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 = NH 4 + , NO 2 − , and NO 3 − ) and N functional gene abundances ( amoA , nirK , nirS , hszA ) from 20 to 38 wells across an NH 4 + , hydrocarbon, and SO 4 2− 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 δ 15 N-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 (SO 4 2− , Fe 3+ ), both oxidizing and reducing functional genes, and N oxides within this anoxic zone. Active plume N cycling was corroborated using an NO 2 − dual isotope based model, which found the fastest (∼10 day) NO 2 − turnover within the N and electron donor rich central plume. Findings suggest that N cycling is not always O 2 limited within chemically complex contaminated aquifers, though this cycling may recycle the N species rather than attenuate N.