ARCHIVED REPORTS_2019 SOIL MGMT PLAN (JULY-REVISED)

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Brusca File No. 137-002 Page 7 May 23, 2018 Murphy Parkway Property Waste Materials Reuse Workplan To further evaluate the leaching potential of arsenic in the stockpiled materials removed from WU1, we have compared the estimated mass of the DI soluble arsenic contained in the stockpiled materials to a potential volume of receptor groundwater beneath the site. Based on an average DI WET arsenic concentration of 0.02 milligrams per Liter (mg/L), and recognizing that the DI WET test utilizes 50 grams of materials and 500 milliliters of water as an extractant, we calculate the total mass of DI soluble arsenic in the stockpiled materials to be approximately 4,120 grams. Assuming, that the entirety of this mass is leached to groundwater, a saturated zone void ratio of 0.5, and a twenty-foot depth of dispersion and diffusion within the saturated zone, the soluble arsenic mass in the stockpiled materials would represent 0.007 mg/L beneath the 48-acre property; this value is below the drinking water standard for arsenic (0.01 mg/L). This mass calculation likely substantially overestimates potential arsenic impact to groundwater because the DI WET data utilized is from the samples with the highest total metals results (the actual average DI WET arsenic concentration in the stockpiled materials likely is less than 0.02 mg/L), and the mass calculation assumes that all soluble arsenic in the stockpiled materials leaches to groundwater. Despite the foregoing that suggests that the soluble arsenic, chromium and lead detected in the stockpiled materials removed from WU1 is not likely to represent a significant leaching threat to shallow groundwater quality, as a highly protective and conservative measure we propose specific procedures to be implemented during the onsite reuse of these materials to mitigate leaching potential. Specifically, we propose that the stockpiled materials removed from WU1 be placed beneath the concrete floor slab of the planned building on the property (and to a lesser degree, beneath planned concrete pavements) so that these materials are not subject to rainwater infiltration and related leaching potential. Construction plans indicate that the concrete slab-on-grade floor for the planned building on the property will be seven inches thick and reinforced with #4 reinforcing bars at 24 inches on center, each way; floor slab concrete will be designed to achieve a minimum 28-day unconfined compressive strength of 3,000 pounds per square inch (psi). The building will be completely surrounded by concrete and asphaltic-concrete pavements. Site pavements will be sloped to drain and onsite storm water will be directed toward drain inlets that discharge to the storm water system. As such, rainwater infiltration and associated leaching potential to materials directly below the future building’s floor slab is considered to be negligible. Once the building pad is constructed utilizing the materials removed from WU1 within the pad subgrade, a comparatively minor amount of these materials will subsequently be excavated during foundation and underground utility trench excavation within the building pad. We suggest that these materials be placed below concrete pavements that will adjoin the building (i.e. the planned concrete-paved dock areas northerly and southerly of the planned building). The concrete pavements are designed to be six inches thick and reinforced with #4 reinforcing bars at 24 inches on center, each way; pavement concrete will be designed to achieve a minimum 28-day unconfined compressive strength of 4,000 psi. The concrete pavements will substantially prevent rainwater infiltration and associated leaching potential to underlying materials. We propose that the materials removed from WU1 be placed within the upper one-foot of the subgrades beneath the floor slab and concrete pavement sections mentioned above since these subgrades will be cement treated as a part of construction activities to enhance the engineering/strength properties of the subgrades. The cement treatment of these materials would further reduce the leaching potential of metals in the materials. It is well recognized that cement treatment of soils can mitigate leaching potential of heavy metal ions by providing both physical