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6 0 GROUNDWATER MONITORING The following section describes the conversion of three testhole borings to groundwater monitoring wells, the collection of water samples, and a discussion of the analytical results On July 28 and 29, 1993 AGI installed and developed the monitoring wells during site investigation activities, and on August 25, 1993, purged and sampled the groundwater monitoring wells The County representative witnessed both the installation and the sampling of the wells 6 1 Groundwater Monitoring Well Installation Three soil bonngs, TH-4b, TH-5, and TH-fib, were selected as groundwater monitoring well locations, and renamed as MW-1 MW-2, and MW-3, respectively (Exhibit 2) Monitoring wells MW-1 and MW-2 were advanced to a depth of approximately 45 feet (bsg) and also encountered groundwater at a depth of approximately 29 feet (bsg) Monitoring well MW-3 was advanced to a depth of approximately 50 feet (bsg) and encountered groundwater at a depth of approximately 29 feet (bsg) Monitoring well MW-3 was advanced to 50 feet (bgs) after the first attempt to install casing at 45 feet (bgs) was aborted due to heaving sands The additional 5 feet of drilling allowed the heaving sand to fill the base of the boring thus allowing the well to be installed with the screened interval appropriately placed The screened interval for all the monitoring wells was installed between the depth interval of approximately 22 to 42 feet (Exhibit 3) Note The screened interval was originally planned from 20 to 40 feet; however, groundwater was encountered at 29 feet to 30 feet and would have placed the top of the groundwater across the blank area between the interlocking 10 foot screened sections Therefore, we placed the base of the screen at 42 feet with the blank (6 inches) at 32 feet The screen consisted of 0 020-inch factory slotted, flush threaded, 2-inch Schedule 40 PVC tubing Blank, 2- inch Schedule 40 PVC casing completed the wells to the surface and a bottom end- cap was placed below the screen No glues, solvents, or chemicals were used in the construction of the wells The annular filter pack consisted of Number 2/12 Monterey Sand, placed between the interval from total depth to two feet above the top of the screen Three feet of bentonite pellets were placed above the sand to form a seal The pellets were hydrated with clean potable water and allowed to set A 6-sack neat cement slurry was placed on top of the plug and extended to six inches below surface grade The wells were secured by a locking safety cap and traffic-rated well cover finished slighting above grade to allow surface drainage away from the cover On September 7, 1993 Crisp Tank Testing & Construction Company surveyed the final elevations of monitoring wells MW-1 , 2 & 3 The wells were surveyed at the north side and top of casing All elevations were referenced to an arbitrary benchmark (Exhibit 4) assumed to be 100 feet above mean sea level 11 of 17