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.