Hydrogeologic controls of nitrogen dynamics along flow paths within a tidal freshwater zone
Abstract
Groundwater and surface water flow through riparian aquifer soils and streambed sediment, providing opportunities for microbial communities to transform nitrogen (N) and carbon. Nitrate removal efficiency is generally thought to decrease from headwaters to coasts, however is poorly constrained near coasts, particularly within tidal freshwater zones (TFZs). Using field observations and numerical models, we show that rapid cycling of nitrogen occurs within nitrification and denitrification hot spots along flow paths through the riparian aquifer. Near the fluctuating water table ( 7-20 m from the stream channel) nitrate production dominates, with pore water nitrate-N reaching concentrations greater than 15 mg/L. In the shallow, saturated riverbed and in deeper portions of the riparian aquifer, nitrate-N concentrations are often low or non-detect. Concentrations vary more in space than time (over the tidal cycle), suggesting that nitrogen processing is driven largely by substrate characteristics. Numerical modelling results support the conclusions drawn from nitrate isotopes. Models indicate the importance of nitrogen mineralization and oxygen supply rates. Indeed, high (>6 mg/L) measured nitrate concentrations occur at ports where water is well oxygenated and moving through low organic matter substrate. In contrast, nitrate removal dominates through large portions of the aquifer that are more hydrologically stable and/or in regions with greater (>2%) organic matter content.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.H31D..08B
- Keywords:
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- 0408 Benthic processes;
- BIOGEOSCIENCESDE: 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCESDE: 1806 Chemistry of fresh water;
- HYDROLOGYDE: 1830 Groundwater/surface water interaction;
- HYDROLOGY