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• All nine onsite monitoring wells. Four of these wells(MW-3,MW-6,MW-7 and MW-9 <br /> had the top of their screen below the groundwater table. Four monitoring wells, MW-10, <br /> MW-16, MW-23 and MW-24,have had no or low, sporadic petroleum hydrocarbon <br /> detections and were previously removed from the sampling program. Well MW-22 was <br /> removed because of site redevelopment plans. <br /> Well SW-10 could not be located. The 23 air sparging wells (made from 1-inch PVC tubing) <br /> were plumbed underground, without well boxes, and were not visible from the surface. <br /> According to representatives of Advanced GeoEnvironmental, who installed tir wells, and had a <br /> representative visit the site during the attempt to locate the wells,the well locations were never <br /> surveyed, and no as-built drawings of the well network was prepared. Treadwell& Rollo used a <br /> backhoe to follow subsurface PVC piping from the old sparging system location to the sparging <br /> wells, and excavated the area around SW-10. During the planned subsurface demolition in April <br /> 2004, an additional attempt will be made to locate this well. <br /> 6.0 PROPOSED REMEDIAL WORK <br /> The data indicate that high concentrations of gasoline, and possibly free product, are present in <br /> soil and groundwater around the former UST location. From the early to mid 1990s,free product <br /> was consistently measured at a thickness greater than one foot in well MW-3- Since <br /> groundwater analyses began in MW-3 in 1996 (after the top of the well screen was submerged <br /> and product was no longer observed), the TPHg concentrations have not shown a significant <br /> change, and have averaged approximately 180,000 µg/L. This is in spite of the fact that, for <br /> several years, in-situ remediation systems (air sparging, vapor extraction and ozone sparging) <br /> operated at the site. During recent groundwater sampling, boringsl6B-6b and 16B-8 had similar <br /> TPHg and benzene concentrations to MW-3,and a hydrocarbon sheen was observed on <br /> groundwater in these borings. <br /> Soil sampling conducted during historic installation of wells around the former UST location, <br /> and during our investigation, identified high TPHg concentrations in soil above the water table <br /> 8 <br /> 25971014.DGD 7 April 2004 <br />