Richard L. Smith Daniel N. Miller J.K. Böhlke 2006 <p><span>Ammonium (NH</span>₄⁺<span>) is a major constituent of many contaminated groundwaters, but its movement through aquifers is complex and poorly documented. In this study, processes affecting NH</span>₄⁺<span><span>&nbsp;</span>movement in a treated wastewater plume were studied by a combination of techniques including large‐scale monitoring of NH</span>₄⁺<span><span>&nbsp;</span>distribution; isotopic analyses of coexisting aqueous NH</span>₄⁺<span>, NO</span>₃⁻<span>, N</span>₂<span>, and sorbed NH</span>₄⁺<span>; and in situ natural gradient<span>&nbsp;</span></span>¹⁵<span>NH</span>₄⁺<span>tracer tests with numerical simulations of<span>&nbsp;</span></span>¹⁵<span>NH</span>₄⁺<span>,<span>&nbsp;</span></span>¹⁵<span>NO</span>₃⁻<span>, and<span>&nbsp;</span></span>¹⁵<span>N</span>₂<span><span>&nbsp;</span>breakthrough data. Combined results indicate that the main mass of NH</span>₄⁺<span><span>&nbsp;</span>was moving downgradient at a rate about 0.25 times the groundwater velocity. Retardation factors and groundwater ages indicate that much of the NH</span>₄⁺<span><span>&nbsp;</span>in the plume was recharged early in the history of the wastewater disposal. NO</span>₃⁻<span><span>&nbsp;</span>and excess N</span>₂<span><span>&nbsp;</span>gas, which were related to each other by denitrification near the plume source, were moving downgradient more rapidly and were largely unrelated to coexisting NH</span>₄⁺<span>. The δ</span>¹⁵<span>N data indicate areas of the plume affected by nitrification (substantial isotope fractionation) and sorption (no isotope fractionation). There was no conclusive evidence for NH</span>₄⁺<span>‐consuming reactions (nitrification or anammox) in the anoxic core of the plume. Nitrification occurred along the upper boundary of the plume but was limited by a low rate of transverse dispersive mixing of wastewater NH</span>₄⁺<span><span>&nbsp;</span>and O</span>₂<span><span>&nbsp;</span>from overlying uncontaminated groundwater. Without induced vertical mixing or displacement of plume water with oxic groundwater from upgradient sources, the main mass of NH</span>₄⁺<span><span>&nbsp;</span>could reach a discharge area without substantial reaction long after the more mobile wastewater constituents are gone. Multiple approaches including in situ isotopic tracers and fractionation studies provided critical information about processes affecting NH</span>₄⁺<span><span>&nbsp;</span>movement and N speciation.</span></p> application/pdf 10.1029/2005WR004349 en American Geophysical Union Ammonium transport and reaction in contaminated groundwater: Application of isotope tracers and isotope fractionation studies article