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i The comer of the bag is opened and the detector probe imnediately placed within the headspace. The highest observed reading is recorded. 1 Monitoring Well Construction The borehole diameter for a monitoring well will be a mirinium of four inches larger than the outside diameter of the casing. I i A monitoring well is typically cased with threaded, factory erforated and blank Schedule 40 PVC. The perforated interval consists of slotted casing, generally with 0.01 or 0.02 inch-wide by 1.5-inch-long slots, with 42 slots per foot. A threaded or slip PVC cap is secured to the bottom of the casing. The slip cap can be secured with stainless steel screws or friction. No solvents or cements are used. Centering devices may be fastened to the casing to ensure even distribution of filter r 3Laterial and grout within the borehole annulus. The well casing is thoroughly washed and/or steam cleaned. It may be purchased as pre-cleaned prior to i completion. Setting the casing inside the hollow-stem auger, sand or grav 1 filter pack material is poured into the annular space to fill from boring bottom to generally one foot above fie perforated interval. The filter pack material in the well is selected to permit the development of a zone o higher hydraulic conductivity adjacent to the well screen but not allow piping of the finer-grained formation materials into the well. The slot size of the well screen is selected so that it will retain a minimum of 5 percent of the filter pack material. Before placement of the bentonite plug,the well is surged to set the filter pack. After surging,the top of the filter pack is measured and, as necessary, additional filter pack material is added. The well is then surged again. This procedure is repeated until the filter pack will not settle further. After setting the filter pack, a one to two foot thick bentonite plug is set above the filter pack to pr vent grout from infiltrating into the filter pack. i A regulatory approved annular filling material such as neat cement, cement with five percent (by volume) ' i bentonite or sand-cement grout will be used to fill the annul from the bentonite plug to within one foot of the ground surface. The annular filling material is placed b a method approved by the regulatory agency overseeing the site. The remaining foot of the well will be completed using a traffic-rated vault that is installed around each wellhead for wells located in parking lots or driveways,while steel(or other material) "stovepipes" are usually set over wellheads in landscaped are s. A traffic-rated vault is typically set 112-inch above grade to minimize surface water from entering the vault, In areas that may be plowed for snow removal,the vault is set flush with the surface to prevent dame ge to the vault by a snowplow. i 3 i