of the underground storage tanks during October 1993. Excavation activities lodged an obstruction in the casing, rendering the monitoring well unusable. The well will be replaced by drilling out the casing and constructing a new monitoring well with like material and screened internal. The monitoring well will be drilled out with augers of sufficient diameter to remove the well construction materials and some native material on the outside of the monitoring well. After r drilling out the damaged well, the new well will be installed. A 2-foot-thick surface seal of concrete will be installed at the surface to inhibit the downward migration of surface water into the annular space of the new monitoring well. F I k � ACTON • MICKELSON • LRPTOOS.MAF { van DAM,INC. Consulting Scientists, Engineers,and Geologists