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a Former Countryside Market,Stockton, California Feasibility Study and Interim Remedial Action Plan May 20,2013 groundwater, and the groundwater gradient carrying those aqueous phase COPCs outside of this zone and subsequently into smear zone area/capillary fringe soils as groundwater levels rise and fall). According to California Environmental Protection Agency guidance (Cal/ PA 2005), humans may be exposed to the upper 10 feet of soil, although direct contact is most likely to occur in surface soils (U.S. Environmental Protection Agency 2002). Thus, since the former UST area was covered with clean soil and pea gravel following excavation activities to a depth great ir than 10 feet bgs, surface and near surface soil related exposure via dermal contact, ingestion and/or inhalation is not considered a complete pathway. 3.1.2 Ambient Air COPCs in soil can become airborne through the disturbance of soil by wird or mechanical means and the subsequent entrainment of soil particles in the air. Airborne particulates (i.e., dust) are available for exposure through inhalation when a receptor is outdoors. Chemicals could also potentially be released to outdoor air via vapor migration from COPCs in the subsurface Based on previous environmental investigation results at the Site (Sectio 2.0) that indicate maximum soil concentrations being present at a depth of 50 feet bgs back in 2001 c oupled with the fact that the former UST area is not covered (i.e., open grass covered lot with n asphalt and/or concrete), infiltration from rainfall and irrigation has continued unobstructed for approximately two decades since the tanks were removed in 1993. Additionally, the top 10 feet of soil in the former UST area was removed and replaced with clean fill in 1995. The above factors have effectively served to both limit the presence of COPCs in the shallow subsurface and drive them to greater depths. Thus, the vast majority of OPCs at the Site are either located in the deeper soil horizon beneath the former UST area (i.e., grDater than approximately 50 feet bgs) or within residually impacted soil at or near the groundwater table. Given this present condition and the fact that the depth to groundwater at the Site is located at over 80 feet bgs, diffusion from the subsurface to ambient air is deemed to be an insignificant expos ire pathway. 3.1.3 Indoor Air Volatile chemicals can enter indoor air via subsurface vapor migrations (surface and/or subsurface soils) and/or from impacted groundwater that is pumped to the surface. ' This pathway is relevant to only volatile COPCs and to receptors spending time indoors. For the reasons described above such as the depth to groundwater and given that no overlying structures are p esent over or in proximity to the former UST area (anticipated future use of the property is for it to re nain residential/agricultural), diffusion from the subsurface to indoor air is only considered to be an insignificant exposure pathway. Similarly, due to the extremely low concentrations of CPOCs that have been detected in groundwater (predominantly below groundwater screening levels for evaluation of potential vapor intrusion concerns per Table E-1 of the February 2013 Environmental Screeni g Levels, CRWQCB - San FewbilM Study and lFeasibity Study&[W TeM FinaLCoc 3-2 The Source Group,Inc.