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4.0 GROUNDWATER INVESTIGATION AND MONITORING 4.1 Groundwater Monitoring ftstern Improvement EMCON previously installed three groundwater monitoring wells at the site. MARK noted that the condition of the surface wellhead boxes might allow introduction of petroleum hydrocarbons from the ground surface to the groundwater In addition, review of the well construction logs suggested that the well screen intervals might allow non-impacted groundwater to rise above a groundwater-confining clay layer (base at 41 to 45 feet bgs) and into a zone of soil that was impacted by hydrocarbons Thus, actual hydrocarbon concentrations in groundwater might be lower than those concentrations observed in the EMCON wells MARK, therefore, recommended destruction and sealing of the wells in question. 42 Groundwater Monitoring Well Replacement On February 3, 1993, the three EMCON monitoring wells were destroyed and sealed with grout by West Hazmat Drilling Corp On May 24 and 25, 1993, MARK installed three new monitoring wells (MW-1, MW-2, and MW-3). To avoid introducing hydrocarbons into the groundwater, 12-inch diameter steel conductor casing was first installed that extended to a depth just above the water-bearing zone, approximately 35 feet bgs The borings were then drilled through the protective casing to a total depth of 45 feet bgs Four-inch diameter polyvinyl chloride (PVC) casing was then placed through the conductor casing and the wells were constructed On October 14, 1993, monitoring well MW-4 was installed, at the request of the EHD, in the same manner as the other wells at a location downgradient of the former diesel fuel dispenser island The well locations arc shown on Drawing 1-2 The boring and well • FASCLOSE 4 4-1 92-1177102 36