River water infiltration enhances denitrification efficiency in riparian groundwater

Trauth, Nico; Musolff, Andreas; Knöller, Kay; Kaden, Ute Susanne; Keller, Toralf; Werban, Ulrike; Fleckenstein, Jan H.

doi:10.1016/j.watres.2017.11.058

Cited by 79 publications

(72 citation statements)

References 77 publications

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“…The fraction of river water in these samples was determined as

F_{river} = \frac{(][, {Cl}^{-}_{italicrip}) - (][, {Cl}^{-}_{italicdist})}{(][, {Cl}^{-}_{italicriver}) - (][, {Cl}^{-}_{italicdist})}

where [Cl − rip ], [Cl − dist ], and [Cl − river ] denote the chloride concentrations of the riparian groundwater sample, distant groundwater end‐member, and river water end‐member, respectively, on each sampling day. The F river values in this study deviate from those in Trauth et al () due to the incorporation of analytical uncertainties in the model (see section ).…”

Section: Mixing and Transformation Modelscontrasting

confidence: 85%

“…Water Resources Research proximity of the Selke River due to the meandering river channel depending on hydrologic conditions and channel morphology (Nixdorf & Trauth, 2018). Chloride concentrations in riparian groundwater and the river suggest that the A and B-N transects are less impacted by infiltrating river water compared to the other well clusters (Figure 1b; Trauth et al, 2018). The aquifer is mainly composed of alluvial sand and gravel deposits transported by the river from the Harz Mountains to the alluvial plains.…”

Section: 1029/2019wr025528mentioning

confidence: 99%

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How Important is Denitrification in Riparian Zones? Combining End‐Member Mixing and Isotope Modeling to Quantify Nitrate Removal from Riparian Groundwater

Lutz

Trauth

Musolff

et al. 2020

Water Resources Research

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Riparian zones are important buffer zones for streams as they are hotspots of nitrate transformation and removal in agricultural catchments. However, mixing of water from different sources and various transformation processes can complicate the quantification of nitrate turnover in riparian zones. In this study, we analyzed nitrate concentration and isotope data in riparian groundwater along a 2-km stream section in central Germany. We developed a mathematical model combining end-member mixing and isotope modeling to account for mixing of river water and groundwater and quantify nitrate transformation in riparian groundwater. This enabled us to explicitly determine the extent of denitrification (as process leading to permanent nitrate removal from riparian groundwater) and transient nitrate removal by additional processes associated with negligible isotope fractionation (e.g., plant uptake and microbial assimilation) and to perform an extensive uncertainty analysis. Based on the nitrogen isotope data of nitrate, the simulations suggest a mean removal of up to 27% by additional processes and only about 12% by denitrification. Nitrate removal from riparian groundwater by additional processes exceeded denitrification particularly in winter and at larger distance from the river, underlining the role of the river as organic carbon source. This highlights that nitrate consumption by additional processes predominates at the field site, implying that a substantial fraction of agricultural nitrogen input is not permanently removed but rather retained in the riparian zone. Overall, our model represents a useful tool to better compare nitrogen retention to permanent nitrogen removal in riparian zones at various temporal and spatial scales. Plain Language SummaryNitrogen is an important nutrient for agricultural crops. However, excessive nitrogen input into surface water in the form of nitrate can lead to algae blooms and lack of oxygen. The riparian zones of rivers are important buffer zones where groundwater is connected to soils, which are rich in soil organisms and organic matter pools fueling reaction processes. Hence, plants and bacteria can remove nitrate from riparian groundwater before it reaches the river. Bacterial consumption of nitrate (denitrification) leads to complete removal of nitrogen via release of nitrogen gas into the atmosphere. In contrast, other biogeochemical processes such as nitrate uptake by plants merely result in nitrogen retention within riparian zones. To quantify the role of denitrification relative to other processes, we developed a novel model combining concentration and isotope data of nitrate and applied it to a groundwater study site in Central Germany. We found that nitrate removal from riparian groundwater by additional processes largely exceeded denitrification. Hence, a major fraction of nitrogen inputs was retained in the riparian zone and may eventually end up in the river. Such information is highly relevant for many river ecosystems at risk of eutrophication because of high nitr...

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“…The fraction of river water in these samples was determined as

F_{river} = \frac{(][, {Cl}^{-}_{italicrip}) - (][, {Cl}^{-}_{italicdist})}{(][, {Cl}^{-}_{italicriver}) - (][, {Cl}^{-}_{italicdist})}

Section: Mixing and Transformation Modelscontrasting

confidence: 85%

Section: 1029/2019wr025528mentioning

confidence: 99%

How Important is Denitrification in Riparian Zones? Combining End‐Member Mixing and Isotope Modeling to Quantify Nitrate Removal from Riparian Groundwater

Lutz

Trauth

Musolff

et al. 2020

Water Resources Research

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“…This vertical stratification depends on lithology, the depth and characteristics of fractured rocks, and ultimately the vertical connectivity (Kolbe et al, 2019; Zhi et al, 2019) of subsurface systems. The “flushing” export patterns has been observed not only for nitrate but also for other solutes that are enriched in shallow soils and depleted in deeper subsurface, including dissolved organic carbon (DOC) (Moatar, Abbott, Minaudo, Curie, & Pinay, 2017; Musolff et al, 2017; Trauth et al, 2018; Zarnetske, Bouda, Abbott, Saiers, & Raymond, 2018). At the Naizin catchment, however, nitrate shows a unique pattern that has both high (May–June) and low (September–October) nitrate concentrations at low flow (Figure 1b).…”

Section: Discussionmentioning

confidence: 99%

Nitrate removal and young stream water fractions at the catchment scale

Benettin

Fovet

2020

Hydrological Processes

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Despite extensive research on nitrate export and removal, nutrient contamination remains a major threat to water bodies worldwide. At the local scale, nitrate removal is governed by biogeochemical conditions that vary in space and time, making integration to entire landscapes critical. Water transit times have often been used to describe solute transport, but the relation between water age and nitrate removal at the catchment scale is still poorly understood. We test the hypothesis that nitrate removal peaks when the fraction of young water in discharge is at its minimum, because nitrate removal occurs mostly under dry conditions where deeper, older groundwater dominates streamflow. We tested this hypothesis by exploring a detailed water quality record from the Kervidy-Naizin catchment (FR) and comparing the dynamics of nitrate to those of a conservative solute (chloride). We find that estimates of nitrate removal are consistent with previous estimates at the site and they show a good (inverse) correlation with the fraction of streamflow that is younger than 2.5 months. However, this young water fraction cannot be used to predict nitrate removal in the winter-spring period, when no removal is observed regardless of streamflow age. While this leads us to reject our hypothesis during the winter period, it also suggests that water age distributions and their correlation with nitrate removal can possibly reveal distinct sources of stream water at different hydrologic regimes and relevant biogeochemical reactions. K E Y W O R D Sagricultural catchment, chloride transport, denitrification, Naizin, nitrate removal, SAS function, water age, young water fraction

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“…The only process able to permanently remove N-input from the catchment is denitrification in soils, aquifers (Seitzinger et 20 al., 2006;Hofstra & Bouwman, 2005) and at the stream-aquifer interface such as in the riparian (Vidon & Hill, 2004;Trauth et al, 2018) and hyporheic zones (Vieweg et al, 2016). As the riverine exports are signals of the catchment or subcatchment processes, integrated in time and space, separating legacy of NO 3 from removal via denitrification is difficult.…”

Section: Catchment Scale N-budgeting 25mentioning

confidence: 99%

Decadal trajectories of nitrate input and output in three nested catchments along a land use gradient

Ehrhardt¹,

Kumar²,

Fleckenstein³

et al. 2018

Preprint

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Increased anthropogenic inputs of nitrogen (N) to the biosphere during the last decades have resulted in increased groundwater and surface water concentrations of N (primarily as nitrate) posing a global problem. Although measures have 10 been implemented to reduce N-inputs especially from agricultural sources, they have not always led to decreasing riverine nitrate concentrations and loads. The limited response to the measures can either be caused by the accumulation of slowly mineralized organic N in the soils acting as a biogeochemical legacy or by long travel times (TTs) of inorganic N to the streams forming a hydrological legacy. Both types of legacy are hard to distinguish from the TTs and N budgets alone. Here we jointly analyze atmospheric and agricultural N inputs with long-term observations of nitrate concentrations and discharge 15 in a mesoscale catchment in Central Germany. For three nested sub-catchments with increasing agricultural land use, we assess the catchment scale N budget, the effective TT of N. In combination with long-term trajectories of C-Q relationships we finally evaluate the potential for and the characteristics of an N-legacy. We show that in the 42-year-long observation period, the catchment received an N-input of 42 758 t, of which 97 % derived from agricultural sources. The riverine Nexport sums up to 6 592 t indicating that the catchment retained 85 % of the N-input. Removal of N by denitrification could 20 not fully explain this imbalance. Log-normal travel time distributions (TTD) for N that link the input history to the riverine export differed seasonally, with modes spanning 8-17 years. Under low-flow conditions, TTs were found to be systematically longer than during high discharges. Systematic shifts in the C-Q relationships could be attributed to significant changes in N-inputs resulting from agricultural intensification and the break-down of the East German agriculture after 1989 and to the longer travel times of nitrate during low flows compared to high flows. A chemostatic export regime of 25 nitrate was only found after several years of stabilized N-inputs. We explain these observations by the vertical migration of the N-input and the seasonally changing contribution of subsurface flow paths with differing ages and thus differing N-loads.The changes in C-Q relationships suggest a dominance of hydrological N-legacy rather than a biogeochemical N-fixation in the soils, which should result in a stronger and even increasing dampening of riverine N-concentrations after sustained high N-inputs. Despite the strong N-legacy, a chemostatic nitrate export regime is not necessarily a persistent endpoint of intense 30 agricultural land use, but rather depends on a steady replenishment of the mass of N propagating through the catchments subsurface. The input-output imbalance, the long time-lags and the lack of significant denitrification in the catchment let us Hydrol. Earth Syst. Sci. Discuss., https://doi.

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River water infiltration enhances denitrification efficiency in riparian groundwater

Cited by 79 publications

References 77 publications

How Important is Denitrification in Riparian Zones? Combining End‐Member Mixing and Isotope Modeling to Quantify Nitrate Removal from Riparian Groundwater

How Important is Denitrification in Riparian Zones? Combining End‐Member Mixing and Isotope Modeling to Quantify Nitrate Removal from Riparian Groundwater

Nitrate removal and young stream water fractions at the catchment scale

Decadal trajectories of nitrate input and output in three nested catchments along a land use gradient

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