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y ATC Associates Inc. .. v 1117 one Palm Avenue, Suite 201 v Modesto, California 95351 209-579-2221 y ASS O C I ATE S I N C . fax: 209-579-2225 .r accurately represent adsorbed-phase petroleum concentrations, if present, due to the water component of the sample. r ,: .F 1, , _ wrs �- - J ve 7 bw Historically, elevated adsorbed TPHg was encountered in the shallow soils (six to ten feet bgs) beneath the site. The soil sampling proposed during this investigation will attempt to further bw define the vertical and lateral extent of TPHg(and BTEX, where applicable) both on-site and off- site. Figure 10 depicts the TPHg in soil isoconcentrations from six to ten feet bgs from data collected during previous subsurface investigations. Refer to Table 3, which is a summary of the soil analytical results. The monitoring wells will be constructed of 2-inch inside diameter, Schedule 40 PVC casing and tAW 0.010-inch slot screen PVC. `The wells will be installed to approximately 20 feet bgs with 15 feet of 0.010-inch slot screened PVC. A sand pack will be placed in the annulus of each well extending from the bottom of the well screen to approximately one foot above the top of the screen. Approximately one foot of bentonite will be placed on top of the sand pack and hydrated with potable water. The remaining annulus will be completed to surface with cement grout. To Wo protect the integrity of the wells, locking, watertight well plugs will be installed on each well and a watertight wellhead labeled "monitoring well" will be set in concrete over each well. The proposed monitoring well locations are shown on Figure 11. Refer to Attachment 1, which `„ depicts a typical groundwater Zone A monitoring well construction diagram. Zone B and Zone C Vertical Delineation r As previously indicated, a grab groundwater sample collected from CPT-2 at 41 feet bgs on October 14, 2004, i gated t resence of dissolved MTBE, BTEX, and TP at45��f bw concentrations of 2, µg/1�' .1 µg/l, 2IR µg/1, 0 µg/1, 2�D /S and 7,600 µg/l, " respectively. MTBE, BTEX, anS TPHg were detected in a grab groundwater sample collected from CPT-2 at a depth of 67 feet bgs at the following respective concentrations: 120 ttg/l, 2.7 y µg/1, <0.5 µg/1, 7.9 µg/l, 20 µg/1, and 300 µg/l. Two groundwater monitoring wells are proposed to be installed at the site to evaluate groundwater quality in Zone C. These wells will be installed adjacent to the USTs and MW7 to a depth of approximately 75 feet bgs. The monitoring wells will be installed inside conductor y casing extending to approximately 55 feet bgs in an effort to minimize potential vertical contaminant migration from shallower groundwater. The monitoring wells will be constructed of WWI' 2-inch inside diameter, Schedule 40 PVC casing and five feet of 0.010-inch slot screen PVC. A IBM sand pack will be placed in the annulus of each well extending from the bottom of the well screen to approximately three feet above the top of the screen. Approximately three feet of bentonite will be placed on top of the sand pack and hydrated with potable water. The remaining 6.0 annulus will be completed to surface with cement grout. To protect the integrity of the wells, 6.0 7 ATC Associates Inc. S:\Environmental\25847.0037\Work Plan additional vert-hor delineationTH2.doc y