Abstract
The fluvial nitrogen dynamics at locations around weirs are still rarely studied in detail. Eulerian data, often used by conventional river monitoring and modelling approaches, lags the spatial resolution for an unambiguous representation. With the aim to address this knowledge gap, the present study applies a coupled 1D hydrodynamic–water quality model to a 26.9 km stretch of an upland river. Tailored simulations were performed for river sections with water retention and free-flow conditions to quantify the weirs’ influences on nitrogen dynamics. The water quality data were sampled with Eulerian and Lagrangian strategies. Despite the limitations in terms of required spatial discretization and simulation time, refined model calibrations with high spatiotemporal resolution corroborated the high ammonification rates (0.015 d−1) on river sections without weirs and high nitrification rates (0.17 d−1 ammonium to nitrate, 0.78 d−1 nitrate to nitrite) on river sections with weirs. Additionally, using estimations of denitrification based on typical values for riverbed sediment as a reference, we could demonstrate that in our case study, weirs can improve denitrification substantially. The produced backwater lengths can induce a means of additional nitrogen removal of 0.2-ton d−1 (10.9%) during warm and low-flow periods.
Highlights
The European Water Framework Directive (WFD) stipulated that by 2027 at the latest, all surface water bodies must achieve good ecological and chemical status [1]
This study provides a detailed description of spatiotemporal nitrogen dynamics along the lowermost Freiberger Mulde (FM) river section within the time period 2017–2018
High ammonification was evidenced at river sections without weirs (0.015 d−1 organic nitrogen fraction (Org-N) → NH4-N) and high nitrification was evidenced at river sections with weirs present (0.17 d−1 NH4-N → NO2-N, 0.78 d−1 NO2-N → NO3-N)
Summary
The European Water Framework Directive (WFD) stipulated that by 2027 at the latest, all surface water bodies (e.g., lakes, rivers, coastal waters) must achieve good ecological and chemical status [1]. Only 40% of approximately 111,000 surface water bodies across Europe have so far fulfilled the WFD objective [2]. Monitoring programs are an essential part of WFD management cycles and they provide the key basis for ecological and chemical status assessment. River Basin Management Plans (RBMP) report status deficits caused by pollution and hydromorphological pressures, and they propose and prioritize different restoration measures [2,3,4]. River pollution pressures include emissions of nutrients, organic matter and hazardous chemicals from diffuse and point sources (e.g., agriculture, sewage system discharges). Massive production and application of fertilizers have led to continuous excess N in soil and water, which results in other harmful processes such as soil acidification and eutrophication [7,8]
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