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� y GROUNDWATER MONITORING WELLS Monitoring Well Drilling. Groundwater monitoring wells will be drilled by the method described in the section entitled Borehole Drilling, with the exception that they will be drilled to a depth of 15 or 20 feet below the water table or piezometric surface, depending upon known conditions and upon unanticipated conditions encountered during drilling. The borehole will then be completed as a monitoring well by the method described below. Monitoring Well Completion. Groundwater monitoring wells will be completed with PVC screen and casing. On-site monitoring wells will be completed with 4-inch-diameter PVC screen and blank casing, while off-site wells will be completed with 2-inch-diameter PVC. A PVC cap will be installed at the bottom of each well. The well screen typically will be 0.020-inch slotted flush-threaded PVC, extending from a depth of approximately 15 or 20 feet below the water table or piezometric surface to 5 to 10 feet above. The well design is intended to accommodate seasonal water-level fluctuations within the slotted interval. The well design may be modified in the field during the drilling operation, depending upon conditions encountered during drilling. In no case does the monitoring well penetrate clay zones greater than 5 feet thick, which might allow downward migration of contaminants into lower water-bearing zones. Blank casing extends from the top of the perforated section to the ground surface (or above, in the case of above-ground completion). The annular space surrounding the well screen and casing will be packed with No. 1/20 (or equivalent) sand, from the bottom of the screen to approximately 2 feet above the top of the screen. The well will then be bailed or surged to settle the filter pack, and more sand will be added as necessary to return the level to a height of 2 feet above the top of the screen. A 2- to 3-foot-thick seal of bentonite pellets will be placed above the sand pack. The well will then be grouted with bentonite-cement slurry or neat cement, from the top of the bentonite seal to the surface. Groundwater monitoring wells commonly will be completed below grade and protected with a water-tight locking cover. A typical well construction is illustrated on Figure 1. The top of casing (TOC) will be surveyed by a licensed surveyor, and all depths to water will be measured in relation to the surveyed mark on the TOC. Monitoring Well Development. Groundwater monitoring wells will be developed by surging, bailing, or pumping until clean, sediment-free water is produced from the well. The length of development time varies, depending upon field conditions. Development water is contained in 55-gallon barrels, which will be stored temporarily on site until disposal.