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tLVA Li0 A S S O C I A T E S I N C 2_3 Groundwater Sampling and Analysis One groundwater sample was collected from borings SB28 through SB37 at the depth groundwater was first encountered, approximately 9 to 14 feet bgs A Geoprobe® Screen Point ' 15 Groundwater Sampler was threaded onto the leading end of the Geoprobe after groundwater had been encountered When the sampler was advanced, O-ring seals and an expendable drive point provided a watertight system to ensure sample integrity When the tool was advanced to the desired depth, extension rods were sent downhole to brace the ' bottom of the sample screen as the tool casing was retracted When the casing was retracted, approximately 41-inches of screen with slot sizes of 0 004 inches was exposed Teflon tubing with a check valve attached to one end was inserted down the casing until it was immersed in groundwater Water was then pumped through the tubing and to the ground surface by oscillating the tubing up and down Groundwater samples were placed on ice and delivered under chain-of-custody documentation to Sequoia or Argon for chemical analysis Following soil and groundwater sample collection, the borings were backfilled with neat cement grout to the ground surface All groundwater samples were analyzed for TPHg, TPHd, BTEX, and oxygenate fuel additives with the exception of the water sample collected from boring SB30 Due to the slow rate of extraction, not enough water was collected to perform analysis for TPHd 30 SUBSURFACE CONDITIONS ' 3.1 H dro eolo is Conditions ' Stockton, California is located in the San Joaquin Valley, the southern extension of the California Great Central Valley (Figure 1) The Great Central Valley is a deep alluvial plain ' extending nearly 500 miles from the Siskiyou Range of the Cascade Mountains in the north to the Tehachapi Range to the south At Stockton, the San Joaquin Valley is approximately 40 miles wide The western Valley boundary consists of the low, rolling foothills of the California ' Coast Range Mountains (maximum elevation due west is less than 3,000 feet) and the eastern boundary consists of the more rugged foothills of the Sierra Nevada (maximum elevation due cast is over 10,000 feet) The Great Central Valley contains a thick sequence of sediment which, in places, reaches a depth of 10 miles These sediments range in geologic age from Jurassic (205 million years before ' present) to Recent (present time) and include both marine and continental deposits The site's immediate subsurface geology consists of PIeistocene and Recent alluvial deposits These alluvial deposits consist of heterogeneous sequences of sand and gravel originating from active ' stream channels, and silt and clay originating from overbank and marsh depositional environments w 1625141reponslsummv doc 4 1