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Peter MacNicholl <br /> Page 4 <br /> May 17, 2006 <br /> 5. Sample Depths: <br /> 5.1. Surface Soil: Surface soil will be sampled by combining soil from 0-1 feet below <br /> ground surface (bgs) at a location. Some chemicals can be tightly bound to surface <br /> soil. Therefore, mixing the top one foot soil in the surface soil sampling may dilute the <br /> concentration of these chemicals and hence underestimate the potential human health <br /> risk from exposure to these chemicals. USEPA risk assessment guidance (USEPA, <br /> 1989) defines surface soil as soil from 0-2 inches below ground surface. HERD <br /> generally accepts surface soil be collected from 0 to Y2 ft bgs. We recommend <br /> amending the sampling plan to reflect the appropriate soil depth (0 to Y2 ft bgs)for <br /> surface soil. Although we occur with collecting individual surface soil sample by <br /> combining soil from 0 to Y2 ft bgs at a location, HERD would like to remind USACE that <br /> results of composite soil samples collected by mixing soil from different locations cannot <br /> be used in risk assessments. <br /> 5.2. Subsurface Soil: The Draft SAP states that shallow subsurface soil samples will be <br /> collected at 5 ft bgs. Mixing of surface and subsurface soil may occur during <br /> construction and excavation activities. HERD considers that human exposure to soil at <br /> 0 to 10 ft bgs is complete under the current residential exposure scenario and all future <br /> human exposure scenarios (residential and industrial worker). Therefore, we <br /> recommend collecting subsurface soil samples at both 5 and 10 ft bgs. <br /> 5.3. Soil Gas: The Draft SAP does not include information on depth to groundwater <br /> onsite. We understand that the Draft SAP is designed for soil and soil gas investigation. <br /> Soil gas samples are designed for evaluation of indoor air vapor intrusion risk under the <br /> residential and industrial exposure scenarios. According to the DTSC vapor intrusion <br /> guidance (DTSC, 2005), it is recommended to collect soil gas at several depths (5 feet <br /> bgs to top of the water table). In the absence of groundwater depth information, HERD <br /> concurs with collecting soil gas at 5 ft bgs. In addition, we also recommend collecting <br /> deep soil gas at 15 ft bgs and at the top of the water table to minimize barometric <br /> influence on the soil gas data. <br /> 6. Conceptual Exposure Model: According to Figure 4, the updated HHRA will <br /> include the following complete exposure pathways under all human receptors onsite: <br /> — inhalation of VOCs in indoor air, <br /> — inhalation of VOCs in outdoor air, <br /> — ingestion of non-VOCs and inorganics from home-grown produce, <br /> — incidental ingestion of all COPCs in surface and subsurface soil, and <br /> — direct dermal contact to all COPCs in surface soil and shallow subsurface soil. <br /> HERD recommends including the exposure to non-VOCs and inorganics through <br /> inhalation of fugitive dust from surface soil for current receptors and subsurface soil for <br /> residents, construction workers and future industrial workers. <br />