The most common source of arsenic contamination in ground water is the mobilization of naturally occurring arsenic on sediments. Given the right chemical conditions in the subsurface, arsenic can dissolve into ground water used for drinking water. Arsenic can have adverse health effects in people who drink water high in arsenic. U.S. Geological Survey (USGS) scientists and their colleagues are conducting field experiments on Cape Cod, Mass., to understand the biogeochemical processes that control arsenic mobility in ground water. The results of one experiment, recently reported in Journal of Contaminant Hydrology, demonstrate that chemical reactions between nitrate, iron and oxygen can affect the mobility of trace amounts of arsenic.

Their experiments consisted of injecting a solution of ground water with dissolved arsenic added into the subsurface through an injection well. Monitoring wells in the resulting arsenic plume then were sampled and analyzed to study the chemical processes controlling arsenic mobility. The resulting information will help water-resource managers understand what conditions would favor the mobility of arsenic in ground water supplies.

Arsenic Tracer Experiment

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 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. Findings showed concentrations of various species over time at a distance of approximately one-meter downgradient from the injection well.

During the period from about nine 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.