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Arsenic Tracer Experiment

Conceptual diagram of tracer test.
Conceptual diagram of tracer test. Suboxic ground water with arsenic(V) added to it was injected into anoxic ground water. The resulting plume of suboxic ground water monitored downgradient of the injection well.
Graph of cissolved concentrations of iron(II), nitrate, arsenic(V), and arsenic(III)
Dissolved concentrations of iron(II), nitrate, arsenic(V), and arsenic(III) measured one meter downgradient from the injection point. Concentrations of iron(II) are relative to the background concentration in the anoxic ground water. Concentrations of nitrate and arsenic species are relative to the injected concentration.

USGS scientists and their colleagues have been conducting tracer experiments to observe the biogeochemical processes, such as oxidation, reduction, and adsorption, that control arsenic mobility in ground water. The current tracer experiment, reported on in the Journal of Contaminant Hydrology, was designed to study the biogeochemical processes that control one important reaction pathway for arsenic—the fate of arsenic(V) (arsenic in the plus five oxidation state, also known as arsenate) in anoxic (without oxygen) ground water with iron and nitrate present. For a period of four weeks, the scientists pumped ground-water containing nitrate, small amounts of oxygen (suboxic conditions), and no iron; added arsenic(V) to it; and injected it into the subsurface in an area where there was no nitrate, no oxygen (reducing conditions), and iron(II) (iron in the plus two oxidation state, also know as ferrous iron) was present. The scientists then monitored ground-water quality at various distances downgradient from the injection well. The tracer experiment took place at the Cape Cod Toxic Substances Hydrology Program Research Site. During the injection period, dissolved and adsorbed iron(II) was oxidized to hydrous ferric oxide by nitrate. This can be seen in the graphs, which show concentrations of various species over time at a distance of approximately one-meter downgradient from the injection well. During the period from about 9 days to 17 days, nitrate concentrations near zero and decreasing concentrations of iron(II) provide evidence that dissolved and adsorbed iron(II) was being oxidized to hydrous ferric oxide by nitrate. After 17 days, at which point all of the iron(II) had been oxidized, the nitrate concentration increased back to that in the injected ground water. Adsorption of arsenic(V) onto the freshly precipitated hydrous ferric oxide removed essentially all of the injected arsenic(V) until approximately three weeks into the experiment. As the nitrate plume passed by the observation well, reducing conditions were reestablished by the native anoxic ground water, and the reduction of dissolved and adsorbed arsenic (arsenic(V)), most likely by arsenic-respiring microorganisms, resulted in large increases in a more mobile form of arsenic (arsenic(III), arsenic in the plus three oxidation state, also known as arsenite). These findings illustrate how the biogeochemical conditions (oxidation or reduction) controls the mobility of arsenic in ground water, as well as the potential importance of nitrate in arsenic transport in ground water.

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Höhn, R., Isenbeck-Schröter, M., Kent, D.B., Davis, J.A., Jakobsen, R., Jann, S., Niedan, V., Scholz, C., Stadler, S., and Tretner, A., 2006, Tracer test with As(V) under variable redox conditions controlling arsenic transport in the presence of elevated ferrous concentrations:  Journal of Contaminant Hydrology, v.88, p. 36-54.

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Additional Reference

Amirbahman, A., Kent, D.B., Curtis, G.P., and Davis, J.A., 2006, Kinetics of sorption and abiotic oxidation of arsenic(III) by aquifer materials: Geochimica et Cosmochimica Acta, v. 70, no. 3, p. 533-547, doi:10.1016/j.gca.2005.10.036.

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