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ivow f� CHAPTER 2 FIELD METHODS The existing groundwater monitoring wells were sampled on May 11 and 12, 1987, by Brown and Caldwell (BC) personnel. Prior to sampling, approximately 25 to 30 gallons of water , representing three to four well volumes, were purged from each well using a centrifugal pump. During purging, the pH, specific conductance, and temperature of the discharged water were monitored. After these parameters had stabilized (when three consecutive readings, taken 5 minutes apart, are within 10 percent of each other) , a sample was collected with a Teflon bailer equipped with a bottom-emptying valve. Each sample was then drained directly from the bottom valve into four 40-milliliter glass bottles with Teflon-lined septums and into one 1/2-liter plastic bottle containing nitric acid preservative. On May 15 , 1987 , BC personnel sampled the City of Stockton Municipal Utility District (City) wells 20M2, 20F1, and San Joaquin County Flood Control District (District) well 20Ll. All three wells are currently in operation, and were running at the time of sample collection. Each sample was collected from the tap located on the discharge line of the pump. The tap was allowed to run for 15 to 30 minutes prior to sampling. Each sample was collected in four 40-milliliter glass bottles with Teflon-lined septums and into one 1/2-liter plastic bottle with nitric acid preservative. The samples were stored on ice from the time of collection through delivery to BC' s California Department of Health Services state-certified analytical laboratory in Emeryville , California. Prior to each use, all groundwater sampling equipment was washed with laboratory detergent and tap water , then rinsed with deionized water . Pending results of the analytical tests, all groundwater produced during well purging and sampling was contained and temporarily stored on site in 55-gallon drums. Photoionization detector (PID) measurements were taken from each well to identify relative hydrocabon vapor concentrations. On May 26, 198741 seven boreholes were drilled. The boreholes were drilled by P.C. Exploration, Inc. , with a truck-mounted drilling rig equipped with 4-inch-inside-diameter and 8-inch-outside-diameter hollow-stem augers. P.C. Exploration, Inc. , has a C-57 contractors license. The locations of the boreholes are shown on Figure 1-3. Each borehole was advanced to approximately 10 feet. Soil samples for lithologic classification and laboratory analyses were collected at 5-foot intervals by using an 18-inch-long by 2-inch- diameter Modified California-type sampler with 6-inch-long brass tube liners. The drive sampler was driven ahead of the auger into undis- turbed soil Each borehole was logged from the samples taken at 5-foot intervals, using the Unified Soil Classification system. i - t