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SECTION 4.0 SITE INVESTIGATION ACTIVITIES Site Investigation activities conducted by URSGWC for this additional groundwater study included (1) a local well survey, (2) discrete vertical sampling of groundwater in two monitoring wells, and (3) estimation of hydraulic conductivity through the performance of three slug tests 4.1 LOCAL WELL SURVEY A review of public well records maintained by the CDWR and the USGS was conducted for the Stockton Yard site A list of public wells recorded at either the CDWR or USGS is provided in Appendix D The listed wells are more than 1,000 feet from the site The California Water Service Company, the primary provider of water to residents in Stockton, was also contacted regarding the water supply wells in the area According to the California Water Service Company, a water well is located less than 1,000 feet south of the site The District Manager of the California Water Service Company, Eric Mar, indicated that he is not aware of any hydraulic communication between the shallow aquifer and the deeper aquifer(Mar, 1998) Approximately 37 percent of the water supply for the California Water Service Company is currently derived from groundwater Subsequent conversations with Mr Mar and a field reconnaissance place the well, identified as pump station 46-01, on 4'' Street, approximately 600 feet south of the southern property boundary of Stockton Yard The reported depth of the well is 450 feet, and is screened from 209 to 426 feet Additional lie information is provided in Appendix D A local well driller, Clark Well and Equipment Co (Clark Well), located at the corner of South B Street and Charter Way, was contacted on August 24, 1999 Mr Mike Clark indicated that they have two wells on their property one well is screened from 50 to 180 feet bgs and is used to supply water to support their drilling operations, and the second well is screened from 250 to 300 feet bgs and is used to supply drinking water for their property These wells are located approximately 500 feet north of northern property boundary of the Stockton Yard site The grout seals for the two wells extend from the surface to the top of the respective screen intervals Mr Clark further indicated that he and his father have drilled the majority of residential, agricultural, and industrial wells in the area, and that they are aware of a number of residential wells more than 500 feet from the Caltrans site Mr Clark indicated that the majority of residential wells in the vicinity were installed before 1972 and that the wells either did not have grout seals or had 20-foot grout seals After 1972, when well permits became a requirement, a 50-foot grout seal was required Ten years ago, a 100-foot grout seal became the requirement (Clark, 1999) Based on conversations with Mr CIark in February 2000, their shallow well was used occasionally until 1999, and has since been shut off The deeper well is reportedly used occasionally and when used, is pumped at approximately 60 gpm Mr CIark also indicated that • there have been private wells located on the Charter Mobile Home Park property, located adjacent to the south of Clark Well and adjacent to the north of Stockton Yard, and on the San S 4PROJ441040100041SYYAROIMA]NTYAROSA.000 4-1 URS Greiner Woodward Clyde