Anoxic nitrogen cycling in a hydrocarbon and ammonium contaminated aquifer

Abstract

Nitrogen fate and transport through contaminated groundwater systems, where N is both ubiquitous and commonly limits pollutant attenuation, must be re-evaluated given evidence for new potential microbial N pathways. We addressed this by measuring the isotopic composition of dissolved inorganic N (DIN = NH4+, NO2−, and NO3−) and N functional gene abundances (amoA, nirK, nirS, hszA) from 20 to 38 wells across an NH4+, hydrocarbon, and SO42− contaminated aquifer. In-situ N attenuation was confirmed on three sampling dates (0, +6, +12 months) by the decreased [DIN] (4300 - 40 μM) and increased δ15N-DIN (5‰–33‰) over the flow path. However, the assumption of negligible N attenuation within the plume was complicated by the presence of alternative electron acceptors (SO42−, Fe3+), both oxidizing and reducing functional genes, and N oxides within this anoxic zone. Active plume N cycling was corroborated using an NO2− dual isotope based model, which found the fastest (∼10 day) NO2− turnover within the N and electron donor rich central plume. Findings suggest that N cycling is not always O2 limited within chemically complex contaminated aquifers, though this cycling may recycle the N species rather than attenuate N.