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I� I Updated Human Health Risk Assessment Natural Attenuation Analysis and Additional Site Investigation Work Plan California Tank Lines, Inc. since 1954 �I 3105 South El Dorado Street, Stockton, California The updated HHRA will include an evaluation of the following potential exposure pathways. • On-Site ingestion of surface soil; C • On-Site dermal contact with surface soil; • Off-Site ingestion of groundwater; vj�' J ti • Off-Site dermal contact with groundwater: and • On-Site inhalation of indoor air. An adult worker and child resident will be selected as potential receptors. A cumulative carcinogenic risk will be calculated for each receptor to account for potential risks from all potential exposure pathways. In the similar manner, a cumulative hazard quotient will be r calculated for each receptor. The HHRA will be updated in conformance with the June 16, 2009 Interim Guidance, Evaluating Human Health Risks from Total Petroleum Hydrocarbons (TPH), Human and f Ecological Risk Division, California Department of Toxic Substances Control. 2.3 Natural Attenuation Assessment Taber Consultants will prepare an assessment of the potential for natural attenuation to effectively degrade impacted ground water in the vicinity of MW-4. This assessment will use soil and groundwater analytical results from samples secured in the CPT study in September 2008. re- rrY r,; 2.4 Soil Borings p • ,1 �[^ . "� Taber Consultants propose to advance two (2) Geoprobea borings to an approximate ' n 0 "za e depth of 50-feet bgs, in the locations shown on Figure 3. Borings will be advanced to the north and south of monitoring well MW-4 to further delineate the extent of groundwater impacts in 7 a�J these areas. ° ` jO/b .., To provide a preliminary indication of petroleum hydrocarbons in the borings, a portable photo-ionization detector (PID) will be used to monitor for the presence of organic vapors in drill cuttings collected from the boring at five-foot intervals. This field screening method will consist of filling a sealable plastic bag to about one-third capacity with soil and sealing the container. After allowing sufficient time for the soil vapor to equilibrate with the container's headspace, the bag will be slightly opened or pierced to allow for the insertion of the PID probe. The concentrations of organic vapors detected by the PID will be recorded on the boring log. Field screening will also include documenting visual indications for the presence of petroleum hydrocarbon impacts, such as staining, discoloration, and/or chemical sheens. Soil lithology will be logged using the Unified Soils Classification System (USCS) method. 3 Taber Project No. 051087 I i