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Mr. Gabriel Ka, P.E. Project 939-01.09 November 1, 1 b- Page 2 SOIL-GAS WELL INSTALLATION PROCEDURES Twelve tri-level soil-gas wells_ (GW-1 through GW-12) were installed out- side the limits of the refuse fill at the locations shown in figure 2. The installation work was performed from October 8 to October 10, 1991. Exploratory borings for the wells were drilled by a track-mounted CME 650 drill rig, equipped with 6-inch-diameter continuous-flight augers. An EMCON geologist supervised the drilling and logged the subsurface mate- rials by examining soil cuttings. Logs of exploratory borings are presented in attachment A. After being drilled to the designated depth, each exploratory boring was converted into a tri-level gas well consistent with the design previously / approved by the California Integrated Waste Management Board (CIWMB). Gas well construction details are summarized in table 1 and illustrated in figure 3. The upper part of each gas probe casing consisted of a specified length of 1/2-inch Schedule 40 polyvinyl chloride (PVC) pipe. The lower 5 feet of each casing was a sampling screen consisting of 1/2-inch Schedule 80 slotted PVC pipe. Each screen section penetrated the top 5 feet of a predetermined subsurface sampling interval. At the top of each casing is a monitoring port consisting of a PVC labcock with 1/4-inch hose barb fitting. To identify the probe casings, the labcocks were staggered vertically to differentiate between shallow, intermediate, and deep probes. The deep probe is set the lowest, the shallow probe is set the highest, and the intermediate probe is in between. The labcock handles were also coded with colored tape to further identify the monitored intervals. The deep probe handle is blue, the intermediate probe handle is red, and the shal- low probe handle is yellow. The gas probe wells were constructed by installing each section of slotted PVC screen in a high-permeability gravel pack separated by intervals of low-permeability bentonite. First, gravel was placed in the bottom of each boring to serve as the base for the deep gas probe. After the deep probe was installed, the annular space surrounding the screened interval was filled with 1/4-inch gravel. A 5-foot-thick seal of granular bentonite was poured over the gravel pack and hydrated with clean water. The interme- diate level gas probe was then installed above the bentonite seal, and the well construction procedure was repeated. When the gravel pack for the shallow gas probe was completed, a bentonite seal was placed up to the ' PJ9 9390109.GOW