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05/23/95 14:34 '0801 272 0430 nONTGOWWY WATSN IMOD2 • DDRW-Tracy OU 1 Design Field Work Scope of Work for Infiltration Study (Task 2C.1) 2.l.l.l. Background. The selected remedial alternative for OU 1 involves the disposal of treated groundwater by infiltration into the Upper Tulare Formation. The use of infiltration galleries are currently recommended as the preferred facility for treated water discharge. An infiltration study is proposed to provide necessary site-specific design data for design of the infiltration facilities. 2.1.1.2. Objectives. The objectives of the Infiltration Study are to: 1) confirm the feasibility of infiltration galleries for the proposed locations,and 2)collect data necessary for the design of the infiltration galleries. 2.1.1.3. Task Description. Infiltration tests are proposed for the eight proposed infiltration gallery locations as shown on Figure 1. These locations were selected based on OU 1 hydraulic objectives(plume containment),land availability,and groundwater modeling results as presented in the Pre-Design Data Report(Montgomery Watson 1995). A certified geotechnical subcontractor will beretained to perform the field work for the infiltration gallery tests with oversight by a qualified Montgomery Watson hydrogeologist/engincer. Field infiltration tests will be performed and soil samples will be collected for geotechnical laboratory analyses. The field infiltration tests will be performed using borehole falling and constant-head test methods. A pressure transducer connected to a data logger will be used to collect water-level data for the infiltration tests. Geotechnical data will also be collected for infiltration facility design. 2.1.1.4. Specific Work Activities. The following specific work activities are proposed for the Infiltration Study. • Twelve 8-inch diameter boreholes will be drilled using the hollow-stem auger(HSA)technique to depths of approximately 20 feet. Once the fatal depth of the borehole is reached,drill rods will be removed and a 15-foot section of 2-inch diameter slotted casing threaded to a 5-foot section of blank casing will be suspended through the center of the auger from the ground surface. Monterey 12 x 20 mesh filter pack will be slowly hemied through the augers to fill the annular space between casing and the borehole wall up to a depth of 4 feet. A 2-foot bentonite seal will be placed on top of the filter pack. The bentonite will be hydrated by adding at least 10 gallons of clean potable water. A temporary flush-mounted traffic-rated well vault will be installed over the we1L The top of the infiltration test well will be secured with a watertight well cap. • Sight 8-inch diameter boreboles will be drilled using the HSA technique to a depth of 40 feet. Split-spoon samples will be collected at approximate intervals of 7.5, 12.5, 15,20, 30,and 40 feet below the ground surface. The boreholes will be backfilled to approximately 25 feet bgs in preparation for well construction. Soil samples will be visually classified in accordance to the Unified Soil Classification System(USCS)on a standardized boring log. All sample sleeves will be sealed with teflon- lined end caps,labeled with the sample designation,sealed in plastic baggies,and stored a ice-cbilled cooler. Selected soil samples will be submitted for geotechnical analyses depending on type of lithology and bad W-cm be :