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Nitrate Addition Enhances Arsenic Immobilization in Groundwater

USGS scientist collecting a groundwater sample
U.S. Geological Survey scientist collecting groundwater samples from a Cape Cod test site to study iron(II) oxidation by added nitrate. Photo Credit: R.L. Smith, USGS.

The addition of nitrate in a low oxygen groundwater resulted in the immobilization of naturally occurring dissolved arsenic and the conversion of nitrate to innocuous nitrogen gas.

Groundwater contaminated with organic carbon from wastewater can contain naturally occurring chemicals such as arsenic, manganese, and iron, which are mobilized under low oxygen conditions when biodegradable carbon is present. The mobilization of these chemicals is particularly a concern where groundwater is a source of drinking water. Scientists at the U.S. Geological Survey (USGS) did combined field and laboratory studies to determine how nitrate affects iron transformation and ultimately how these transformations affect the mobility of arsenic in groundwater.

Three test tubs with filter paper inside
Photograph of iron(III) oxyhydroxides (yellow-brown color in two left-hand tubes collected on folded filter paper) that was produced in iron(II)-containing groundwater by the addition of nitrate. The tube on the right is a filter collected from the same groundwater before nitrate was added. Photo Credit: R.L. Smith. USGS.

Samples were collected from a well-characterized sand and gravel aquifer at a long-term USGS research site at Cape Cod, Massachusetts, that was contaminated with wastewater. Iron, arsenic, nitrate, and phosphate concentrations were measured in a region of the aquifer with previously documented iron- and arsenic-containing groundwater. These measurements were done as part of natural gradient tracer tests to measure chemical changes in response to additions of nitrate and to monitor the effect on groundwater chemistry for 120 days. Laboratory incubations were also done with aquifer sediments from the site to confirm reactions indicated by the field experiments.

Scientists determined that the addition of nitrate to anoxic groundwater enhanced microbially mediated processes in the aquifer that ultimately resulted in arsenic immobilization. More specifically, the injected nitrate oxidized reduced iron in the absence of oxygen, producing iron oxyhydroxides, which then immobilized the contaminant compounds, such as arsenic (V), arsenic (III), and phosphate through sorption. The nitrate additions resulted in decreased total arsenic concentration in groundwater from approximately double the U.S. Environmental Protection Agency drinking water limit (10 micrograms per liter) to levels less than one-half the drinking water limit for as many as 30 days after the nitrate was added. However, there was a finite amount of oxyhydroxides produced following the nitrate additions, which resulted in increased arsenic concentrations similar to those measured in the groundwater prior to nitrate addition. The nitrate itself was removed in the process, being reduced first to nitrous oxide gas and then to nitrogen gas.

This study is the first field demonstration of the effectiveness and rapid rate at which nitrate reduction can stimulate iron oxidation and subsequent arsenic removal. The arsenic attenuation processes were rapid but would require continued and carefully estimated nitrate additions to continue to immobilize arsenic without increasing nitrate concentrations in the groundwater. These results provide a basis for pilot scale tests of practical, long-term solutions to arsenic contamination in groundwater. The process also has the potential for remediation of other contaminants that can be sorbed from solution by iron oxyhydroxides such as manganese, chromium, certain radionuclides, and phosphate.

The USGS Toxic Substances Hydrology Program funded this study, as well as the Hydrologic Research and Development Program.

Reference

Smith, R.L., Kent, D.B., Repert, D.A., and Böhlke, J.K., 2017, Anoxic nitrate reduction coupled with iron oxidation and attenuation of dissolved arsenic and phosphate in a sand and gravel aquifer: Geochimica et Cosmochimica Acta, v. 196, p. 102-120, doi:10.1016/j.gca.2016.09.025.

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