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t � Mr Edward C Ralston March 13, 2001 Page 2 • On July 24, 1998, one Geoprobe® (GP-1) was advanced to a depth of approximately 40 feet below ground surface (bgs) GP-1 delineated the extent of on-site soil impacts On August 21, 1998, two groundwater monitoring wells (MW-1 and MW-2) were installed During installation of Well MW-4, the drill rig auger broke a mismarked City of Lathrop water main Approximately 600,000 to 1,000,000 gallons of water were discharged from the broken water main On September 14, 1998, IT returned to the site to install wells MW-3 and MW-4 Based on recent groundwater monitoring data from the site, depth to water is encountered at depths between 13 and 15 feet bgs Historical depths to groundwater obtained from the Department of Water Resources report a historical shallow depth of 6 feet bgs and a maximum depth to groundwater of 33 50 feet bgs The groundwater quality data from on- site monitoring wells indicates that dissolved petroleum hydrocarbon concentrations have been as high as 67,000 parts per billion (ppb) for TPH-g and 350 ppb for benzene The extent of the groundwater impacts is not delineated SCOPE OF WORK The scope of work for this investigation included the drilling and sampling of five soil borings and conversion to groundwater monitoring wells to delineate the lateral extent of hydrocarbons in the soil and groundwater . The specific scope of work is discussed below • Soil Boring and Monitoring Well Installation. On December 4 and 5, 2000, under the direction of an IT geologist, five soil borings (MW-5 through MW-9) were drilled and converted to groundwater monitoring wells by Cascade DriIlmg, Inc (License# C57-717510) Monitoring well locations are shown on Figure 2 All five borings were drilled to a total depth of approximately 25 feet bgs and soil samples were collected at five-foot intervals using a modified California split- spoon sampler The monitoring wells were constructed with 2-inch diameter Schedule 40 PVC casing and screen intervals were placed from approximately 15 feet below to 5 feet above first-encountered groundwater Screen material consists schedule 40 PVC casing with 0 02-inch machine cut slots A number 3 sand filter pack was placed from the bottom of the borings to approximately one foot above the screen interval, followed by a one-foot thick bentonite transitional seal A two-foot thick concrete seal was placed above the bentonite A traffic rated vault box was placed in the concrete above the casing material All work was conducted in accordance with the permits obtained from the San Joaquin County Public Health Services/Environmental Health Division (SJCPHS/EHD) and the City of Lathrop Field and laboratory methods are presented m Attachment A. Boring logs and well details are presented in Attachment B 911005 1C/1205wp 0301 doc IT Corporatron