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- weave - WESTON GE0-ENGINEERS The annular space between the outside of the slotted well casing and the borehole will be filled with a clean sand pack (medium aquarium sand) to variable depths, but not less than 4 feet above the slotted pipe section. A 1 foot neat cement seal followed by a 1 foot bentonite seal will be placed above the sand pack to form an impermeable stationary plug. A grout mixture of Portland cement and bentonite (5i1 mixture) will be placed above the bentonite plug up to a depth of approximately 4 feet below the ground surface, Concrete will be poured, to slightly above the paving surface, incorporating.the protective well box, The groundwater monitoring wells will be sealed with tight fitting caps which can be locked to prevent tampering, A typioal groundwater monitoring well design proposed for this site is illustrated on Figure 5. A 2 inch casing (ID) will be used for all wells to enable necessary equipment' installation and use. The minimum thickness of the sand/gravel pack placed between the slotted casing interval and aquifer formation material (the annulus) will be at least 2.5 inches, The minimum thickness of the formation/sanitary seal will be at least 2.5 inches also, The minimum thickness of the reinforced concrete slab used for surface protection will be 4 inches and will slope away from the casing and the protective box. Elevations and top-of-casing measurements will be aocurately surveyed and recorded to within 0.01 vertical foot and surveyed location will be accurate to within 0,5 horizontal foot with respect to established reference point(s) . Well Development Because a monitoring well must be properly constructed and develpped to avoid subsequent sampling problems, the design criteria fox- the intake part of the well should allow for easy inflow of water from the geological formation. The casing slot width should be small enough to keep the gravel pack sand and the natural formation materials out, The well will be cleaned of drilling debris and properly developed as indicated by a consistently clean water sample during the deueiopment process. To remove any silts which may be caked on the sides of the borehole and reduce the amount of naturally occurring fine material in the vicinity of the well screen, a high--velocity water jet or other similar' deviee will be used to wash this material from the formation walls into the wells. Air lift pumping, surging and/or- bailing of the wells will later remove this material. Well development will continue until completely depleating well of water and/or when water pH, temperature and resistivity have stabilized and the groundwater is cQnsietently clean with no visible turbidity. The well will then he ailgwe d to equilibrate for a minimum of 48 hours before sampling,