ARCHIVED REPORTS XR0006100 - Laserfiche WebLink

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The property owner has installed eleven monitonng wells in the course of investigating the soil and groundwater conditions at the site Table 1 is a tabulation of groundwater monitoring results Table 2 is a tabulation of soil sample results Geology and Hydrogeology of the Site The site is located on surface sods belonging to the Stockton Adobe Clay and Honeycut Clay series They are derived from recent alluvial fan and floodplain deposits The 40 feet of explored section consisted of clay and silt overlying interbedded lenses of silty and clayey sands below 20 feet The groundwater in the vicinity of the site is assumed to be semiconfined Groundwater elevation at the site vanes from a wet year high of 7 feet bgs to about 35-40 feet bgs dunng the previous drought period Groundwater flow direction at the site has generally been to the west since December 1997 Two water bearing sands have been delineated with the most recent well installations, April 9 and 10, 2001, see Figure 4 This first sandy formation has been designated the A zone aquifer The base of the A zone is at approximately 17 - 18 feet below ground surface (bgs) An aquitard consisting of silty clay approximately 3 to 4 feet thick separates the silty clayey sand A zone aquifer from the B zone aquifer that is found at approximately 21 feet bgs and is approximately 6 to 7 feet thick The second sandy formation (B zone aquifer) consists of cleaner moderately sorted fine to coarse sand that shows graded bedding (fine to coarse with depth) Beneath the B zone aquifer is stiff clay that extends from approximately 27 feet bgs to 40 feet bgs The original five monitor wells did not encounter groundwater in the A zone aquifer when initially drilled and sampled Depth to water measurements taken between 1993 and February 1996 indicate that the A zone aquifer was dry (depth to water greater than 17 feet bgs) NMI through MW5 are screened across both the A and B zone aquifers Sometime after February 1996 depth to groundwater measurements indicated that groundwater was in contact with the A zone aquifer Groundwater flow direction in the A zone at the site has generally been to the west since December 1997 The B zone shows a similar flow direction, which would be expected due to the connection between zones at wells MW 1 through MWS Overview of Workplan The November 27, 2000 workplan was designed to remove soils contaminated with gasoline range hydrocarbons while in the process of installing an air sparginWvapor extraction trench The removal of the contaminated soils will expedite the remediation objectives for this site The 3anaury 10, 2002 Workplan Revision addressed the need to destroy monitor wells MW 1, MW2 and MW3 in an effort to dewater the area to be excavated to approximately the 19-20 foot depth An aquifer test was to be performed prior to dewatering, to evaluate the storativity and transmissivity for both the A and B zone aquifers, see Figure 4 Monitor wells MW1, MW2 and MW3 would interfere with the evaluation of the aquifer test and would most likely snake dewatering the A zone 3 Sanchez Rev WP 2-02