Seasonal Variation and Key Controls of Groundwater Ammonium Concentrations in Hypoxic/Anoxic Riparian Sediments

The seasonal controls of hydrology, temperature, hypoxia, and biogeochemical conditions for groundwater ammonium–N (NH4+) concentrations are not well understood. Here we investigated these controls for riparian groundwaters located upstream of two milldams over a period of 4 years. Groundwater chemistry was sampled monthly while groundwater elevations, hydraulic gradients, and temperatures were recorded sub-hourly. Distinct seasonal patterns for NH4+ were observed which differed among the wells. For wells that displayed a strong seasonal pattern, NH4+ concentrations increased through the summer and peaked in October–November. These elevated concentrations were attributed to ammonification, suppression of nitrification, and/or dissimilatory nitrate reduction to ammonium (DNRA). These processes were driven by high groundwater temperatures, low hydraulic gradients (or long residence times), hypoxic/anoxic groundwater conditions, and increased availability of dissolved organic carbon as an electron donor. In contrast, NH4+ concentrations decreased in the riparian groundwater from January to April during cool and wet conditions. A groundwater well with elevated total dissolved iron (TdFe) concentrations had elevated NH4+ concentrations but displayed a muted seasonal response. In addition to hydrologic controls, we attributed this response to additional NH4+ contribution from Fe-driven autotrophic DNRA and/or ammonification linked to dissimilatory Fe reduction. Understanding the temporal patterns and factors controlling NH4+ in riparian groundwaters is important for making appropriate watershed management decisions and implementing appropriate best management practices. Plain Language Summary Wetland or riparian soils and groundwaters depleted of oxygen can accumulate ammonium–N (NH4+), a toxic pollutant at high concentrations. NH4+ can be produced and removed via microbial processes that are influenced by seasonal factors such as: temperature, groundwater levels, dissolved oxygen, organic carbon, nitrate, and dissolved iron. In this study, we studied monthly grab samples of riparian groundwater collected upstream of milldams over a period of 4 years. We evaluated the data, identified three main seasons, and generated relationships between physical and chemical factors and NH4+ over time. The main findings from this study show that NH4+: (a) peaks in October and November when water is warm, stagnant, and high in organic carbon, (b) decreases from January – April when temperatures cooled and lowered microbial production, and (c) elevated iron groundwater concentrations can dampen these seasonal trends. Results from this study can help to improve the timing of milldam removal practices and for the management of wetlands that remove nitrogen from the terrestrial environment. Key Points - Groundwater ammonium–N concentrations peaked following warm water temperatures, low hydraulic gradients, and high dissolved organic carbon (DOC) - Ammonium concentrations were likely driven by seasonal changes in ammonification, nitrification, and dissimilatory nitrate reduction to ammonium (DNRA) - Hydrologic conditions and elevated dissolved iron (Fe) concentrations likely shaped the seasonal ammonium patterns

Description

This article was originally published in Journal of Geophysical Research: Biogeosciences published by AGU. Published 2025 American Geophysical Union. Sena, M. G., Peipoch, M., Joshi, B., Rahman, Md. M., Peck, E., Gold, A. J., et al. (2025). Seasonal variation and key controls of groundwater ammonium concentrations in hypoxic/anoxic riparian sediments. Journal of Geophysical Research: Biogeosciences, 130, e2023JG007900. https://doi.org/10.1029/2023JG007900. To view the published open abstract, go to https://doi.org/10.1029/2023JG007900. © 2025. American Geophysical Union. All Rights Reserved. This article will be embargoed until 07/30/2025.

Keywords

ammonium, seasonal, wetland, iron, biogeochemistry, milldams

Citation

Sena, M. G., Peipoch, M., Joshi, B., Rahman, Md. M., Peck, E., Gold, A. J., et al. (2025). Seasonal variation and key controls of groundwater ammonium concentrations in hypoxic/anoxic riparian sediments. Journal of Geophysical Research: Biogeosciences, 130, e2023JG007900. https://doi.org/10.1029/2023JG007900

URI

https://udspace.udel.edu/handle/19716/36074

Collections

Open Access Publications - Department of Plant and Soil Sciences

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