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..i Pilot Study Workplan November 7,2014 Project#54.62563.0001 Page 4 2.4 SENSITIVE RECEPTORS Four municipal wells (WP-1 through WP4) located at the four corners of the facility (Figure 2) were installed in 1955 and 1956 to provide water to the plan and to the community during periods of high demand. These four municipal wells were completed to depths ranging from 420 to 460 feet. Well WP-4 was abandoned in 1989. Historic water quality data indicate the three remaining municipal wells were sampled in 1989 and 1990 and were below the detection limit for total chromium. In 1991, total chromium was detected in WP-2 at 0.054 mg/L, but neither total nor hexavalent chromium was detected in any of these three municipal wells in sampling events conducted through 1992. The municipal wells were not sampled again until 1998 at which time both total and hexavalent chromium were again below detection, and the three wells were below detection limits in the next two monitoring events in 1999 and 2000. The three wells were sampled again in 2001 when hexavalent chromium was detected in WP-1 at a concentration of 0.0044, but total and hexavalent chromium were below detection in WP-2 and WP-3. WP-1 was sampled again in 2002 when both total and hexavalent chromium were detected at concentrations of 0.0061 and 0043 mg/L, respectively. The last time any of the municipal wells were sampled was in 2003 when only WP-1 was sampled, and in that event total chromium was below detection but hexavalent chromium was detected at a concentration of 0.0037 mg/L (ATC, 2008). In a phone conversation on March 12, 2014 between Cardno ATC and Mr. Robert Grandburg with the City of Stockton, Municipal Utilities Department, Mr. Grandburg informed Cardno ATC that the remaining municipal wells WP-1, WP-2, and WP-3 have all been destroyed. 3.0 GEOLOGIC AND HYDROGEOLOGIC SETTING This section describes the regional and site-specific geological setting and hydrogeology. The information presented was based on interpretation of data collected from site assessments and monitoring events. 3.1 REGIONAL GEOLOGY 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 east is over 10,000 feet). The Great Central Valley contains a thick sequence of sediment, which, in places, reaches a depth of ten 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 immediate subsurface in the valley consists of Pleistocene and Recent alluvial deposits, which, in turn, consist of heterogeneous sequences of sand and gravel originating from active stream channels, and silt and clay originating from overbank and marsh depositional environments. 3.2 LOCAL GEOLOGY Data collected from twenty two borings advanced at the site were used to evaluate the local geology. The Iithology across the site generally consists of alternating layers of silt and clay with some sand in the upper approximately 50 to 60 feet, with more sand intervals below approximately 60 feet. Some isolated cemented sand intervals exist between approximately 70 to 75 feet. Between approximately 105 and 120 feet the