Excess nitrogen from intensive agricultural production, atmospheric N deposition, and urban point sources elevates stream nitrate concentrations, leading to problems of eutrophication and ecosystem degradation in coastal waters. A major emphasis of current US-scale analysis of water quality is to better our understanding of the relationship between changes in anthropogenic N inputs within watersheds and subsequent changes in riverine N loads. While most water quality modeling assumes a positive linear correlation between watershed N inputs and riverine N, many efforts to reduce riverine N through improved nutrient management practices result in little or no short-term improvements in water quality. Here, we use nitrate concentration and load data from 478 US watersheds, along with developed N input trajectories for these watersheds, to quantify time-varying relationships between N inputs and riverine N export. Our results show substantial variations in watershed N import-export relationships over time, with quantifiable hysteresis effects. Our results show that more population-dense urban watersheds in the northeastern U.S. more frequently show clockwise hysteresis relationships between N imports and riverine N export, with accelerated improvements in water quality being achieved through the implementation of point-source controls. In contrast, counterclockwise hysteresis dynamics are more common in agricultural watersheds, where time lags occur between the implementation of nutrient management practices and water-quality improvements. Finally, we find higher tile-drainage densities to be associated with more linear relationships between N inputs and riverine N. The empirical analysis in this study is bolstered by modeled simulations to reproduce and further explain drivers behind the hysteretic relationships commonly observed in the monitored watersheds.

How to cite: Van Meter, K., Basu, N., and Byrnes, D.: Memory and Management: Competing Controls on Long-Term Nitrate Trajectories in U.S. Rivers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13731, https://doi.org/10.5194/egusphere-egu24-13731, 2024.

High-frequency observations of solute and particulate concentrations (C) and associated discharge (Q) measurements allow the analysis of C-Q relationships on the event-scale. Together with catchment attributes and event properties, C-Q relationships can be used to disentangle the fine-grained dependencies between catchment properties, hydrologic processes and water quality.

We collected 72 high-frequency (sub-hourly), continuous C and Q time series across Germany in catchments ranging from 8 to 122.000km² (median of 6.051km²). Besides discharge at all 72 locations, the database contains water temperature (60 locations), turbidity (34), conductivity (59), oxygen (57), pH (53), ammonia (16), nitrate (22), phosphate (10) and chlorophyll (12) in varying lengths over a maximum period of 20 years. Event filters were used to extract single discharge events. Hysteresis indices and classes of each event were used to describe the C-Q relationships. Event properties such as season, magnitude, variability, length, antecedent conditions, rise- and fall characteristics and physical catchment characteristics were assigned to each C-Q relationship.

We used a Random Forest model to explain the hysteresis properties based on the event- and catchment characteristics. Particularly the variables turbidity and conductivity revealed spatio-temporal dependencies, which we relate to interplays of land use and soil characteristics. We further found that the C-Q patterns are significantly impacted by the identifiability and definition of the extracted discharge events across the heterogeneous catchments as well as the resolution and quality of the measurements of the different C variables.

How to cite: Kiesel, J., Houska, T., Müller-Hillebrand, J., and Fohrer, N.: German-wide analysis of high-frequency, continuous concentration-discharge relationships, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9538, https://doi.org/10.5194/egusphere-egu24-9538, 2024.

Peatlands are an important natural terrestrial carbon sink. Any impact on the drivers of hydro-biogeochemical processes in these ecosystems can be particularly severe. Climate change and degradation by drainage and ditching are dramatically changing peatlands. Degraded peatlands turn from effective carbon sinks to emitters. They can also threaten drinking water supplies, as (heavy) metals can leach from degraded peatlands together with dissolved organic carbon (DOC). However, quantifying DOC fluxes from terrestrial to aquatic ecosystems is challenging. The hydro-biogeochemical processes at the soil-aquatic interface are not only complex but also occur at different spatial and temporal scales. These processes depend on a variety of constantly changing external conditions such as temperature, nutrient and oxygen availability. In addition, there is no sensor that can directly measure DOC concentrations in streams in situ.

Here we investigated the DOC concentration in two nested catchments of two adjacent streams in the Ore Mountains of southern Saxony, Germany. One stream is dominated by mineral soils, the other by (degraded) peat soils. Each of the four sites is equipped with YSI-EXO fDOM sensors. Other data include discharge, water temperature, turbidity and electrical conductivity. A machine learning algorithm (Random Forest) was trained to predict DOC concentration from the available data set (validation r² between 0.85 and 0.98). The 15-minute resolution DOC data were analysed for potential driving factors. Interestingly, the area-specific loads of the peat-dominated catchment with 3.4 g C m^-2 a^-1 were not significantly different from those of the mineral soil-dominated catchment with 1.8 g C m^-2 a^-1. However, the annual loads were almost twice as high as previously determined from monthly data. With the high-resolution DOC data, we can identify periods of extreme DOC concentrations (up to 40 mg l^-1) after heavy rain events in summer and constant high DOC concentrations of 20 mg l^-1 during snowmelt in winter. By applying the algorithm to DOC:DON ratios, we were also able to quantify the different sources contributing to streamwater DOM with plant-derived material from peat and microbially-derived material from the mineral soil.

Previous DOC measurements, mostly based on 2-week to monthly measurements, are likely to greatly underestimate the contribution of DOC to C fluxes in ecosystems. This is particularly important for C-rich ecosystems such as peatlands.

How to cite: Houska, T., Müller, I., Kaiser, K., Knorr, K.-H., Lau, M., Jackisch, C., and Kalbitz, K.: Increasing stream water DOC concentrations in peat-affected catchments: insights from high-resolution water quality analysis, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10390, https://doi.org/10.5194/egusphere-egu24-10390, 2024.

Riparian wetlands are major sources of dissolved organic carbon (DOC) to streams. Increasing DOC concentrations were observed for many northern streams during the last decades, with potential implications for carbon (C) storages of wetland soils and streamwater quality. Drivers behind these trends, and particularly the significance of redox processes in wetland soils, are still incompletely understood. In soils, organic C is often associated with or bound to iron (oxy) hydroxides. These associations of iron (Fe) and organic C may immobilise and protect soil organic matter from mineralisation under oxic conditions. However, organic C can be remobilised if ferric Fe is reduced under anoxic conditions, a process which also increases pH further enhancing DOC solubility. Redox processes are therefore presumably important drivers of DOC dynamics in both wetland soils and the adjacent streams. We hypothesised that in-stream DOC concentrations are mainly driven by redox conditions within riparian organic soils, where DOC mobilisation is controlled by reduction of DOC associated Fe. We further propose that these DOC mobilising redox processes are particularly relevant for periods of rewetting of riparian soils, e.g. in autumn. In this study, were used monitoring data following a drought experiment conducted in summer 2017 in a sub-catchment of Krycklan in northern Sweden. For the experiment, a drought was simulated for a sub-catchment in Krycklan by damming a lake outlet that feeds a small stream. For the rewetting period after the drought experiment, daily time series of discharge, DOC and Fe feeding into the manipulated stream section were calculated from data measured at the top and the bottom of the stream section. Discharge was measured by flumes. From discharge time series, baseflow feeding into the stream section was computed using a baseflow separation filter. Time series of baseflow was assumed to represent average watertable dynamics in riparian wetlands. Both Fe and DOC concentrations were obtained from absorbances measured across different wavelength using a portable ultraviolet–visible probe. Adsorbances were converted into aquatic concentrations using a partial least-squares regression model calibrated on Fe and DOC concentrations measured in the laboratory. Furthermore, concentration time series were corrected for discharge and in-stream retention (for DOC only). We found that Fe increased with increasing baseflow with a time lag of 5d indicating delayed iron reduction in riparian areas in response to elevated watertables. Dynamics of DOC were weaker related to baseflow than to Fe, but DOC was significantly correlated to Fe. From rules to obtain directed acyclic graphs it can be inferred that changing baseflow - as a proxy of watertables in riparian wetlands - caused changing discharge corrected Fe concentrations in the stream, which can be understood as a proxy of Fe concentrations in riparian wetlands. Changing Fe concentrations caused changes of computed in-stream DOC concentrations, which can be seen to represent mobilised DOC pools in riparian wetlands. It is concluded that redox driven mobilisation of DOC is a plausible process for rainfall periods in autumn, when riparian soils are rewetted after summer.

How to cite: Selle, B., Hortmann, A., Knorr, K.-H., and Laudon, H.: Redox driven mobilisation of DOC from riparian wetlands in Krycklan (N Sweden) after a drought experiment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15329, https://doi.org/10.5194/egusphere-egu24-15329, 2024.

Sustained increments of organic carbon concentrations in northern freshwaters have triggered concerns about the impacts of water browning and raised questions on the underlying mechanisms causing this phenomenon. In addition to the key role played by reduced sulfate deposition, hydrologic mechanisms have been put forward as possible concurrent causes of the observed trends of organic carbon concentrations. How the suggested hydrologic controls act is however still unclear. In this study we analyze long data series (> 30 years) of daily discharge and weekly to biweekly Total Organic Carbon (TOC) concentration for four reference acid-sensitive rivers in Norway, whose locations span the entire length of the country, to clarify hydrologic changes which may be promoting freshwater browning. In all cases we observe stable values of the slopes of double logarithmic relations between concentration and discharge, as well as a steady growth along the years of the intercepts of these relations. These joint observations enable sorting out previously proposed biogeochemical mechanisms for the observed trends of TOC concentrations (i.e., less sulfate deposition versus higher soil temperature). Decreasing ratios of concentration and discharge variability along the years, observed in all watersheds during the autumn season, point at growing stores of organic carbon produced in summer and suggest that the spatial distribution of the sources is becoming more homogeneous. In detail, analyses of the runoff frequency, which is typically higher in wetter and more hydrologically connected watersheds, suggest that sources are more homogeneously connected to streams than before. In fact, increasing trends throughout the years of the runoff frequency, as well as strong relations between runoff frequency and increasing concentrations of aquatic organic carbon, are detected in all cases. More connected sources together with more frequent runoff events, which multiply the chances for the organic carbon to reach streams, may hence contribute to the observed rise of organic carbon concentrations in northern freshwaters.

How to cite: Basso, S. and de Wit, H.: Increasing hydrologic connectivity contributes to browning of northern freshwater, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2137, https://doi.org/10.5194/egusphere-egu24-2137, 2024.

In upland watersheds, depletion of essential nutrients due to physical erosion and chemical weathering can be compensated by exogenous inputs such as aeolian dust deposition. The presence and chemical composition of exogenous dust arriving in natural environments is commonly analyzed in soil profiles using a suite of geochemical and isotopic tracers. However, it remains an outstanding challenge to describe the impacts of dust on the reaction rates that produce these profiles and how this cascades into ecosystem function and water chemistry. As increasingly intense and episodic periods of drought and aridity are promoted by a warming climate, the role of dust production and deposition in Critical Zone structure and function requires improved modeling techniques to facilitate rigorous quantification and prediction. Here we present a newly developed process-based reactive transport framework by modifying the open source CrunchTope software in order to quantitatively interpret the impacts of dust deposition and solubilization in stream water chemistry, regolith weathering rates, and ecosystem nutrient availability. We describe two simulations: (1) a generic model demonstrating a simplified system in which bedrock uplift and soil erosion occur in tandem with solid phase dust deposition at the land surface; (2) a case study based on a small (0.54 km²) upland Mediterranean watershed located on Mont Lozère in the National Park of Les Cévennes, France. In the absence of an exogenous dust input, long-term field observations of calcium in stream water, rain, bedrock, soil, and plant samples cannot be produced from reactive transport simulations of the weathering profile. By adding a carbonate-rich depositional input consistent with the composition of Saharan dust, both stream water chemistry and elemental mass-transfer coefficients in the soil profile better align with field observations, suggesting that dust has become a significant input to this field site in the last ~10 ka. Over this period, the deposition of exogenous carbonates has introduced far more calcium into the system than what could be supplied by the Ca-poor granitic bedrock. This highly soluble carbonate also limits the reactive potential of infiltrating precipitation, ultimately inhibiting chemical weathering rates and hence the component of elemental export fluxes derived from local bedrock. This is the first demonstration of solid-phase dust deposition incorporated into a multi-component reactive transport framework. Our update to the CrunchTope source code allows us to show how dust incorporation affects geochemical cycling across upland watersheds beyond the prohibitive limitations of simplified steady-state assumptions, a feature that will allow further research of a variety of Critical Zone systems subject to the effects of environmental change scenarios. 

How to cite: Aranda Reina, C., Bouchez, J., Golla, J. K., Ayral, P.-A., and Druhan, J. L.: Quantifying the impacts of an exogenous dust input to the soil and stream chemistry of an upland Mediterranean watershed using a reactive transport modeling framework, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6339, https://doi.org/10.5194/egusphere-egu24-6339, 2024.

Applying road salt to melt ice on urban roads and pavements has raised the chloride levels in streams and rivers in the U.S. over time, which can damage aquatic life. High chloride levels in the waterbodies of the Fox River watershed, where one-third of the land is urban, are mainly caused by road salt application for deicing. This study analyzed how the chloride levels changed over the years and seasons in the Fox River watershed – from Stratton Dam to Illinois River. The Seasonal Kendall Tau (SKT) method was used to estimate the annual and seasonal trends in chloride concentration at 44 monitoring sites on the Fox River and its tributaries for three time periods (2017–2021, 2012–2021, and 1997–2021), after conducting exploratory data analysis and assessing the data suitability. The chloride concentration in the watershed varied over time, space, and season. From 2012 to 2021, it declined or remained stable at most of the monitoring sites, but it rose slightly from 1997 to 2021. The 5-year trend from 2017 to 2021 was similar, except that some sites showed an increase in summer and fall. The chloride concentration along the Fox River and Tyler Creek showed a longitudinal pattern, decreasing from upstream to downstream in most seasons and periods, except for the 5-year annual and fall trends, which increased. Weighted Regression on Time Discharge and Season (WRTDS) models were developed to estimate the trends in flow-normalized chloride flux for one site on the Fox River and two sites on its tributaries with daily flow data. The resulting trends indicate that the chloride fluxes dropped significantly at the Fox River and Polar Creek sites, mainly in the winter of 2012–2021. However, the site on Blackberry Creek had an opposite trend of increasing chloride flux, except for the winter flux, which also declined. Trends in selected streamflow statistics including mean, 7-day minimum, and 1-day maximum flows were also analyzed for the three monitoring sites to provide insight into how hydrologic variability affects chloride trends. The trends in annual and seasonal flow statistics exhibited a steep slope for low flows but a gradual slope for high flows, indicating more variability in the low flow statistics during the periods of analysis. The changes in chloride concentration and flux were partly related to the changes in flow, but other factors affecting water quality, such as watershed conservation, may also play a role. Assessing trends over distinct periods provides a nuanced understanding of how mitigation strategies may influence water quality improvements through the years and serves as a crucial guide for initiatives aimed at enhancing the overall health of the Fox River ecosystem. Choosing deicing methods that balance cost, performance, and environmental impacts should be a vital part of a mitigation plan. Moreover, monitoring and evaluating trends can help assess the current status of chloride levels in the watershed and inform future actions.

How to cite: Getahun, E. and Zavelle, A.: Chloride Trends in the Fox River watershed: Stratton Dam to Illinois River , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2921, https://doi.org/10.5194/egusphere-egu24-2921, 2024.

Pollution control and environmental protection of the Yangtze River have received major attention in China. However, modeling the river’s pollution load remains challenging due to limited monitoring and unclear spatiotemporal distribution of pollution sources. Specifically, anthropogenic activities’ contribution to the pollution have been underestimated in previous research. Here, we coupled a hydrodynamic-based water quality (HWQ) model with a machine learning (ML) model, namely attention-based Gated Recurrent Unit, to decipher the daily pollution loads (i.e., chemical oxygen demand, COD; total phosphorus, TP) and their sources in the Middle-Lower Yangtze River from 2014 to 2018. The coupled HWQ-ML model outperformed the standalone ML model with KGE values ranging 0.77–0.91 for COD and 0.47–0.64 for TP, while also reducing parameter uncertainty. When examining the relative contributions at the Middle Yangtze River Hankou cross-section, we observed that the main stream and tributaries, lateral anthropogenic activities, and parameter uncertainty contributed 15%, 66%, and 19% to COD, and 58%, 35%, and 7% to TP, respectively. For the Lower Yangtze River Datong cross-section, the contributions were 6%, 69%, and 25% for COD and 41%, 42%, and 17% for TP. The primary drivers of the anthropogenic pollution sources, in decreasing order of importance, were temperature (reflecting seasonality), date, and precipitation. This study emphasizes the synergy between physical modeling and machine learning, offering new insights into pollution load dynamics in the Yangtze River.

How to cite: Huang, S., Xia, J., Wang, Y., Wang, G., She, D., and Lei, J.: Deciphering pollution loads in the Middle-Lower Yangtze River by coupling water quality models with machine learning, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3559, https://doi.org/10.5194/egusphere-egu24-3559, 2024.

China has been a traditional aquaculture powerhouse, contributing over one-third of the global production. The Guangdong-Hong Kong-Macao Greater Bay Area stands out as a primary region for aquaculture. Pond aquaculture is a significant method employed in this area, benefiting from its geographical advantages with a widespread and numerous distribution of fish ponds within the Greater Bay Area. The primary production units for aquaculture ponds are predominantly household-based, decentralized, and lack a significant intensive production effect. Aquaculture personnel often rely on experiential judgment to assess water quality. In recent years, increased human factors and a lack of effective management in the aquaculture pond industry have exacerbated water pollution issues. This has resulted in a growing severity of water pollution problems, with governmental departments unable to conduct large-scale monitoring of aquaculture pond water quality. Dissolved oxygen serves as a crucial indicator reflecting the water quality of these aquaculture ponds. Only dissolved oxygen concentrations within suitable ranges can facilitate the growth of aquatic products; concentrations that are either too high or too low can adversely affect aquatic product growth. This study utilized Landsat 8/9 OLI satellite images, employing atmospheric correction based on Rayleigh reflectance. It combined machine learning and water body index methods to establish a dissolved oxygen Support Vector Regression (SVR) inversion model (R²=0.67). This model determined the trends in dissolved oxygen concentration changes and spatial distribution patterns in aquaculture ponds within the Greater Bay Area over the past decade. The results indicate that from 2013 to 2023, there was a marginal decrease of 0.04% in dissolved oxygen concentration. Concentrations decreased during 2014-2016 and 2018-2020, while they increased in other years. Seasonally, concentrations were higher in spring and autumn and lower in summer and winter. Summer exhibited the lowest dissolved oxygen concentration throughout the year, with the smallest concentration difference and relatively concentrated numerical distribution. Dissolved oxygen concentrations varied significantly in other seasons. Aquaculture ponds in low latitude coastal areas generally had lower dissolved oxygen concentrations, while those in northern mountainous and upstream river regions had relatively higher dissolved oxygen concentrations.

How to cite: Mao, K. and Yang, X.: Remote Sensing Retriving of Dissolved Oxygen Concentration in Aquaculture Ponds in the Guangdong-Hong Kong-Macao Greater Bay Area, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4925, https://doi.org/10.5194/egusphere-egu24-4925, 2024.

The Danish EPA has in the 3^rd River Basin Management Plan (RBMP) under the Water Framework Directive set target nitrogen loads for each coastal water for how to reach the reduction needed from coastal catchments to be implemented in 2027. In this context four locally based pilotprojects have been initiated to engages stakeholders to find local solutions for the RBMP. One of these new pilots are focusing on the Hjarbæk estuary situated in Limfjorden being one of the coastal water bodies in Denmark that needs the highest reductions in nitrogen loadings to be achieved before 2027 (ca. 65 %). This new project involving a coastal water board with all main stakeholders in the region being represented was initiated in February 2023 and has delivered proposals for 2 scenarios by the end of 2023 that can assure that the Hjarbæk estuary reach the target of achieving good ecological conditions.

Because of the high reductions in nitrogen loadings needed it is necessary to reduce all sources and both nitrogen and phosphorus to reach the goal. Focus in the RBMP has so far been to reduce the total nitrogen (TN) loadings. In the locally based scenarios phosphorus has gained greater focus. Our calculations show that every ton of phosphorus that is removed corresponds to removing 22 tons of nitrogen in Hjarbæk Fjord.  To be most cost-effective the effort will be carried out based on the principle of achieving the greatest possible effect per area unit. For that a detailed mapping of nitrogen (N) attenuation in the catchment have been conducted at a scale of ca. 15 km2 (ID15 sub-catchments) including mapping of both N-retention in groundwater and surface waters as well as N-delays in groundwater in Karst sub-catchments. The mapping shows huge differences in N-retention in both groundwater and surface waters within the ID15 sub-catchment (<20 % to >80 %).

The local engagement of stakeholders representing all sectors in the catchment and estuary have worked together to set up two scenarios that includes: i) marine mitigation measures such as mussel farming and eelgrass planting; ii) reductions in point source loadings; iii) use of a new portfolio of N mitigation measures to be adopted at source (e.g. catch crops, early seeding, set a side, afforestation, etc.); iv) use of transport mitigation measures from field to surface water (several types of constructed wetlands, riparian buffers and restored wetlands); v) the possible use of different phosphorus mitigation strategies in the catchment (lowering bank erosional P-losses, buffer strips, afforestation, etc.).

How to cite: Windolf, J., Piil, K., Jørgensen, T. B., Andersen, H. E., Dalgaard, T., and Kronvang, B.: The establishment and use of local coastal water boards is tested in Denmark to find bottom-up solutions for RBMP 2027, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14984, https://doi.org/10.5194/egusphere-egu24-14984, 2024.

To counteract soil subsidence and greenhouse gas emissions, groundwater levels in agriculturally used peatlands are increased in summer (e.g. by subsurface irrigation). This rewetting could lead to increased nutrient mobilization under anaerobic conditions in nutrient-rich soils, which will lead to eutrophication in ditches and lakes. However, rewetted peatlands can also be used to purify surface water and utilize the available nutrients by cultivation of wet crops like Typha and Phragmites, which is called “paludiculture”. These wet crops can provide raw materials for fiber based products (e.g. insulation and building materials). Paludiculture can also be implemented in multifunctional buffer zones along streams in sandy landscapes.

This multifunctional land-use can create a win-win situation that combines biomass production of wet crops with the provision of ecosystem services, such as peat preservation and water purification. To underpin what the water purification potential of paludiculture is, measurements have been done in several mesocosm experiments and field-scale paludiculture pilots within national and European projects (e.g. VIP-NL, KLIMAP, Carbon Connects and CINDERELLA). These pilots and experiments were used to learn how to cultivate paludiculture crops under different hydrological circumstances (water level and fluctuations), nutrient loads, water quality, soil types and field configurations. We quantified the nutrient uptake by Typha and Phragmites and the change in water quality between inlet and outlet in different situations. The results are also used to investigate which combination of factors will give the most efficient combination of water purification, nutrient uptake and biomass production.  In the end, this contributes to developing new ways of sustainable and economical feasible farming on wet peat soils and in brook valleys.

How to cite: Geurts, J., Van der Snoek, M., Fritz, C., and Van Duinen, G.-J.: Paludiculture: multifunctional land-use to decrease nutrient loading, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17087, https://doi.org/10.5194/egusphere-egu24-17087, 2024.

In recent years, intensive agricultural practices have been adopted to enhance crop yields for food production. Therefore, the issue of agricultural non-point source pollution attracts attention due to the increasing use of fertilizer. The non-point source pollution control is diverse and difficult to manage because of the physical landscape and agricultural practices (e.g., topography, soil texture, farming, and irrigation). In addition, excessive fertilizer used to enhance crop yields would lead to land degradation. During a rainfall event, nitrogen and phosphorus from fertilizers would be washed into surface water and infiltrated into groundwater, resulting in the degradation of the aquatic environment.

In this study, the use of slow-release fertilizer, compared to the reference of conventional chemical fertilizer, and a constructed wetland were adopted at a pilot-scale study. During a 40-day growing cycle, Brassica Chinensis L.(Pak-Choi) was chosen as the model crop, and two types (i.e., chemical fertilizer and slow-release fertilizer) of fertilizers were applied under pre-designed conditions. Two simulated intense rainfalls were operated on the 26^th and 33^rd days, and the surface runoff was introduced to the constructed wetland for further nutrient removal. The soil and water samples from the soil and wetland were analyzed for nutrient concentration variation, and the nutrient distribution and removal efficiency were assessed. The results showed that the chemical fertilizer has a higher nutrient loss rate. The nitrogen (N), phosphorus (P), and potassium (K) contents in the soil increased rapidly after top dressing and decreased significantly after the simulated rainfall. In contrast, slow-release fertilizer has a relatively steady nutrient loss rate during the growing cycle. Meanwhile, the chemical fertilizer has a higher total N, P, and K loss via infiltration and runoff than slow-release fertilizer. For the wetland treatment, the N removal for chemical fertilizer and slow-release fertilizer after 15 days was 24.4 % (i.e., from 19.99 ppm to 15.11 ppm) and 29.5 % (i.e., from 20.81 ppm to 14.68 ppm), the P removal amount was 38.1 % (i.e., from 0.21 ppm to 0.13 ppm) and 87.5 % (i.e., from 0.08 ppm to 0.01 ppm), respectively. Opposing to the conventional chemical fertilizers, the use of slow-release fertilizers could reduce nutrient loss. The constructed wetland demonstrated a positive effect on removing the nutrients in neighboring water, which reduced the impact of agricultural non-point source pollution.

Keywords: Conventional farming, Best management practices, Slow-release fertilizer, Constructed wetlands

How to cite: Huang, C.-J. and Fan, C.: Non-point source pollution control in farmland by source-reduction strategies coupled with wetland treatment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7171, https://doi.org/10.5194/egusphere-egu24-7171, 2024.

The agricultural sector is a large contributor to poor water quality in our freshwater systems. One potential pathway for agriculturally sourced pollution to enter the freshwater environment is through drainage ditches, which can be either open surface drains or subsurface pipes, or a combination of both. While large scale tile-drainage systems with a central output point are common in some countries, Irish agricultural drainage networks tend to be comprised of a complex network of drains, each with varying levels of connectivity to freshwater systems. Recent research by Moloney et al. (2020) categorised these drains in terms of their connectivity, finding that those with a direct connection between a farmyard and river/stream were at the greatest risk for transporting the highest concentrations of nutrients (phosphorus and nitrogen). Therefore, to target the most ideal location to mitigate against nutrient transport through drainage ditches, drains with direct farmyard connectivity provide the most resource- and cost-effective option.

This study investigated the effectiveness of sediment ponds installed in drainage ditches which had a direct connection between a farmyard and a river. Three case study farms were selected, two in the south (Cork) and one in the south-east (Wexford) of Ireland. All ponds were installed by 2021, with sampling commencing in December 2022. Grab water samples were collected weekly (Wexford farm) and fortnightly (Cork farms) at multiple points upstream and downstream of the ponds, and were analysed for nitrogen, phosphorus and dissolved organic carbon. Sediment samples were extracted from within the drainage ditches in summer 2023 and analysed for Mehlich-3 P, pH, Morgan’s P and particle size distribution. Initial results suggest ponds provide limited attenuation of nutrients, with no significant decreases at the downstream sample points. While sediment phosphorus concentrations are marginally elevated downstream, suggesting potential accumulation in the soil, further sampling is needed to confirm this trend. This study provides valuable insights into nutrient dynamics within agricultural drainage ditches and contributes to a better understanding of the effectiveness of sediment ponds as a potential mitigation measure for nutrient retention.

How to cite: Heerey, L., Fenton, O., Regan, F., White, B., Kent, N., and Daly, K.: Sediment ponds; an effective mitigation measure for reducing nutrient export in agricultural drainage ditches? , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12746, https://doi.org/10.5194/egusphere-egu24-12746, 2024.

It is known from recent international research that woodchip bioreactors can be an effective measure to reduce emissions of nitrate from agricultural drainage water. In the Netherlands, up till now no experience was present with woodchip bioreactors. Therefore, a field pilot was started at an agricultural test location situated in a lowland catchment in the south of the Netherlands (Vredepeel). A woodchip bioreactor was installed to treat drainage water from 4 ha arable land on sandy soil. Nitrate was measured in the in- and effluent of the bioreactor to estimate nitrate removal efficiency over time. Also, water chemistry and discharge were monitored. 
With a series of sampling points in the woodchip bioreactor, biogeochemical processes in the reactor are investigated that can explain the performance of the reactor. The goal is to not only determine the removal efficiency, but also potential side effects and effects of temporarily limited flow rate (e.g. sulfide and ammonia production and oxygen demand). We aim to give practical guidelines for practical design and application for agricultural fields in sandy lowland catchments. In this contribution we will present the monitoring results of one drainage season.

How to cite: Jansen, S., Van Driezum, I., Rozemeijer, J., Van Loon, A., and Van Herpen, F.: Field scale optimization of woodchip bioreactors for nitrate removal from drainage water in the Netherlands, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9606, https://doi.org/10.5194/egusphere-egu24-9606, 2024.

As in other areas with intensive agriculture in Europe, Flanders struggles with bringing surface water quality in line with the EU Nitrates Directive. At about 25% of the 874 surface water measurement locations in Flanders monitored by the Flanders Environment Agency (VMM), the 90^th percentile from monthly measurements conducted during 2018-2023 exceeded a NO₃^- concentration of 50 mg/L.

A large fraction of the nitrate that leaches out of the root zone is denitrified in the groundwater. However the denitrification rate varies spatially, so mitigation measures are best targeted to zones from where NO₃^- is transported to the surface water without undergoing significant denitrification in the aquifer. We used a novel methodology to predict NO₃^- concentrations in the outlets of small catchments that explicitly considers hydrochemical variation within aquifers and is based on the thickness of oxidized and reduced zones in an aquifer.

The depth of the redoxcline, the boundary between the oxidized and reduced zone, was determined from the phreatic groundwater monitoring network of VMM consisting of 2089 multilevel groundwater wells in Flanders. Analysis of the time series of hydro-chemical data (redox potential and dissolved NO₃^-, O₂, Fe, Mn) allowed us to classify the filters as being in oxidized or reduced zone. For each well, the depth of the first ‘reduced’ filter was taken as the depth of the redoxcline.

Assuming a spatially uniform nitrate concentration in the groundwater recharge, the nitrate concentration of the water reaching the catchment outlet can be estimated as:

NO₃^- at catchment outlet = (thickness of oxidized zone / equivalent aquifer thickness ) × NO₃^-   in recharge water        (1)

This simple approach assumes that all nitrate is denitrified once the groundwater flowline crosses the redoxcline. Instead of the true aquifer depth, we used the aquifer's equivalent thickness, utilizing the Hooghoudt equation based on the average distance between watercourses. The nitrate input in the recharge water for this calculation was taken from VMM’s NEMO model, which provides the nitrate leachate from agricultural fields to groundwater.

Our investigation covered 68 small agricultural catchments (0.4-20.4 km²). The ratio of oxidized to entire aquifer thickness varied from 0.03 to 1, averaging 0.33. Therefore, on average, 33% of the agricultural areas would show high nitrate vulnerabilities because flowlines originating from there do not cross the redoxcline. However, the ratio of the average observed NO₃^- concentration in surface water to that in recharge water is 0.46, indicating an overestimation of denitrification. The estimated nitrate concentrations in surface water, calculated using Equation (1), showed a reasonable agreement with observed values (R² = 0.32). A much lower R² (0.08) is observed when replacing the ratio of thicknesses in Equation (1) with the average thickness ratio of 0.33. This suggests that the variation in nitrate concentration at the catchment outlet is predominantly governed by the relative thickness of the oxidized zone.

This method identifies nitrate-vulnerable areas along water courses in a catchment that can be used to better target mitigation measures across Flanders.

How to cite: Khan, A. H. A. N., Vanderborght, J., Smolders, E., and Diels, J.: Identifying nitrate-vulnerable zones for surface water pollution in Flanders based on the depth of the redoxcline and aquifer thickness, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6444, https://doi.org/10.5194/egusphere-egu24-6444, 2024.

 Cascade reservoirs construction has modified the nutrients dynamics and biogeochemical cycles, consequently affecting the composition and productivity of river ecosystems. Cascade reservoirs in different rivers typically exhibit distinct variabilities in the retention characteristics of different nutrients. The Jinsha River, as the predominant contributor to runoff, suspended sediment (SS), and nutrients production within the Yangtze River, is a typical cascade reservoir region with unclear transport patterns and retention mechanisms of nutrients (nitrogen and phosphorus). Therefore, we monitored monthly variations in nitrogen and phosphorus concentrations from November 2021 to October 2022. The results demonstrated that the concentrations and fluxes of total phosphorus (TP) and particulate phosphorus (PP) significantly decreased as they moved downstream along the cascade of reservoirs, primarily due to PP deposited with SS, while total nitrogen (TN) and dissolved total nitrogen exhibited opposing trends. Moreover, the positive average annual retention rates for TP and PP were 9.64% and 15.64%, respectively, in contrast to the negative averages of -8.38% for TN and -10.51% for particulate nitrogen. A higher proportion of TP and PP was retained by the reservoirs in the flood season compared to the non-flood season. Additionally, the variability in runoff-sediment and hydraulic retention time (HRT) of cascade reservoirs played crucial roles in the retention of TP and PP. A stronger relationship between HRT and TP retention rate during the flood season suggested that the cascade reservoirs could effectively transport or intercept TP downstream when HRT was either less than or greater than 5.3 days. Consequently, the HRT of these reservoirs could be managed to control nutrients delivery, which was of particular significance for watershed government institutions. This study enhances our comprehension of how cascade reservoirs influence the distribution and transport patterns of nutrients, offering a fresh perspective on nutrients delivery regulation. 

How to cite: Zeng, Q.: Impact of cascade reservoirs on nutrients transported downstream and regulation method based on hydraulic retention time, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14010, https://doi.org/10.5194/egusphere-egu24-14010, 2024.

Diffuse pollution of shallow groundwater as a result of leaching of substances from agricultural soils has a negative impact on groundwater quality. In the drinking water sector, the focus has traditionally been on nitrogen (nitrate) and crop protection products, which are subject to strict standards. Geochemical buffering processes in the subsurface convert a large part of the nitrate load that leaches to groundwater from agricultural soils. However, these processes can often lead to an increase in the hardness (sum of the calcium and magnesium concentrations) of groundwater, which is undesirable for drinking water and domestic use.

Recent research shows that 70% of phreatic groundwater extraction locations in the Netherlands show a significant increasing trend in hardness. However, quantitative insight into the relationship between spatial characteristics (land use, soil type and geochemical composition of the subsoil) and hardness has so far been lacking. In this research we analyzed a long time series of data (since 1900) from shallow (< 25 m below ground surface) phreatic and semi-confined groundwater extraction locations in the Netherlands. The trends in hardness and partial pressure of CO₂ (P_CO2) and relationship with spatial characteristics of the extractions is presented.

A clear influence of agriculture activities in groundwater protection areas was observed; the hardness in agricultural-dominated extraction sites was 2.5 times higher compared to nature-dominated extraction sites, while the trend in the increase in hardness was almost 3 times higher. The trend is observed in both calcium carbonate-rich and calcium carbonate-poor soils. In carbonate-rich areas, the hardness of groundwater is determined by the addition of acid, from atmospheric deposition, agricultural activities such as fertilization and crop harvesting and by weak acid (CO₂) contributions from root respiration and mineralization of organic matter. In carbonate-poor soils hardness sources are the use of calcium and magnesium salts and the application of manure on agricultural land. 

In carbonate-rich systems, slightly less than half of the groundwater hardness was found to be due to limescale weathering due to strong acid input, while slightly more than half of the hardness was due to weathering from CO₂. Higher P_CO2 levels and trends in agricultural-dominated extraction sites comparted to nature-dominated sites reveals an impact of intensive agricultural production on the CO₂ production is soils, and thereby on groundwater quality, that have not been considered so far.

To minimize hardness as a result of soil acidification it is recommended to reduce nitrogen deposition, limiting nitrate leaching and limit the use of fertilizers that acidify the soil. To minimize hardness as a result of weak acid (CO₂) weathering, more extensive agricultural practices to reduce root respiration should be adopted, and the degradation of soil organic matter can be limited by preventing (short-term) lowering of groundwater levels in organic rich soils. The results indicate that land use has a significant effect on the hardness and P_CO2 in groundwater. Mitigating measures should consider an area-based approach, taking into account the land-use, soil type and geochemical characteristics of the subsurface to limit the impacts of increased hardness and P_CO2 as a result of agricultural activities.

How to cite: Hockin, A., van der Grift, B., and Scheper, D.: Unexpected impact of agricultural land-use practices on the concentration and trend in hardness of groundwater abstracted for drinking water supply, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20251, https://doi.org/10.5194/egusphere-egu24-20251, 2024.

Since the introduction of the EU Nitrates Directive (91/676/EEC), nitrogen concentrations have gradually declined. The recent summer droughts (2018-2020) in the Netherlands, have caused an increase in nitrogen concentrations in water leaching from agricultural soils, exceeding standards. It is expected that with increasing numbers of climate extremes, summer droughts will occur more often in the Netherlands. In order to develop possible strategies, it is important to better understand the underlying mechanisms and consequent impact of droughts on water quality. 

In rural areas of the Netherlands the water quality of shallow groundwater and surface water (leaching water) is strongly influenced by agricultural land use. The EU Nitrates Directive aims to protect waters against pollution caused by nitrates from agricultural sources. In the Netherlands, leaching of nitrogen to shallow groundwater and surface water is monitored for over 30 years by the Dutch Mineral Policy Monitoring Programme (LMM). Within the LMM, the Netherlands is divided in 4 main and 14 subregions based on soil type. The water sampling at participating farms, enables to analyse the effect of farming practices on water quality in the different LMM regions. 

Nutrients applied during the growing season can leach to shallow groundwater and surface water in the following autumn and winter. Summer drought inhibits the uptake of nutrients by crop, hampers the process of denitrification and leads to thickening of the soil moist and upper groundwater, leading to accumulation of excess nitrates in the soil. These excess nitrates can potentially increase nitrogen concentrations in leaching water in winter. Not every soil region responds similarly to a drought period. The aim of this study is to investigate why certain regions experience a stronger impact of drought on nitrogen leaching than others. 

We used monthly spatial Standardized Precipitation Evaporation Index data provided by the Royal Meteorological Institute (KNMI) over the period 1990-2022 to identify where (region) and when (year) summer droughts occurred. The LMM dataset was used to analyse the change in groundwater levels, nitrogen concentrations in leaching water and soil nitrogen surplus following a summer drought. Furthermore, we investigated the role of soil type and land use on the change in nitrogen concentrations in leaching water caused by drought.

Our findings reveal that the magnitude of the increase in nitrogen concentration in leaching water following a summer drought is determined by the duration and intensity of the drought. Furthermore, soil type and agricultural practices influenced the variation of the impact by droughts between regions. These results can be used to identify areas that are more sensitive to impacts of droughts on water quality based on their soil and land use characteristics and thus support the development of adaptation strategies by farmers, water authorities and national government. 

How to cite: Oosterwoud, M., Wismans, H., Vrijhoef, A., van Duijnen, R., and Wuijts, S.: Impact of drought on nitrogen concentrations in leaching water from agricultural areas in the Netherlands, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17979, https://doi.org/10.5194/egusphere-egu24-17979, 2024.