HS5.7

Managing water flow at the outlet of subsurface drainage networks is an important issue for water authorities aiming to preserve freshwater quality. As buried drains are directly connected to arterial ditches, dynamic and specific hydrological functioning of drained plots contribute to export part of chemical compounds derived from the farm inputs application, such as the exceed pesticides or fertilizers. Provided the necessary changes of agricultural practice at the farm scale, intercepting water at the outlet of subsurface drainage network in artificial wetland is considered as an efficient mitigation measure and constitutes a nature-based solution that addresses multiple ecological objectives.

From the pilot experimental subcatchment of Rampillon in France (355ha of intensive farming in the agricultural region of Brie, rural Parisian basin), we propose a synthesis of the results obtained from the 10 years of performance assessment of this artificial wetland in buffering nitrates and pesticides from drained fields. We also provide a feedback from the 15-years collaboration with the local and national stakeholders strongly involved from the early beginning of this long-term project. After an initial stage of dialog and co-construction with the stakeholders (5 years), the experimental artificial wetland (0.5ha or 0.15% of upscale watershed) was constructed in 2010. The full monitoring of the wetland for water quality and quantity started in 2012 and was complemented by biodiversity surveys in 2017 and 2021.

We will present yearly and seasonal variations of removal efficiency of nitrate and pesticides and discuss the limits and interests of artificial wetland in this context, including the high potential for removal with high seasonal variability due to thermal sensitivity and hydrological effects. Concerning heavy metals, a former study showed that agricultural fields do not significantly contribute to their transfer and that the artificial wetland retained the major part of them. In a hydraulic study using conservative tracers, we highlight the role of vegetation patches on the hydraulic performance of the wetland and on the pollutant mitigation. All those results allowed us to develop a modeling approach based on Tank In Serie (for nitrate and pesticides) to define a standard design aiming 50% annual removal efficiency.

After ten years, involved farmers are now the best ambassadors to disseminate the role of nature-based solutions in helping mitigate the unintentional pollutions from agricultural activities, not considering artificial wetland as a right to pollute but a complementary tool to Best Management Practices.

Finally, part of our works focuses on the role of artificial wetland in biodiversity maintenance in agricultural landscape by monitoring the ecosystem dynamics and the seasonal exposure of different vertebrate and invertebrate communities using ecotoxicological approach. In fine, this will allow us to better understand the intricacy of the ecological functions ensured by these artificial ecosystems and to assess their potential in supporting multiple ecosystem services such as water quality preservation, biodiversity protection, landscape connectivity and recreational activities to local populations.

How to cite: Tournebize, J., Chaumont, C., Jeliazkov, A., Lebrun, J., Bahi, A., Michel, A., Lemaire, B., and Henine, H.: Artificial wetland for mitigation of non-point source agricultural pollution in a French drained context: lessons from a 10-years monitoring., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7487, https://doi.org/10.5194/egusphere-egu22-7487, 2022.

Nutrient losses from agricultural areas constitute a major cause for the degradation of aquatic ecosystems worldwide. Sixty percent of the area in Denmark is arable land and thus there is a huge need for mitigation measures to decrease the transport of nutrients from agricultural areas to downstream recipients. We discuss results, experiences and challenges for an optimised implementation of nutrient transport mitigation measures targeting agricultural nutrient losses to fresh and marine water. In 2016 the Agricultural Package was adopted by the Danish Parliament and Danish farmers were again allowed to fertilise their crops to economic optimum. To compensate for the consequent increase in fertilisation rates and the potential negative consequences on water quality, a new nitrogen (N) and phosphorus (P) management plan was introduced. This plan consists of measures to mitigate N losses in smaller catchments (≈ 15 km²) and knowledge of the N-retention capacity of the individual catchments is used for optimisation of the implementation of mitigation measures. A series of nutrient transport mitigation measures has been scientifically approved for use in this new regulation, and more measures are currently undergoing scientific testing. This study focuses on documenting the nutrient retention effects of already approved nutrient transport mitigation measures, such as restoration of riparian wetlands, lowland fens and swamps, re-establishment of shallow lakes, constructed wetlands (surface flow and subsurface flow), as well as measures not yet approved and still under development such as integrated buffer zones, saturated buffer zones and controlled drainage.

How to cite: Hoffmann, C., Zak, D., Kronvang, B., Carstensen, M., and Audet, J.: Mitigation measures for improvement of agricultural drainage water and surface water quality in Denmark, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4101, https://doi.org/10.5194/egusphere-egu22-4101, 2022.

Floodplains, especially along large rivers in central Europe, have been modified heavily during the last centuries. Not only floodplain losses by embankment and dikes, but also a decoupling of river hydrology by river bed incision and embankments occurred. Nevertheless, floodplains are known to act as nutrient sinks when inundated by river water during floods. Nutrients, such as nitrate, are nowadays known to be of higher concentrations than under natural conditions and can cause water quality issues. How much of this retention function is left in several highly modified European rivers?  

With this study, we provide an overview of our activities to quantify denitrification along large rivers in Germany and the Danube in Austria as well as in its river basin by applying models as well as field and laboratory measurements. We explore this key ecosystem function of endangered floodplain ecosystems and their potential to act as nature-based solutions to mitigate the effects of nitrate pollution.

Therefore, we modeled denitrification by a semi-empiric model for the rivers Rhine, Elbe, Main, and Weser as well as the Danube. For the latter, a comparison with a statistical model based on in-site measurement on nutrient concentrations was carried out. Furthermore, we are currently estimating the denitrification potential of the Danube River by applying the semi-empirical model for in-stream retention and for floodplains, considering controlling soil physical and chemical parameters as well as flooding probabilities.

On the measurement side, we applied the acetylene inhibition method for 6 locations (113 plots) along the rivers Elbe, Rhine, Main, and Weser together with calculated average inundation time to determine upper-bound soil denitrification potential estimates. Through this combined model- and measurement-based approach, proxy-based statements were further developed on a large scale.

From all our studies we conclude that floodplains still contribute to nitrate retention in large modified rivers. Depending on the level of connectivity between rivers and their adjacent floodplains, the occurrence of frequent inundation during the year, and the way floodplains are inundated (large spatial extent vs preferential pathways carrying the water quickly through the floodplain), averaged modeled N-retention is about 400 kg N/ha/yr which is mid to upper range of reported removal rates in other systems. However, compared to the large nutrient loads and the small regular inundation extent, current floodplains alone limited in their current connectivity level are not capable to improve water quality significantly. Based on the laboratory measurements resulting in lower retention rates of 25-100 kg N/h/yr we conclude that the pH value is a key parameter that influences denitrification: thus, even in well-connected floodplains denitrification potential is comparably low due to low pH values as proved for the Elbe River. Further field studies and laboratory experiments under closer in-situ conditions are necessary to better understand which processes limit and foster denitrification. Therefore, we adapt our methods to assess actual/quasi-in-situ soil denitrification on a study site along the Havel River in spring 2022.

How to cite: Natho, S., Hein, T., Kaden, U. S., Kra, A.-C., and Tschikof, M.: The role of floodplains for denitrification along large rivers in central Europe – measurements and modeling for a comprehensive overview, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3761, https://doi.org/10.5194/egusphere-egu22-3761, 2022.

Restoration of wetlands as well as other measures aimed on increasing water and nutrients buffering capacity along the streams are becoming increasingly proposed as solutions addressing problems of water pollution. We observed, however limited integration of this type of measures in  river basin management plans. We assume, that it is caused mainly by lack of knowledge (and tools) about   effectiveness assessment in case of numerous measures applied in a particular catchment. In other words, upscaling of restoration measures should be possible, when we can provide water managers with more accurate estimates of cumulative effects of different measures. It can be achieved by  hydrological model application in appropriate scale.

We have tested different approaches in modelling of several river basins where restoration activity or buffering  measures where applied or planned. This includes Kamienna River in upland landscape, Słupia River in costal settings and Pilica River in lowlands. There were also modelling experiments conducted on rivers in Finland, Sweden and Lithuania.   Based on the modelling  experience from these catchments using Soil and Water Assessment Tool (SWAT), where different types of river buffer zones and wetlands  implementation has been tested, a number of challenges can be emphasized. At a general level, three major sources of challenges dominate: (1) spatial extent and location of measures, (2) their accurate parametrization and (3) simplification of processes in modelling scheme. Most commonly, in semi-distributed models, principal calculation units for which water and nutrient balance is calculated, are lumped and non-contiguous geographic units within each sub-catchment. Particular model setup may consist of thousands of such units, and each of them may represent one field, a portion of a field, or, more likely, portions of many fields. It becomes problematic when we aim to simulate measures for particular river reach or wetland, but can only define it at coarser sub-catchment level. The second issue is related to proper parametrization of empirical/physical sub-models, simulating processes of nutrients adsorption and settling. In most cases, uncertainty related to parametrization of buffer zones and wetlands  is significant. No simple calibration procedure for setting the optimal parameters’ values can be applied, and the process itself is more expert-dependent. Another issue is related to simplifications of processes. For instance, in SWAT, nutrient transformations simulated in wetlands are limited to the removal of nutrients by settlings however transformations between nutrient pools are ignored. For the buffer zones, model only affects contaminants that are present in surface runoff and neglects the potential effects of buffer zones on shallow groundwater.

There is a big room for improvement by providing the best monitoring results of particular measures and applying them in a modeling scheme. This can be done based on wetland restoration projects which were monitored by our group in Kampinos National Park, Biebrza NP,  Słowiński NP as well as in Lithuania and Norway. Showing the results of our models we would like to rise discussion on inter-calibration of the particular measures in the modelling settings.

How to cite: Okruszko, T., Grygoruk, M., Marcinkowski, P., and Piniewski, M.: Challenges in model based estimation of mitigation measures to improve water quality on catchment scale, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10377, https://doi.org/10.5194/egusphere-egu22-10377, 2022.

Non-point pollution by pesticides affects the quality of drinking water supplies and aquatic environments. Ponds collecting runoff and erosion fluxes offer a complementary tool to mitigate pesticide transfer to aquatic ecosystems. Pesticides mitigation results from various physicochemical processes operating in the water column, the sediment layer, the vegetation, and the suspended particles of ponds. Many studies on ponds dissipation potential focus on nitrates and suspended sediments, but very little is known about the behavior of pesticides. In order to predict and enhance the dissipation of pesticides in ponds, a new 0D time-dependent model was designed, “PESTIPOND.” The dissipation function of ponds is due to a combination of transport, transfer, and transformation processes of pesticides in an interplay between pond compartments. A previous review helped us identify the main processes of pesticides dissipation in ponds and their controlling factors. A time-dependent environmental fate model is currently developed to describe the behavior of dissolved and particulate pesticides at the pond scale. We tested the performance of this process-based model by simulating the behavior of four molecules with contrasted physicochemical properties (i.e., Aclonifen, chlorotoluron, s-metolachlor, and tebuconazole) in a synthetic scenario representing an experimental pond study. PESTIPOND enabled to compute the outlet concentration of pesticides in ponds based on their inlet concentration. The model simulated the behavior of pesticides in water, sediments, vegetation, and suspended particles of a pond due to sorption, settling/resuspension, volatilization, and degradation processes. The conceptual model is based on pesticide mass-budget equations in pond compartments depending of climate and hydraulic conditions. Sensitivity analysis provides a framework to recognize important variables in the computational model. We adopted the Morris and Sobol methods for the sensitivity analysis of PESTIPOND to identify dominant parameters influencing the model outputs. A preliminary result showed that the prevailing processes determining pesticide fate were sorption and biodegradation for dissolved pesticides and settling for particulate pesticides. The identification of effective mechanisms can be helpful to hierarchize processes based on their contribution to the dissipation function of ponds. This hierarchization may improve the estimation of ponds efficiency with respect to pesticide dissipation. In the future, implementing the PESTIPOND model in SWAT is expected to extend the prediction of ponds dissipation to the catchment scale. PESTIPOND will be particularly helpful to set up dimensioning criteria to design performant and efficient ponds to mitigate pesticides transfer into the environment.

How to cite: Bahi, A., Sauvage, S., Payraudeau, S., Imfeld, G., and Tournebize, J.: PESTIPOND: A fate model of pesticides in ponds, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7572, https://doi.org/10.5194/egusphere-egu22-7572, 2022.

Over the past two decades, lakes in Yangtze Plain have suffered from serious eutrophication, in some regions with increased frequency of cyanobacteria blooms over years. In this study, we investigated the underlying causes of eutrophication using a combination of process-based ecosystem modelling and statistical data analysis. We found that terrestrial nutrient exports with runoff have significantly increased from 1979 to 2018 in Yangtze Plain, directly linked to the enhanced usage of chemical fertilizer for crops. Based on statistical analyses of environmental variables, terrestrial nutrient exports and satellite-observed probability of eutrophication occurrence (PEO), we separated the studied fifty lakes into five classes with similarities in environmental and nutrient variations, and attributed key factors in controlling the temporal changes of PEO. The results showed that the satellite-observed PEO trends in five classes could be largely linked to the terrestrial nutrient exports and environmental changes. Specifically, we found agricultural activities can explain the observed eutrophication trends in western lakes where lake catchments are dominated with arable and natural land, and the reduced discharge of industrial wastewater was found to be linked to the declining trends in eutrophication for eastern lakes where the green growth of industrialization were promoted from 2003 to 2011. These findings highlight the importance of sustainable management of agriculture and industrialization to overcome eutrophication issues in this region.

How to cite: Guan, Q., Tang, J., Feng, L., Olin, S., and Schurgers, G.: Terrestrial nutrient exports and environmental changes explain eutrophication trends in fifty large lakes of Yangtze Plain, China, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2204, https://doi.org/10.5194/egusphere-egu22-2204, 2022.

We present a detailed field study on the dynamics between processes retaining nitrogen (denitrification, plant uptake) and processes producing greenhouse gasses (incomplete denitrification, methanogenesis) in integrated buffer zones (IBZ). Integrated buffer zones are novel systems designed to be integrated within the riparian zone to retain nitrogen and other pollutants that otherwise bypass the riparian buffer zone via drainpipes. However, the anaerobic conditions established within the IBZ to enhance denitrification, can lead to production of the greenhouse gasses N₂O and CH₄. We investigated both the atmospheric emission of N₂O and CH₄, and waterborne losses of dissolved nitrogen, N₂O and CH₄ from two IBZ sites in Denmark for one year. The study showed that the emission of N₂O was relatively low, and that IBZ can even work as a sink of N₂O. On the contrary, the IBZ were sources of CH₄, although the emissions were comparable to those of natural wetlands and other drainage transport mitigation measures. The hydrology was identified as the key driver of emission pathways and their relative importance, as well as the nitrogen retention efficiency.

How to cite: Vodder Carstensen, M., Zak, D., van't Veen, S., Wisniewska, K., Ovesen, N., Kronvang, B., and Audet, J.: A field study of nitrogen removal and N2O and CH4 fluxes from integrated buffer zones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13435, https://doi.org/10.5194/egusphere-egu22-13435, 2022.

ReWet is currently establishing four observatories on drained peatlands in Denmark. These observatories will serve as platforms for ecosystem monitoring, experimental research, technological development and demonstration. The objective of ReWet is to facilitate climate smart management and land use change related to agriculture and forestry on peat soils  The ReWet observatories will focus on measurements of fluxes of greenhouse gases (GHG), energy, water and matter (including major nutrients and dissolved carbon) in the interface of the upper groundwater, soil, vegetation and the atmosphere under different rewetting strategies and land use combinations. The ecosystem monitoring will include biodiversity namely vegetation composition and soil microbial communities. The vertical movement of the peat surface will be monitored using dynamic radar reflectors. The monitoring and research carried out at the observatories will, in combination with nationwide  soil databases, enable development of science based national strategies for rewetting of temperate wetlands like in Denmark to achieve substantially lower GHG emissions at national scale, less nutrient pollution of aquatic ecosystems and increased landscape biodiversity.

How to cite: Petersen, R. J., Zak, D., Maagaard, A., Hoffmann, C., Hans, A., Jesper, C., Lars, E., Johannes, P., and Paul, L.: Wetland observatories for rewetting of drained peatlands (ReWeT-DK), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13454, https://doi.org/10.5194/egusphere-egu22-13454, 2022.

Constructed wetlands (CW) treating runoff from agricultural catchments can efficiently reduce nitrate contamination, however, most wetlands are inherently net sources of methane (CH₄), which is of environmental concern due to its potent global warming capacity. For the mitigation of this negative aspect of CWs we need investigations on the CH₄ emission dynamics and environmental conditions governing CH₄ production and consumption in CWs.

This study integrates results from 4-years (2014-2017) investigations in an off-stream CW in Rampillon, France (0.53 ha, depth 0.3-0.8 m, est. 2010) and from 4-years (2018-2021) studies in an in-stream CW in Vända, Estonia (0.45 ha, depth 0.1-0.6 m, est. 2015). In Rampillon, during four 2-weeks measurement campaigns throughout all seasons CH₄ fluxes were measured using floating automated chambers connected to the QCLAS laser system. In addition, gas was sampled twice day from manual floating chambers for further analysis in lab. In Vända in-stream CW, CH₄ fluxes were measured twice a month using manual chambers and gas-chromatographs.

The average annual CH₄ emission in Rampillon for 2014-2015 was 7.7 g CH₄-C m^-2 yr^-1, showing highest values from deeper (0.5-1.0 m) parts in summer and autumn. The highest values reached up to 180 mg CH₄-C m^-2 h^-1, mainly due to ebullition. The emissions in winter and spring were up to 10 times lower, however no negative values were observed. There was an increasing trend in CH₄ fluxes: in 2017 the average emission reached to 12.0 g CH₄-C m^-2 y^-1. Differences between the emission values gathered from authomated chambers were about 10% higher than those measured from manual chambers.

In Vända in-stream CW, a clear increase in average annual emissions was found: from 0.4 in2018 to 10.5 g CH₄-C m^-2 yr^-1 in 2021. It was correlated with increasing Typha latifolia-dominated vegetation cover. Emissions showed strong correlation with air and water temperature while no clear relationship was found with the depth of various parts.

Large CH₄ emission from CWs is a major concern and therefore a smart management is needed. Our previous studies in surface flow CWs treating nitrate-contaminated runoff demonstrate that above-ground biomass harvesting of plants can decrease the CH₄. The end of growing season is likely the best time for biomass harvesting while avoiding the excessively high CH₄ emissions that the summer harvest may produce.

How to cite: Mander, Ü., Soosaar, K., Tournebize, J., Kill, K., Chaumont, C., Maddison, M., and Kasak, K.: Climate impact of constructed wetlands treating nitrate-rich agricultural runoff: The methane problem, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13507, https://doi.org/10.5194/egusphere-egu22-13507, 2022.

Vegetated buffer strips have been introduced in some European countries since the 1980s to mitigate the deterioration of watercourses by surface runoff from intensively managed agricultural land. However, the effectiveness was proven to be less for the retention of dissolved nutrients than expected, as agricultural drainage water was directly charging streams via tile drainage pipes. Therefore, a new drainage mitigation measures was introduced in Denmark to lower the nutrient load of freshwater and eventually marine systems, called saturated buffer zone (SBZ). Drainage pipes are disconnected at the sloping field margin and to the riparian zone by diverting drainage water to a buried, lateral distribution pipe running parallel to the stream. Results of a 2-year monitoring study unravel a high performance of the investigated SBZ as nitrate and phosphate removal efficiency was as high as 87% and 76 %, respectively. However, these high efficiencies must still be interpreted with caution since subsurface water flows was rather heterogenous varying by two orders of magnitude within the investigated transects. None the less, SBZs are a promising mitigation measure for removing nutrients from farmland.

How to cite: Zak, D., Maagaard, A., Kronvang, B., Hoffmann, C., Audet, J., and Petersen, R.: Saturated buffer zones as novel drainage mitigation measure in Denmark, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13510, https://doi.org/10.5194/egusphere-egu22-13510, 2022.