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New Information on the Long-Term Fate of Ammonium in Ground Water

USGS scientist is monitoring a bladder used to hold the injection fluid during a tracer test
A USGS scientist is monitoring a bladder used to hold the injection fluid during a single-well tracer test designed to study the transport on ammonium in ground water on Cape Cod, Massachusetts. The bladder is placed in a small pool of water to control the temperature of the injection fluid during the test.

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 Ammonium

A 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:

Diagram of a single-well tracer test
Single-well tracer tests involve injecting a tracer into one port on a multilevel sampling well and then monitoring the same well for the tracer.
  • Tracer tests (both single-well injection tests and small-scale natural gradient tracer tests, see diagrams) using bromide and 15N-labeled ammonium (15N is a stable isotope of nitrogen) can determine the ability of the resident microorganisms to conduct nitrification (the oxidation of ammonium to nitrate). The data generated by the tests also can be used to estimate nitrification rates by comparing the concentrations of 15N in ammonium to 15N in other forms of nitrogen.
  • Nitrification occurs to a limited extent along the margins of the plume where anoxic (oxygen-starved) plume waters mix with oxygenated ground water. However, there was no conclusive evidence for reactions in the center of the plume that could consume ammonium, suggesting that ammonium could persist in the subsurface for decades.
  • Dissolved ammonium, nitrate, and nitrogen gas (N2) in the same location of the plume were not due to common origins. Nitrate and nitrogen gas are much more mobile than ammonium, and most likely came from distant upgradient locations (see nitrate cross section).
  • The bulk of the ammonium in the center of the plume is moving downgradient at a much slower rate (about 0.25 times the groundwater velocity) than other more mobile wastewater constituents, such as boron (see boron cross section). The ammonium in the center of the plume is very old and most likely dates back to wastewater that was disposed of early in the history of the infiltration beds.
Diagram of a multiple-well natural gradient tracer
Multiple-well natural gradient tracer tests involve injecting a tracer in an upgradient well and then monitoring downgradient wells for the tracer.

Potential Long-Term Implications

The 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.


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