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Geospte& consultants 1.3 Site Geology & Hydrogeology The subsurface under Building 817B has characteristics of deltaic sediment deposits, similar to the rest of RRI. These deposits contain unconsolidated clay, silt, and sand. Boring logs from Geosyntec's 2019 investigation indicate that a fill layer extends from ground surface to a maximum depth of 5 feet bgs and is underlain by unsaturated clay. Review of historical boring logs indicate that an unsaturated, organic, silty clay layer is present from approximately 4 to 9 feet bgs,underlain by an inorganic, silty clay layer from approximately 9 to 18 feet bgs. Below this clay layer, there is silty sand and sand from approximately 18 to 27 feet bgs, silty clay from approximately 27 to 34 feet bgs, and well-sorted, fine-to medium-grained sand from approximately 34 to 40 feet bgs (ERS 2008). Historical potentiometric surface maps indicate that shallow groundwater generally flows radially from the former UST, predominantly to the east and west. According to ERS, the former tank pit of UST 817B may have acted as a recharge area to the shallow groundwater water(ERS 2016). Groundwater monitoring wells in the vicinity of UST 817B are screened between 4 to 14 feet bgs (917-01 through 917-06 and 917-10 through 917-12) and 18 to 28 feet bgs (917-07 through 917- 09, 917-13, and 917-14). Depth to groundwater measured during the 2023 annual groundwater monitoring event at RRI (Geosyntec 2023) from the UST 817B wells was on average approximately 6.50 feet below mean sea level (msl) (approximately 7.30 feet bgs). 2. INVESTIGATION OBJECTIVES AND APPROACH The objective of this Work Plan is to collect sufficient data to evaluate the Site's suitability for closure under the LTCP. The LTCP requires general and media-specific criteria to be met in order for Site closure to be considered. The Site meets all general criteria of the LTCP, with the following exceptions. • A conceptual site model (CSM)that assesses the nature, extent, and mobility of the release has not been developed. Additional soil, soil vapor, and groundwater data collected during the subject investigation will allow for a CSM to be developed, specifically a CSM that evaluates the potential presence of secondary source material and better characterizes petroleum hydrocarbon concentrations in soil, soil vapor and groundwater. • Secondary source material may not have been removed to the extent practicable. The LTCP defines secondary source material as petroleum impacted soil or groundwater located immediately beneath the point of release from the primary source. One boring location advanced in 2022 at the location of the former USTs and within the presumed 2010 excavation area detected TPH-g above the capillary fringe at a concentration of 1,900 mg/kg (i.e., sample collected from 2 to 4 feet bgs from boring SS-817B-06A) (Figure 3). Additional soil samples will be collected as part of the subject investigation to delineate the extent of elevated TPH-g concentrations around the former tank excavation. Investigation Work Plan—UST 817B 4 December 2024