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Review of the Draft CPT Groundwater Investigation Work Plan of the Off-Depot TCE Plume DDJC Sharpe Site, Lathrop, California February 2007 SPECIFIC COMMENTS 1. Table 2-1,Rationale for Locations of Proposed CPTs for Definition of Off-Base Plumes, Page 2: An additional cone penetrometer testing(CPT) location to the west of proposed location OD-1 may be needed to establish horizontal extent downgradient o contamination at MW-510C. Section 1.1 states that the groundwater investigation objective"is to estimate the downgradient extents of trichloroethene(TCE) contamination exceeding detection limits in the C aquifer zone..." According to Tab.e 2- 1,the purpose of CPT location OD-1 is to find the vertical extent of TCE exceeding tie detection limit in the C zone near monitoring well MW51OC. However, Table 2-1 al o states the TCE concentration in MW510 is 0.2927 micrograms per liter(ug/L) (near the detection limit), so the horizontal extent of TCE exceeding detection limits in the C aquifer zone has not been defined. Given the location of downgradient wells PWO12 PWO13 and AG12, it maybe important to define the horizontal extent of TCE. Pleas consider adding a CPT location to the west of proposed location 013-1 to find the downgradient horizontal extent of TCE exceeding the detection limit in the C aquifer zone, or explain why an additional location is not needed. 2. Table 2-1,Rationale for Locations of Proposed CPTs for Definition of Off-Base Plumes, Page 3: For proposed CPT location OD-11, it appears the proposed maximum sample depth should be deeper than 180 feet below ground surface (bgs). Table 2-1 states OD-11 will be sampled only if samples from 013-7, OD-8 or OD-10 have results greater than detection limits. However,proposed location OD-10 may be sampled to a depth of 200 feet bgs, according to Table 2-1. If TCE concentrations at OD-10 excet d detection limits at depths greater than 180 feet bgs, it appears that the Work Plan should include an option for sampling OD-11 at approximately the same depth. Please cons der extending the maximum sample depth for proposed CPT location OD-11 from 180 feet to 200 feet bgs. 3. Section 2.4, Page 3: It appears the Study Boundaries described in the data quality objectives (DQOs) should be greater than 150 feet bgs,which is shallower than the deepest proposed samples in Table 2-1 and the depth of the C aquifer zone. Table 211 shows two CPT sampling locations (OD-10 and OD-12)with proposed sampling depth intervals to 200 feet bgs, and Section 2.6 on page 4 implies that the C-zone interval extends to 180 feet bgs. Please revise the vertical extent of the Study Boundaries to be consistent with the proposed field work or explain why 150 feet bgs was chosen as the lower Study Boundary.