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ARCADIS o Magnesium concentrations increased in post-injection sampling at wells affected by the injections(U-27A, U-27A', U-276')by 8 to 76 mg/L. Magnesium concentrations decreased to below baseline levels in U-27A'and U-2713'in post-injection sampling. In U-27A, magnesium concentrations decreased by 50-percent between the 3-month and 6-month post injection event and are currently 57 mg/L, near-baseline levels (33 mg/L). There is no water quality objective specific to magnesium. o A temporary increase in TDS concentrations occurred following injections. For example, the largest increase in TDS concentrations occurred in U-27A. In this location,TDS concentrations increased to 2,280 mg/L three months following injections, followed by a decrease to 1,200 mg/L six months following injections relative to baseline concentrations up to 866 mg/L. Sulfate is the main component that caused the increase in TDS concentrations,and TDS is expected to decrease as the remainder of sulfate is consumed in this location. o Dissolved iron and manganese concentrations increased 3 months following injections in wells where the greatest concentrations of sulfate were delivered and sustained two months after injections(i.e., U-27A, U-27A', and U-278'). Increases in dissolved iron and manganese concentrations ranged from approximately 3 to 3.5 mg/L. However, between 3 and 6 months following injections iron and manganese concentrations decreased approximately 40-90 percent in U-27A and U-27A',while in U-2713' iron concentrations remained fairly constant and manganese concentrations decreased.The sulfide generated through microbial sulfate reduction is known to chemically reduce iron and manganese present in the soil matrix, making these metals more soluble and temporarily increasing their groundwater concentrations. o Low concentrations of sulfide were generated following injections,with a maximum concentration of 790 micrograms per liter(Ng/L)in U-276. Sulfide concentrations are decreasing in U-27A',which received the highest concentration of sulfate during the injections. Sulfide concentrations are expected to decrease similarly in other wells through oxidation and precipitation reactions. Summary and Proposed Activities In summary, biological oxidation via sulfate reduction of BTEX,TPH-DRO, and TPH-GRO has been stimulated by sulfate injections. Groundwater monitoring will continue and existing data indicates there will be no long-term adverse secondary water quality effects to the regional aquifer as a result of the sulfate injections. Significant indicators of injection solution (sulfate and fluorescein)or secondary water quality effects have not been observed downgradient of the treatment zone in wells U-3 and NP-2-22, demonstrating that transient effects of the injection are limited to site boundaries. Page: gl�mjecPgrvYlietmn\"A6115 'x,112 mmmwka4on`mrespanCe lconespuNen®rlmttc,mnay131KA155im...,M 10102011. 3/4