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• C-Zone—The C-zone is the most continuous sand layer under the Facility and is separated from the other zones by 2 to 10 feet of silty clay (except at location OW-7, where no clay layer was observed between the A/B-and C-zones). The C-zone ranges in thickness from 13 to 27 feet and is first encountered at depths ranging between 21 and 48 feet bgs. • D-Zone—The D-zone consists of thin, laterally discontinuous sand layers interbedded with silt and clay. The D-zone ranges in thickness from 13 to 28 feet and is first encountered at depths ranging between 43 and 65 feet bgs. Approximately 1.5 to 7 feet of silt and clay separates the D-zone from the C-zone. Extensive lithologic data obtained during the vertical groundwater investigation (Ash Creek, 2009a) resulted in reclassifying former C-zone sands in the northern portion of the Facility as D-zone sands. Organic material consisting rootlets and degraded woody debris have previously been observed in the D-zone. Groundwater at the Facility has been gauged since the early 1990s. Monitoring well construction details are shown in Table 1. Recent groundwater elevations are presented in Table 2, and historical groundwater elevations are tabulated in Appendix A. Based on historical gauging, groundwater is typically first encountered in the A-zone at depths of approximately 4 to 10 feet bgs, and can vary up to 3 feet seasonally. Regionally, groundwater flows towards the east and southeast, with local and seasonal variations. Average flow gradients are: 0.0054 in the A/B-zone (historically 0.007 in the A-zone and 0.0038 in the B-zone), 0.003 in the C-zone, and 0.002 in the D-zone. Recent groundwater elevation contours for the A-, B-, C-, and D-zones are shown on Figures 6, 7, 8, and 9, respectively. B-zone contours have continued to be separated from A-zone contours to remain consistent with STTC maps completed for the northern leasehold. Aquifer tests performed by SECOR (2002) indicate A/B-zone hydraulic conductivities ranging between 5.1 x 10-4 centimeters per second (cm/s; 1.5 feet per day [ft/day])to 3.3 x 10-3 cm/s (9.4 ft/day), and C-zone hydraulic conductivities ranging between 10-2 cm/s(30 ft/day)and 3.3 x 10-2 cm/s(100 ft/day). Based on the hydraulic conductivity, gradients, and assumed porosity (25%), the expected groundwater velocity near the Facility for the A/B-zone ranges between 11 and 74 feet per year, and between 131 and 438 feet per year for the C-zone. Aquifer tests have not been performed in D-zone wells within the leasehold (SECOR, 2002b). 2.4 Previous Releases Well ST/MW-1 was installed in July 1992 near AST No. 3302 to investigate whether a petroleum release had occurred at the Facility (Figure 2). Separate-phase hydrocarbons (SPH) were observed in the well shortly after its installation at a thickness of approximately one foot. SPH were observed in the well at thicknesses ranging between about 1/2 and 1 foot until 1994. After 1994, the thickness of SPH observed in the well began to decrease and by 1998 only a sheen or thicknesses of less than 0.06 foot were recorded. Revised Cleanup Plan Page 3 NuStar Stockton Terminal-Stockton,California November 7,2012 1014-12