ASSESSING NITRIFICATION AND ITS EFFECT ON NITROGEN TRANSPORT WITHIN A GROUNDWATER CONTAMINANT PLUME ON CAPE COD USING MULTIPLE ISOTOPIC AND BIOGEOCHEMICAL APPROACHES

Ammonium (NH₄⁺) is a major constituent of many contaminated groundwaters, but its movement through aquifers is complex and poorly documented. In this study, nitrification in a treated wastewater plume was studied using a combination of techniques. These included large-scale monitoring of NH₄⁺ distribution, isotopic analyses of coexisting aqueous NH₄⁺, NO₃^-, N₂, and sorbed NH₄⁺, in situ natural-gradient ¹⁵NH₄⁺ tracer tests, single well in situ tests to measure nitrification potential, and molecular analysis of DNA extracted from aquifer cores. Combined results indicate that the main mass of NH₄⁺ was moving downgradient at a rate about 0.25 ± 0.10 times the groundwater velocity. δ¹⁵N data indicate areas of the plume affected by nitrification (substantial isotope fractionation) and sorption (no isotope fractionation). 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⁺. There was no conclusive in situ evidence for nitrification in the anoxic core of the plume, but DNA and single well tests with added oxygen indicated the potential for nitrification activity existed even though the zone had been anoxic for several decades. Nitrification did occur along the upper boundary of the plume but in situ rates were low (≤0.15 µmol N L^-1 day^-1) and limited by transverse dispersive mixing of plume 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 long after the more mobile wastewater constituents are gone. The multiple approaches including isotopic tracers and fractionation studies provided critical information about both the distribution and rates of processes affecting NH₄⁺ transport.