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Y approximately 0.0004 ft/ft toward PSW. Downward vertical gradients are observed to the east of the Site and increase toward the water supply wells as a result of the pumping in the deep coarse sand and gravel layer overlying the Corcoran Clay (Figure 5). 2.3 Sulfolane Based on a review of the current and historical sulfolane concentrations in monitoring wells, remedy extraction wells, and water supply wells,the CSM indicates that sulfolane in the eastern portion of the Site appears to have migrated vertically downward to the top of the Corcoran Clay and then laterally in the deep coarse sand and gravel layer towards the water supply wells (Figure 3). Sulfolane was first detected in PSW in March 2016 at an estimated concentration of 5.2 micrograms per liter(gg/L), below laboratory reporting limits. In the most recent(May 2018) PSW sample, sulfolane was detected at a concentration of 18 µg/L. Based on this increasing concentration trend, it appears that sulfolane may be migrating east toward PSW and LWD-10. Based on the currently available data it is unknown if pumping at PSW provides hydraulic containment to prevent the movement of sulfolane to LWD-10. 2.4 Data Gaps Identified Based on CSM Review Most of the boreholes east of the Site were terminated prior to encountering the Corcoran Clay and the depth of the Corcoran Clay is uncertain. Additionally, many of the existing boring logs from water supply wells used in the development of the understanding of the general stratigraphic layering to the east of the Site were drilled using the mud rotary drilling method without a geophysical log and/or were logged by drilling contractors and not under the direction of a California licensed Professional Geologist. Developing a better understanding of the detailed stratigraphy and depth of the Corcoran Clay to the east of the Site is important to understanding potential transport pathways for sulfolane and to evaluate potential remedial alternatives needed to protect the water supply wells. The vertical and lateral extent of sulfolane east of the Site is not defined by the existing groundwater data. The detections of sulfolane in PSW with the lack of detections in PW-25 and PW-34 indicate that either the sulfolane plume is narrow and is traveling between PW-25 and PW-34 or is thin and is traveling underneath monitoring wells PW-25 and PW-34. Understanding the vertical and lateral extent of the sulfolane plume between the on-Site source area and the water supply wells is important to understanding the potential transport pathways and evaluating potential remedial alternatives to protect the water supply wells. Site Characterization Work Plan 3 June 22,2018