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SECTION 4.0 SITE INVESTIGATION ACTIVITIES • t,�c sf Joaquin County Fairgrounds, located-e,.a4 of Caltrans Shop 10 Maintenance Station According to Mr CIark, "the two private wells at the mobile home park were closed a couple of years ago," and "the fairgrounds used to have [private] wells " The location and the number of private wells once located on the fairgrounds was not available at Clark Well, but their locations were more than 500 feet from the Caltrans District 10 sites (Clark, 2000) The closed, mobile home park wells were located within 500 feet of Stockton Yard Clark Well provided copies of well logs and pumping rates (where available) for the local, public and private wells in the vicinity of the Stockton Yard site Well logs available through Clark Well and the California Water Service Company indicated general stratigraphy consisting of a clay layer extending from the surface to approximately 40 to 63 feet bgs, intermittent sand and fclay, a clay layer from approximately 75 to 135 feet bgs and extending to approximately 135 to 190 feet bgs, followed by layers of sand and gravel Available water quality information for the two wells on the Clark property, and for the California Water Service Company Well No 46-01 indicated that concentrations of the constituents of concern (TPH-g, BTEX and MTBE) were below their respective detection limits Water quality data from the Clark Well was collected in 1988 and 1989, and data from the California Water Service Company was collected in 1999 Water quality data for mobile home park wells were not available Available information is �. included in Appendix D Based on the results of the local well survey, there does not appear to be any private or public wells within 500 feet of the property boundaries of Stockton Yard, with the exception of the closed wells located on the mobile home park property The two Clark Well wells are approximately 500 feet north of the northern property boundary The mobile home park and Clark Well wells are likely located greater than 500 feet from the former USTs at Stockton Yard Based on the review of 8 years of groundwater sampling data for wells on the Stockton Yard site, the dissolved petroleum hydrocarbon plume does not appear to have migrated to these public and private wells 4.2 GROUNDWATER SAMPLING AND ANALYSIS URSGWC conducted vertical groundwater sampling and analysis in two wells (MW-6 and VW-2) at Stockton Yard on April 7, 1999 The objective of the groundwater quality monitoring and sampling was to collect discrete samples vertically within the developed wells to assess the vertical extent of petroleum hydrocarbons Discrete groundwater samples were collected at three depths within the respective water columns of each well The discrete samples were collected without immediate prior purging of groundwater from the wells at depths of (1) within approximately the top 2 feet of the water • table, (2) at approximately 5 and 10 feet below the water table, and (3) approximately 15 to 20 S 4PROJ441040100041SYYAR01MAINTYAROSA.00C 4-2 URS Greiner Woodward Clyde r