Ammonium (NH₄⁺) is a major constituent of many contaminated groundwaters, but its movement through aquifers is complex and poorly documented. In this study, processes affecting NH₄⁺ movement in a treated wastewater plume were studied by a combination of techniques including large‐scale monitoring of NH₄⁺ distribution; isotopic analyses of coexisting aqueous NH₄⁺, NO₃⁻, N₂, and sorbed NH₄⁺; and in situ natural gradient ¹⁵NH₄⁺tracer tests with numerical simulations of ¹⁵NH₄⁺, ¹⁵NO₃⁻, and ¹⁵N₂ breakthrough data. Combined results indicate that the main mass of NH₄⁺ was moving downgradient at a rate about 0.25 times the groundwater velocity. Retardation factors and groundwater ages indicate that much of the NH₄⁺ in the plume was recharged early in the history of the wastewater disposal. NO₃⁻ and excess N₂ gas, which were related to each other by denitrification near the plume source, were moving downgradient more rapidly and were largely unrelated to coexisting NH₄⁺. The δ¹⁵N data indicate areas of the plume affected by nitrification (substantial isotope fractionation) and sorption (no isotope fractionation). There was no conclusive evidence for NH₄⁺‐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 NH₄⁺ and O₂ from overlying uncontaminated groundwater. Without induced vertical mixing or displacement of plume water with oxic groundwater from upgradient sources, the main mass of NH₄⁺ 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 NH₄⁺ movement and N speciation.