Arsenic Mobility Investigation

 Demonstrated Monitored Natural Attenuation

SSP&A designed a field, laboratory, and modeling study following EPA Guidance on using MNA to achieve appropriate cleanup levels in groundwater within a reasonable timeframe. Included in the work was identification of the active attenuation mechanism at the site, stability of the sequestered forms, and capacity of the aquifer for attenuating arsenic under expected future geochemical conditions. SSP&A performed mineralogical and spectroscopic characterization of source materials to identify the chemical formas of arsenic present in site soils. SSP&A also designed a series of batch tests to quantify the leaching potential of mineral-bound arsenic and the capacity of the underlying aquifer to remove dissolved arsenic from groundwater via adsorption. Finally, SSP&A parameterized and calivrated a reactive transport model for arsenic migration in groundwater that included the following geochemical processes: kinetically-inhibited mineral dissolution and precipitation; redox transformations; equilibrium groundwater chemical reactions; and competitive adsorption based on a surface complexation model. Consistent with EPA Guidance, the study found that an evaluation of MNA performance requires site-specific geochemical characterization of arsenic speciation, mineral solubility, and competitive adsorption. Furthermore, reactive transport modeling is a tool that incorporates groundwater flow and chemistry, and can thus be used to assess long-term porformance of a monitored natural attenuation remedy.

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