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T <br /> was backfilled with bentonite This well construction was chosen <br /> because perched groundwater was encountered in the gravel fill <br /> material Beneath these gravels, at approximately 10 5 feet in depth, <br /> a 3-foot-thick salt layer was encountered As drilling progressed into <br /> the salts, the groundwater level dropped by approximately 1 foot, <br /> suggesting that the water was perched within the confines of the old <br /> excavation which was filled with gravel <br /> ' A split spoon sampler was driven into sands beneath this salt to <br /> obtain a sample. In order to avoid potential cross-contamination of <br /> the lower sand with perched groundwater above the silt layer, it was <br /> ' decided that the well screening should be set within the perched <br /> water Thus, the borehole was filled with bentonite at the silt Iayer <br /> and well screening was set an the gravels above it <br /> Confined groundwater conditions were also encountered an MW-3A <br /> and MW-4A At MW-3A, a 10-foot screened interval is between 15 <br /> and 25 feet below the ground surface This interval was chosen <br /> because no groundwater was initially encountered an this boring <br /> until a depth of approximately 19 feet below the ground surface <br /> After the groundwater was encountered, the water level rose within <br /> ' the boring until a final depth of approximately 3 5 feet below the <br /> ground surface To protect the upper soils from possible petroleum <br /> constituents in the groundwater, blank casing was placed to a depth <br /> of 15 feet The uppermost 15 feet of this boring contained mostly clays <br /> and salts, with manor amounts of sand <br /> ' In MW-4A, groundwater was encountered at an approximate depth <br /> of 19 feet After the groundwater was amtially encountered, the level <br /> subsequently rose to a depth of approximately 10 5 feet below the <br /> ground surface Therefore, the screened interval in this well was <br /> placed between 15 and 25 feet below the ground surface <br /> vinLy <br /> ll m <br /> Each monitoring well was developed immediately after installation <br /> by surging for five minutes The wells were subsequently left to <br /> equilibrate for a period of one week before subsequent purging and <br /> sampling activities took place on April 3, 1992 Well development <br />' records are included in Appendix F <br /> On May 12, 1992, the wells were surveyed for elevation and location <br /> (Appendix F) The elevations of the tops of the well casings were <br /> measured with respect to the same benchmark that had been used <br /> during previous investigations at the former tank site #1 As was <br />' done previously, an arbitrary base elevation of 100 feet was utilized <br /> 3-5 <br />