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SLE ARWATER G R 0 L P I N C accessed 2n a manner to minimize entry of outside air, and is tested for total orgaruc vapor using a calibrated organic vapor meter (OVM) The results of the field screening are noted with the lithologic descriptions on the Field Exploratory Soil Boring Log On encountering an impermeable (clayey) layer three feet or more in thickness below a saturated permeable layer, where the unpermeable layer is considered to be a possible confining layer for an underlying aquifer, drilling is halted until a decision to proceed is obtained from the project manager This process minimizes the chance of introducing contamination to an underlying, clean aquifer Soil Waste Managment Soil cuttings are stockpiled on and covered with plastic sheeting to control runoff, or contained in 55-gallon D O T-approved drums on site Waste soil is sampled to chemically profile it for disposable, and hauled by a licensed waste hauler to an appropnate landfill All waste stored on site is properly labeled at the time of production Soil Bonng Abandonment Soil borings which are not to be converted into monitoring wells are sealed to the ground surface using neat cement or sand-cement slurry in accordance with federal, state and local regulations Native soil may be used to fill the top two to three feet for cosmetic purposes, as permitted Monitoring Well Installation Well Casing, Screen and Filter Tack Construction All well construction is performed in accordance with Department of Water Resources "California Well Standards" and all requirements of local oversight agencies Soil borings to be converted into single-cased monitoring wells are a minimum of eight inches in diameter for 2- inch diameter wells and a minimum of ten inches in diameter for 4-inch diameter wells Monitoring wells are constructed with schedule 40, threaded, polyvinyl chloride (PVC) casing unless site geochemistry or contamination necessitates an alternative material The wells are constructed with factory-slotted screen and threaded end caps The screened interval is placed such that it extends approximately ten feet into the water bearing zone, and at least five feet above the expected maximum water level The screened interval may extend less than five feet above the maximum water level, only to prevent intersection of the screened interval with the top of the confining layer of a confined aquifer, or where the water table is too shallow to allow this construction A graded sand filter pack is placed in the annular space across the screened interval and extended approximately one to two feet above the screen, as site conditions permit, so as to prevent extension of the sand pack into an overlying water-bearing unit The well screen slot size is the maximum size capable of retaining 90% of the filter pack Typically, 0 010-inch screen is used where the formation is predominantly clay and/or silt or poorly-graded fine sand 0 020-inch screen is used where the formation is predominantly well-graded or medium to coazse sand and/or gravel The filter pack grade (mean grain size) is selected according to native sediment type as follows a) for poorly graded fine sand or silt/clay - 4 times the 70% retained grain size of the formation b) for medium to coarse sand, gravel or well graded sediments - 6 times the 70% retained grain size Since results of particle size analysis are not always available, Clearwater often selects CLEARWATER GROUP, INC 2 August 26, 1996