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plunging and purging, and compressed air evacuation. Each of the existing wells is equipped with a dedicated pneumatic displacement pump for sampling. Regional hydrogeologic information indicates a gentle west to southwest trending ground water gradient. Monitoring results indicate a more south trending local gradient. More wells may be installed if necessary to properly monitor the site during the postclosure period. The monitoring wells are currently being sampled quarterly and annually for the parameters listed in the Waste Discharge Requirements under Ground Water Monitoring which were issued by the Central Valley Regional Water Quality Control Board (Appendix A) . Quarterly reports will be forwarded to the LEA, Regional Board, and the Board. 15. INSTALLATION OF LANDFILL GAS MONITORING SYSTEM Twelve tri-level soil-gas wells (GW-1 through GW-12) were installed outside the limits of the refuse fill at the locations shown on sheet 3 . 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 AML drilling and logged the subsurface materials by examining soil cuttings. Logs of exploratory borings are presented in the Landfill Gas Monitoring Well Installation Report (Appendix F) . 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 summa- rized in table 1 and illustrated in figure 3 of Appendix F. 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 a 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 shallow probe handle is yellow. 14