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I 1 11 11 11 11 11 11 11 11 II •: GROUNDWATER INSTALLATION AND SAMPLING 4.1 4.2 4.3 Borehole Advancement for Well Installation Boreholes for monitoring well installation will be advanced and sampled using a hollow - stem auger rig as described in Sections 3.1. The boreholes for the monitoring wells will be drilled to a depth of approximately 20 feet and to a depth of about 27 feet for the extension well provided a continuous aquitard is not encountered. If an aquitard (i.e., a stiff clay layer) is encountered below the water table and is not found to contain elevated levels of petroleum hydrocarbons using field screening method (e.g., PID meter and/or TPH field test kit), then the vertical continuity of this clay later will be verified by sampling and advancing the borehole 5 feet. If the clay layer is found to be greater than 5 feet thick, the clay will be considered a continuous confining layer and the borehole will be plugged with bentonite or cement grout to the top of the clay. The monitoring wells will then be completed in the portion of the saturated zone overlying the clay layer. If the clay layer is found to be less than 5 feet thick, the clay will be considered a localized clay lens and the borehole will be advanced to the target depth. Groundwater Monitoring for Well Installation The groundwater wells will be constructed using either 2 -inch or 4 -inch I.D. casing and screen (see Figure A-1, Typical Well Construction Details). The 2 -inch I.D. monitoring wells will be constructed of Schedule 40 or 80 PVC casing and well screen with 0.02 -inch slots. The proper lengths of screen and blank casing will be steam cleaned and threaded above the ground prior to lowering inside the borehole. The well screen will be about 15 ft in length and will extend across the water table surface. The bottom of the screened interval will be fitted with an end plug and the top of the well casing will be equipped with an expandable, watertight, locking cap. The annular space between the 2 -inch well screen and the wall of the borehole will be filled with clean Lonestar No. 2/12 or 2/16 sand to provide filtration capacity and impede movement of sand or silt into the well. The sandpack will extend from the bottom of the borehole to about 1 foot above the top of the well screen. A 1 -ft thick plug of hydrated bentonite will be placed above the sandpack and the remaining annular space will be backfilled with a neat cement grout containing 5% powdered bentonite. The groundwater extraction well will be constructed of Schedule 80 PVC easing and well screen with 0.02 - inch slots. The well will be constructed in a manner similar to the monitoring wells, however, the well screen will be 20 ft long -with a 2 ft sump (see Figure A-1 for additional details). The wells will be protected from vandalism or the entry of surface fluids. Each well will be finished with either a locking standpipe or a watertight, security traffic box. Well Development Procedures The wells will be partially developed during well completion by surging and bailing water from the wells after installation of the sandpack by prior to installing the well seal. The purpose of this partial well development is to promote settlement of the sandpack I1 402_331-98_63 A-3