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C=M PEA Work Plan Martin Metals Finishing Inc. (Former) Stockton, California 4.2.3 Trench Sediment Sampling Significant sediment was not observed in the trench during Site reconnaissance. To the extent feasible, CDIM will collect up to three grab samples of accumulated sediment in the drainage trench within MMF. The purpose of these samples is to evaluate the presence of residual contaminants in the trench, which is a known historical conduit of illicit wastewater discharge. Proposed trench sampling locations, shown on Figures 5 and 6, were selected due to the historical locations of MMF operations. Field staff may elect to modify or eliminate these locations based on conditions observed in the field (e.g., absents of sediment, safe trench access, areas of significant sediment accumulation, staining, etc.). 4.2.4 Subsurface Soil Sampling CDIM will contract with an environmental drilling company to advance up to 11 direct push soil borings at the Site. Proposed boring locations, shown on Figures 5 and 6, were selected to evaluate potential subsurface impacts related to Martin Metal Finishing operations, including: • Five soil borings in locations of historic operations and adjacent to the storm drain trench within the MMF building; • One soil boring adjacent to the drainage trench north of the MMF building; • Four soil borings adjacent to the storm drainpipes connecting the drainage trench to the Mormon Slough outfall; and • One soil boring in Mormon Slough downgradient of the presumed location of the old stormwater outfall. Each of the soil borings will be advanced to a target depth of 15 ft bgs. CDIM's contractor will perform concrete coring where required, then advance the boring using a hand auger to five ft bgs to ensure the area is clear of subsurface utilities or obstructions". The driller will then utilize a direct-push drill rig with continuous acetate-lined core sampler attachment to advance each boring to the target depth. Field staff will log the soils from each boring and note soil type, estimates of grain size distribution, color, moisture, evidence of contamination (e.g., staining or discoloration, odor, etc.). To measure volatile vapors in soils, field staff will place soil in a zip-top bag, allow it to equilibrate, and utilize a photoionization detector(PID)or organic vapor analyzer to collect a reading. Samples for VOC analysis will be collected with a TerraCore kit from the undisturbed soil core in accordance with USEPA Method 5035. All other samples will be collected into soil containers provided by the analytical laboratory. Samplers will change gloves between collection of each sample to avoid cross- contamination. 11 Soil boring locations to be completed as soil vapor probes will not be cleared using a hand auger to 5 ft bgs. Instead,direct- push will begin from the ground surface to allow the minimum five feet of direct push borehole above the probe, allowing for a two hour equilibration time per DTSC's Advisory-Active Soil Gas Investigations(DTSC, 2015). CDIM—Fairgrounds Industrial Park Page 12