Toxic Substances Hydrology Program
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New Information on the Long-Term Fate of Ammonium in Ground Water
The ammonium (NH4+) dissolved in ground water in the center of a contaminant plume on Cape Cod, Massachusetts, has persisted for decades after it entered the subsurface, while other forms of nitrogen in the plume, such as nitrate (NO3-), have moved on with the ground water. This is the observation of a team of USGS scientists, and they have found that ammonium moves much more slowly than nitrate because of chemical and microbiological processes that retard its movement in the subsurface. Many freshwater resources have been affected by excess nutrients. Although nitrate is the form of nitrogen most commonly associated with ground water contamination, ammonium is also found in ground water, primarily from the discharge of wastewater from sources such as septic systems and wastewater infiltration beds. Unlike nitrate, much less is known about the fate and transport of ammonium in ground water. Fate and Transport of AmmoniumA team of USGS scientists used large-scale water-quality monitoring, two types of subsurface tracer tests, laboratory experiments, analysis of various nitrogen isotopes and numerical simulations of ammonium transport to investigate biological and chemical mechanisms that control the transport and fate of ammonium at the Cape Cod research site. The plume resulted from the disposal of wastewater from a sewage treatment plant into infiltration beds from 1936 to 1995. The USGS investigations and field experiments have shown that:
Potential Long-Term ImplicationsThe above results, published in two recent papers, suggest that ammonium in anoxic wastewater contaminant plumes can persist for a long time, and without exposure to oxygenated ground water ammonium could reach discharge areas, such as lakes or streams, long after other more mobile wastewater constituents are gone. This information can help water resource managers understand the long-term implications of wastewater disposal and deal with problems related to excess nutrients in water bodies fed by ground water. This work was funded by the USGS's Toxic Substances Hydrology Program and National Research Program and by a grant from the U.S. Department of Agriculture. References
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