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January 31, 2024 Risk-Based Evaluation for Reuse of Stockpile Material for the Proposed Treatment Wetland Project – City of San Leandro Water Pollution Control Plant 20240131-Terraphase-SanLeandro-TreatmentWetland-Risk-TM-DRAFT.docx Page 6 of 10 summarizes the DI-WET soil and sludge data relative to Aquatic Habitat Goal ESLs. Only metals whose 95-percent upper confidence limit on the mean (UCLM) DI-WET concentration exceeded at least one of the ESLs are presented. Table: Comparison of DI-WET Concentrations to Aquatic Life ESLs Metal Fresh Water (μg/L) Saltwater (μg/L) 95% UCL on the Mean Stockpile Soil (μg/L) Storage Pond Sludge (μg/L) Cadmium 0.25 9.3 1.0 NC* Copper 9 3.1 23.9 5 Lead 2.5 8.1 7.8 5.45 Mercury 0.025 0.025 0.12 NC* Nickel 52 8.2 2.8 15.8 Selenium 5 0.5 2.1 3.30 Silver 3.4 0.19 NC* 0.732 Vanadium 19 NE 51.1 7.25 Notes: NE = ESL not established, NC* = UCLM not calculated because all results were non-detect. The UCLMs were generally above one ESL but not the other, except for copper and mercury in the stockpiled soil, which were above both the fresh water and saltwater habitat ESLs. However, all of the exceedances were fairly minor. Given that the sludge is underlain by Bay mud, which has a very low permeability and is likely to attenuate the movement of metals, and that the pond is in an area where hydraulic gradients are due to be upward (i.e., in or near a groundwater discharge zone), it is unlikely that dissolved metals would migrate to the underlying groundwater. This is borne out by historical groundwater data. Groundwater samples were collected in 2016 from three wells screened in shallow groundwater immediately next to the storage basin (Fugro 2016). The three wells were located between the storage basin and the tidal slough. The concentrations of dissolved metals detected in the groundwater samples were all below the fresh-water and saltwater habitat ESLs. The potential stabilization of the pond sludge is still in its planning stages; however, it is likely that Portland cement (PC) will be used to stabilize the sludge, and/or sludge-soil mixture, to facilitate the construction of the Project. Mixing of PC with the sludge/soil will result in more alkaline conditions. However, the impact of higher pH on the chemical composition of any potential leachate is considered to be minimal because the leachability of most metals is expected to decrease as a result of PC stabilization.1 An alkaline pH can potentially increase the solubility of arsenic, but the overall arsenic leachability is expected to decrease due to PC stabilization. Furthermore, leaching of COPCs from stabilized soil/sludge is highly unlikely given its placement below clean fill and the impermeable liner over the clean fill. 1 Solidification/stabilization are well-established technologies for reducing the leaching of contaminants, including metals, from soil and sediment (https://clu-in.org/techfocus/default2.focus/sec/Solidification- Stabilization/cat/Overview/)