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Work Plan-Soil and Groundwater Remediation CSUS Multi-Campus Regional Center,Stockton,CA 1 March 3,2003 Page 4 of 8 Sandy lenses occur sporadically throughout the subsurface and range in composition from clayey sands to medium/coarse-grained sands. A relatively continuous sandy horizon occurs between 38 and 45 feet bgs and is shown in Figures 5, 6, and 7, Appendix A. This sandy bed ranges in composition from sandy, silt to clayey sand/sand and varies slightly in depth and thickness between borings, but is apparently continuous across the site. The subsurface soils were dry to approximately 18 feet bgs and became increasing moist with'depth. Groundwater was encountered at approximately 40 feet bgs within the sandy horizon and rose in the ' borings, suggesting the aquifer is under semi-confined conditions because of the overlying clays. The estimated groundwater gradient for the site has varied over the past year from northeast to southeast. The estimated groundwater gradient for the monitoring event conducted on December 27, 2002, was ' 0.0010 ft/ft to the northeast. Historical groundwater gradients estimated for the site were calculated using monitor wells MW-1,MW-2, and MW-3, and are shown in Figure 8, Appendix A. �. 4.0 FIELD AND ANALYTICAL METHODS This section includes a description of the excavation and backfill plan; dewatering activities; soil over- excavation, including soil sampling, and laboratory analytical procedures; and, in-situ groundwater remediation. 4.1 EXCAVATION AND BACKFILL PLAN An Excavation and Backfill Plan (Plan) shall be prepared by the excavating contractor to provide a safe working environment for the contractor's employees and on-site personnel, and to protect the public from the work area. The Plan shall be suitable to prevent failure of the excavation walls, protect the groundwater quality, and prevent the off-site migration of sediment. The Plan shall meet the requirements of the City of Stockton Grading and Building Ordinances and the 1997 Uniform Building Code, Chapter 33 [Site Work, Demolition, and Construction, Appendix Chapter 33 (Excavation. and Grading)]. The contractor is responsible for obtaining the necessary permits to perform the excavation, including any permits required by City of Stockton. The lateral extent of the proposed excavation area is shown in Figure 9, Appendix A. Upon completion of the over-excavation, the excavation will be backfilled with excavated overburden soil and clean imported soil. Excavated overburden soil that does not contain petroleum hydrocarbons at G or above the detection limit of the field screening device (1 part per million total organic vapors) and does not appear to be impacted by petroleum hydrocarbons based on field observations (visual and odor) I will be placed in the excavation first. Imported backfill shall consist of suitable soil meeting the Unified Soil Classification for GC, GM, SM, ML, or SC soils. The backfill will be placed and compacted in six to eight-inch lifts. All, soil and aggregate base backfill materials shall be compacted to a minimum compaction of 90% per ASTM 1557. Density tests will be conducted at two-foot ihtervals. A minimum of two compaction curves will be produced based on the density testing for the excavated soil to be used as backfill. Additional compaction curves will be necessary for imported fill. The information provided in this work plan is for the convenience of the contractor, and it is the , contractor's responsibility to research data, permit requirements, and existing site conditions as needed to } 1 prepare a suitable excavation plan. t 4Ma F CONDOR i, 2