Project 5 - Application of Enhanced Mitigation Methods for Groundwater As at US Superfund Sites

Field work focused on enhancing remediation approaches for arsenic contaminated aquifers.
Enhancing As remediation approaches

Background: With the exception of lead, more National Priorities List (NPL) Superfund sites have arsenic as an issue than any other contaminant of concern. Yet, there are very limited remediation strategies for groundwater systems contaminated with dissolved As that are efficient and time effective. As such more methods are needed. Our work focuses on developing two enhanced remediation strategies which could potentially save time and money at Superfund sites contaminated with arsenic. Our prior work has shown that injection of oxalic acid can substantially increase mobilization of arsenic from sediments at the Vineland Superfund site (from laboratory to pilot field scales), potentially showing a way to greatly enhance the pump and treatment methodology currently in use there as well as at other sites. We are also actively investigating in situ immobilization approaches that might provide cheaper and more effective strategies for certain scenarios or separate portions of Superfund sites. We hypothesized that magnetite will be a good target mineral for arsenic immobilization since it is stable under both oxygenated and reducing conditions found at many sites and has recently been shown to incorporate arsenic into its structure. Our initial experiments carried out on sediments from two Superfund sites showed that additions of ferrous iron and nitrate additions resulted in formation of iron oxides including appreciable amounts of magnetite and that the incubations with the magnetite kept dissolved As conditions low even under reducing conditions.

Current work: We are currently working on aquifer sediments collected from Dover Municipal Landfill Superfund site in NH, where natural As appears to be the primary source of elevated As in the groundwater. Our initial aims for the oxalic acid approach will be to see whether the approach is transferable to a site with naturally-occurring arsenic that is being mobilized by reducing conditions- i.e. does the oxalic acid remove the mobilizable As leaving behind As that is less available. Our initial aims with the magnetite approach will be to carry out preliminary laboratory studies in order to predict when magnetite will form and when it will effectively result in a stable net sink of aqueous arsenic. We are also investigating whether certain geophysical parameters which can be monitored remotely through electron probes can be used to estimate the amount and location of mineral formation. To be able to scale-up to field studies for either of these in situ approaches we will have to be able to carry out the amendment injections in a way that can overcome spatial heterogeneities in both hydraulic flow-paths and arsenic and iron geochemistry. As such, key aims of our work include characterizing the fine scale variability in hydraulic conductivity of the field areas and modeling different injection and pumping scenarios. To validate the models we plan on using continuous, high-resolution geophysical resistivity survey methods during the in situ injection experiments to determine in real-time the advection and dispersion of injected amendments and reactions that occur in situ. We expect to carry out field experiments at the Dover Landfill site in the later years of the grant.

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