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WE/SSASSOCIATES I Q Weiss Associates Procedure for Installing Ground Water Monitoring Wells Boreholes for ground water monitoring wells are commonly drilled with a drill rig capable of using either auger or air/mud-rotary drilling methods. The upper part of each borehole is drilled without fluids using an auger or air rotary to determine the depth to the shallowest ground water. Drilling then is completed using the mud-rotary technique as mud-rotary is capable of handling a wider range of subsurface conditions than augers or air rotary methods. A Weiss Associates geologist continuously logs each borehole during drilling and constantly checks drill cuttings for chemical odors. Soil samples for chemical analysis are collected in thin-walled brass tubes inserted in a 2-in. ID split-barrel drive sampler. Upon removal from the sampler, the tubes are immediately trimmed, sealed with aluminum foil, plastic caps and tape, labeled and refrigerated until delivery to the laboratory. These procedures minimize the potential for cross-contamination and volatilization of VOCs prior to chemical analysis. Other soil samples are collected in a 1.5-in. ID split-barrel drive sampler to document soil stratigraphy and estimate relative permeability of the subsurface materials. All drilling and sampling equipment is steam-cleaned prior to use at each site and between boreholes to minimize the potential for cross- contamination. Monitoring wells are usually cased with threaded, factory-perforated and blank 4 or 4 1/2-in. Schedule 40 PVC. The perforated interval consists of slotted casing generally 0.020 in. slot size by 1.5 in. long with 42 slots per foot. A PVC cap is fastened to the bottom of the casing with stainless steel screws; no solvents or cements are used. Centering devices may be affixed to the casing to assure even distribution of filter material and grout. The well casing is throughly washed and steam-cleaned prior to installation. After installing the casing, sand or gravel filter material is placed in the annular space adjacent to the perforated interval in each well. Where necessary, the drilling fluid is thinned prior to installing the filter material to prevent bridging and creation of void space in the annulus. A 1 to 2 ft thick bentonite seal is placed above the filter material to prevent grout from infiltrating. A mixture of neat cement and about 5% bentonite then is tremied into the annular space above the bentonite seal to the surface. A PVC cap is placed on each well and lockable steel well covers are installed for security and wellhead protection. Traffic rated meter boxes are installed around the well cover for wells in parking lots and driveways. All wells are thoroughly developed to remove residual drilling fluids and mitigate any potential well-bore damage. Well development techniques used include pumping, bailing, surging, swabbing, jetting and flushing. 6/88