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Now .i G E OTECHNICAL AND ENVIRONMENTAL CONS U LTA NTS Project No. S8149-06-01 June 2, 1997 Cella Barr Associates 777 Campus Commons Road, Suite 200 Sacramento,California 95825 Attention: Mr.Jim Nelson Subject: WESTSIDE CHANNEL OUTFALL SYSTEM BYRON ROAD TO THE OLD RIVER TRACY,CALIFORNIA WORKPLAN FOR DETENTION BASIN PIEZOMETERS Dear Mr.Nelson: Based on the request of Mr. Jeff Carruesco with the San Joaquin County Public Health Services, Geocon has prepared this Workplan for the installation of five temporary piezometers in the vicinity of the subject detention basin locations. The purpose of the piezometers is to accurately determine the groundwater elevation at the detention basin locations over a period of approximately six to eight months. The five piezometers will be installed in the vicinity of the intersection of San Jose Road and Middle Road approximately 3 to 4 miles northwest of downtown Tracy in San Joaquin County, California. The site location is depicted on the Vicinity Map, Figure 1 and the approximate locations of the piezometers are depicted on the Site Plan,Figure 2. The piezometers will be constructed by advancing five borings to depths on the order of 19 feet below the ground surface (bgs) using a truck mounted drill rig equipped with eight-inch diameter hollow stem augers. Soil samples will be collected at approximate 2.5-foot intervals within the upper 10 feet of the borings and at 5-foot intervals below a depth of 10 feet bgs. Each piezometer will consist of 20 feet of 2-inch diameter PVC casing[The bottom 15 feet of the casing will be slotted to allow the groundwater to enter the casing The piezometer casing will stick up above the ground surface approximately one foot and will be sealed with a locking cap. The annular space around the casing will be filled with No. 3 Monterey sand to a height approximately 1 foot above the screened zone. A 2-foot-thick hydrated bentonite seal will be placed above the sand. A detail depicting the piezometer construction is attached herein as Figure 3. The top of the piezometer casings will be surveyed by Cella Barr Associates. Geocon will then measure the depth to the groundwater within the piezometers no sooner than 24 hours after their installation. The depth to groundwater will be measured using a battery-powered electronic water depth indicator. Two additional groundwater depth measurements will be conducted by Geocon within the following 6 to 8 months after their initial installation/measurement. The piezometers will subsequently be destroyed after the third measurement. Piezometer destruction will consist of overdrilling the casing/boreholes and backfilling with bentonite-cement grout. 3235 Sunrise Blvd., Suite 6 ■ Rancho Cordova, California 95742 ■ Telephone (916) 852-9118 ■ Fax (916) 852-9132