HS2.3.2

A number of statistical approaches have been developed to quantify the overall trend in river water quality, but most approaches are not intended for reporting separate trends for different flow conditions. We propose an approach called FN_2Q, which is an extension of the flow-normalization (FN) procedure of the well-established WRTDS (“Weighted Regressions on Time, Discharge, and Season”) method. The FN_2Q approach provides a daily time series of low-flow and high-flow FN flux estimates that represent the lower and upper half of daily riverflow observations that occurred on each calendar day across the period of record. These daily estimates can be summarized into any time period of interest (e.g., monthly, seasonal, or annual) for quantifying trends. The proposed approach is illustrated with an application to a record of total nitrogen concentration (632 samples) collected between 1985 and 2018 from the South Fork Shenandoah River at Front Royal, Virginia (USA). Results show that the overall FN flux of total nitrogen has declined in the period of 1985–2018, which is mainly attributable to FN flux decline in the low-flow class. Furthermore, the decline in the low-flow class was highly correlated with wastewater effluent loads, indicating that the upgrades of treatment technology at wastewater treatment facilities have likely led to water-quality improvement under low-flow conditions. The high-flow FN flux showed a spike around 2007, which was likely caused by increased delivery of particulate nitrogen associated with sediment transport. The case study demonstrates the utility of the FN_2Q approach toward not only characterizing the changes in river water quality but also guiding the direction of additional analysis for capturing the underlying drivers. The FN_2Q approach (and the published code) can easily be applied to widely available river monitoring records to quantify water-quality trends under different flow conditions to enhance understanding of river water-quality dynamics. (Journal article: https://doi.org/10.1016/j.scitotenv.2020.143562; R code and data release: https://doi.org/10.5066/P9LBJEY1).

How to cite: Zhang, Q., Webber, J., Moyer, D., and Chanat, J.: Estimating river water-quality trends under different flow conditions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-595, https://doi.org/10.5194/egusphere-egu21-595, 2021.

Understanding the spatial and temporal variation of concentration-flow (CQ) relationships is valuable to enhance understanding of the key processes that drive changes in catchment water quality. This study used a data-driven approach to understand how the CQ relationship is influenced by catchment flow regimes (baseflow versus runoff dominated) throughout the Australian continent. To summarize the CQ relationship, we focus on the b exponent in a power-law relationship (C=aQ^b). We considered six commonly monitored constituents, namely, electrical conductivity (EC), total phosphorus (TP), filterable reactive phosphorus (FRP), total suspended solids (TSS), nitrate–nitrite (NO_x) and total nitrogen (TN), at a total of 251 catchments in Australia. A novel Bayesian hierarchical model was developed to assess a) the impacts of flow regime on CQ relationships, both across catchments (spatial variation) and within individual catchments (temporal variation); and b) how these impacts vary across five typical Australian climate zones – arid, Mediterranean, temperate, sub-tropical and tropical.

We found that for individual constituents: 1) spatial variations in CQ relationships are clearly influenced by the catchment-level baseflow contribution, and these influences differ with climate regions; 2) across climate zones, runoff-dominated catchments (i.e. with low baseflow contribution) have relatively stable CQ relationships, while groundwater-dominated catchments (i.e. with high baseflow contribution) have highly variable CQ patterns across climate zones; 3) within individual catchments, the variations in instantaneous baseflow contribution have no systematic and consistent effect on the CQ relationships. The influence of catchment baseflow contribution on CQ relationships has potential to be used to predict catchment water quality across Australia, with over half the total variability in concentration of sediment, salt and phosphorus species explained by variations in catchment-level baseflow contribution.

How to cite: Duvert, C., Guo, D., Minaudo, C., Dupas, R., Lintern, A., Liu, S., and Zhang, K.: Effects of streamflow regime on the concentration-flow (CQ) relationships across climate zones: a study across the Australian continent, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4394, https://doi.org/10.5194/egusphere-egu21-4394, 2021.

Degradation of aquatic ecosystem health due to the presence of excess nutrients and sediments is a long-standing, leading global environmental concern. Concentration-discharge (C-Q) relationships have been used to disentangle the hydrologic and biogeochemical drivers affecting the transport dynamics of nutrients, sediments, and other constituents of interest. However, C-Q relationships alone are insufficient to provide actionable information to watershed managers and decision-makers. Rather, a comprehensive understanding of the degree of inequality of pollutant transport over time and space is necessary so that appropriate best management practices can be implemented in the “right place” that is effective at reducing targeted pollutants at the “right time”. Such spatial and temporal targeting requires a uniform metric for identifying “hot spots” and “hot moments”. Nutrient and sediment transport are known to exhibit strong spatial and temporal inequality, with a small percentage of locations and events contributing to the vast majority of total annual loads. The processes for determining how to reduce total annual loads at a watershed scale often target spatial, but not temporal, components of inequality, such that “hot moments” are far less understood than “hot spots”. Here, we introduce a framework using Lorenz Inequality and the corresponding Gini Coefficient to quantify the temporal inequality of nutrient and sediment transport across the Chesapeake Bay watershed and couple the inequality analysis with C-Q analysis to disentangle the relative role of hydrologic versus biogeochemical controls on nutrient cycling and transport and categorize nutrient transport across a response gradient. This framework allows for interpretation of the physical factors that influence the extent to which hydrologic and biogeochemical drivers are attenuated or exacerbated, such that a catchment’s ability or lack thereof to buffer highly variable hydrologic and biogeochemical signals can be quantified and understood through the proposed framework.

Data were obtained for 108 sites in the Chesapeake Bay’s Non-Tidal Network from 2010 through 2018. The Lorenz Inequality and Gini Coefficient analyses were conducted using daily-scale data for flow and loads of total nitrogen (TN), total phosphorus (TP), and total suspended sediment (TSS) at each gauging station. We leverage these results to create a “temporal targeting framework” that identifies periods of time and corresponding flow conditions that must be targeted to achieve desired or mandated load reduction goals across the watershed. Among the 108 sites, the degree of temporal inequality for TP and TSS (0.37 – 0.98) was much greater than for flow and TN (0.29 – 0.77), likely due to the importance of overland versus baseflow in the transport pathways of the respective constituents. These findings stress the importance of informed design and implementation of best management practices effective in “hot moments,” and not just “hot spots,” across impaired watersheds to achieve and maintain water quality restoration goals. The “temporal targeting framework” provides a useful and convenient method for watershed planners to create low- and high-flow load targeting tables specific to a watershed and constituent.

How to cite: Preisendanz, H., Veith, T., Zhang, Q., Biertempfel, J., and Shortle, J.: Coupling C-Q and Lorenz Inequality Analyses to Create a Temporal Targeting Framework for Watershed-Scale Decision-Making, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7867, https://doi.org/10.5194/egusphere-egu21-7867, 2021.

We propose a data-driven approach of concentration-discharge (C-Q) relationship analysis, including a new classification of C-Q hysteresis loop at the catchment scale, combined to a simulation of lateral Q and C at the reach scale. We analyse high-frequency, multiple-site records of Q and electrical conductivity (EC) in karst catchment outlets, in which EC informs on water residence time. At the catchment scale, contributions of pre-event water (PEW) and event water (EW) during storm events are investigated through hysteresis loops analysis, which allows inferring hydrological processes. Our new classification of hysteresis loops is based on loop mean slope and hysteresis index. At the reach scale, lateral Q and EC are simulated using a diffusive wave equation model, providing a more spatialized picture of PEW and EW contributions to streamflow during storm events. The methodology is applied to two catchments (Loue river and Cèze river) in France, including 8 gauging stations with hourly Q and EC time series covering 66 storm events.

For both catchments, a conceptual model of water origin and hydrological-processes seasonal and spatial variability is drawn. Regarding Loue catchment, summer and fall storm-events are characterized by contribution of PEW through piston-type flows, whereas decreasing EC values in winter and spring storm-events indicate the major contribution of EW through surface runoff and following fast infiltration in karst. EW contribution is increasing towards downstream. Regarding Cèze catchment, higher contributions of EW are observed, indicating that fast infiltration and surface runoff are the dominant processes, associated to a PEW signature in summer and fall. PEW contribution also increases in karstified areas. Intra-site water origin seasonality is mostly related to karst aquifer saturation state, whereas inter-site variability is linked to karst areas extension. These results are encouraging to extend this approach to a variety of sites, notably influenced by important surface water/groundwater interactions, and groundwater flooding.

How to cite: Le Mesnil, M., Charlier, J.-B., Moussa, R., and Caballero, Y.: Combining  concentration-discharge hysteresis and reach-scale lateral flow modelling to characterize flood water origin and processes: application to karst catchments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14832, https://doi.org/10.5194/egusphere-egu21-14832, 2021.

Understanding discharge and solute responses is pivotal for water resources management and pollution mitigation measures. The few studies that have analysed concentration-discharge relations using high temporal resolution tracer data collected during rainfall-runoff events have shown that these relations may vary for different events and depend on season, event characteristics or antecedent wetness conditions. 

In this study, we used hydrometric and tracer data (stable isotopes, major ions and electrical conductivity (EC)) to i) compare the concentration-discharge relations for different tracers, ii) characterize the hysteretic relations between discharge and tracer concentrations at the event timescale, and iii) determine whether the changes in hysteresis can be explained by event characteristics.

Data collection was carried out in the Ressi catchment, a 2-ha forested watershed in the Italian pre-Alps. The catchment is characterized by high seasonality in runoff response, due to the seasonality in rainfall (high in fall) and evapotranspiration (high in summer). Discharge and rainfall have been measured continuously since August 2012. Stream water, precipitation, shallow groundwater and soil water samples were collected for tracer analyses during 20 rainfall-runoff events between September 2015 and August 2018. All samples were analyzed for EC, isotopic composition (²H and ¹⁸O) and major ion concentrations. To investigate the possible controls on concentration-discharge relations, we determined the main characteristics (e.g., total event rainfall, rainfall intensities, antecedent soil moisture and depth to water table, runoff coefficient) for each selected rainfall-runoff event.

The EC, calcium, magnesium, sodium and sulfate concentrations in stream water decreased during rainfall events, due to the dilution by rain water. The concentration-discharge relations for these tracers with a dilution behavior were stronger and more significant than for the tracers that were mobilized during the event. Interestingly, nitrate, potassium and chloride, concentrations sometimes increased at the onset of events, likely due to a rapid flushing of solutes from the dry parts of the stream channel and the riparian area, and then decreased during the event. These temporal dynamics in solute concentrations resulted in different hysteretic relations with discharge. Clockwise loops (i.e., discharge peaked later than the tracer concentrations) were common for the isotopes, chloride and potassium, whereas anti-clockwise hysteresis loops were more typical for EC, magnesium, calcium, sulfate, sodium and nitrate. A preliminary correlation analysis suggests that event characteristics alone cannot explain the changes in hysteresis, except for the hysteresis area for the relations between discharge and calcium concentration that depends on the magnitude of the rainfall event (i.e., the larger the rainfall amount and the runoff coefficient, the smaller the hysteresis loop). 

These results highlight the importance of the first flush and indicate that runoff processes and solute sources can change when the catchment becomes wetter and connectivity of the hillslopes to the stream increases.

Keywords: concentration-discharge relation; major ions; electrical conductivity; stable isotopes; hysteresis; forested catchment.

How to cite: Zuecco, G., Marchina, C., Gelmini, Y., Amin, A., van Meerveld, I., Penna, D., and Borga, M.: How do concentration-discharge relations vary among rainfall-runoff events? An analysis for the Ressi experimental catchment (Italian pre-Alps), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15045, https://doi.org/10.5194/egusphere-egu21-15045, 2021.

Over the past decades, intensive agriculture has altered surface water and groundwater resources quality. Nutrient surplus increased nitrate concentrations in groundwater and rivers resulting in eutrophication or drinking water risk having ecosystem, sanitary and economic repercussions. Legislations led to a reduction of agricultural inputs of nitrogen since 1990’s followed by a decrease of nitrate concentrations in rivers, but still difficult to predict and evaluate. Indeed, the incomplete knowledge of the spatial variability of climate and nitrogen inputs, cumulated to the unknown groundwater heterogeneity,  leads to hydrological and biogeochemical processes difficult to model. This study deals with the long-term variations (~decades) of nitrate concentrations in three rivers (~30 km² catchment) located in Brittany. Thus, we focus on groundwater modelling because they constitute the bigger hydrological reservoir. We developed a parsimonious equivalent hillslope-scale groundwater model. The model parameterization, which controls hydrological functioning such as mean groundwater residence times, young water contribution to the river or denitrification, relies on long-term monitored streamflow and nitrate river concentrations. In addition, dissolved CFC were sampled in the catchments. Finally, we found that uncertainty on simulated nitrate river concentrations is low. The physically-based model also brings information on temporal and spatial variability of groundwater residence times highlighting the relative importance of young (1-5 yr) and old waters (~decades) for nitrate river concentrations. Moreover, calibrated models show similar trends looking at two fictive input scenarios from 2015 to 2050.

How to cite: Guillaumot, L., Aquilina, L., de Dreuzy, J.-R., Marçais, J., and Durand, P.: Using hillslope-scale groundwater model to bridge the gap between basin inputs and river concentrations. The case of nitrates in Brittany (France)., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10962, https://doi.org/10.5194/egusphere-egu21-10962, 2021.

The agricultural source pollution, such as nutrient and pesticides, affect the quality of surface water and groundwater. The agricultural nonpoint source pollution due to the excessive land fertilization is considered by researchers and governments as a concerning and sensitive issue. At the scale of agricultural catchments, the modeling of nitrate-leaching losses has been widely addressed in several studies. However, most of developed models require a large number of input data and parameters. Some of them include a complex process of biogeochemical nitrogen process or a full agronomic module and could be computationally time-consuming. Moreover, the quality of the input data makes the model calibration less efficient.

The objective of this study is to present a new conceptual and reservoir model (SIDRA-N), developed to better access the time-variation of nitrate concentrations [NO3-] at the outlet of subsurface drainage network. The model represent a simplified scheme of subsurface flow and nitrate transfer processes in the soil profile, between the drain and the mid-drain. The soil profile is decomposed into three interconnected compartments: the first compartment represents the rapid transfer of water and nitrate through the soil macroporosity; the two other compartments describe the progressive contribution of the horizontal transfer.

The input data to the nitrate module consists on the Remaining pools of Nitrate at the Beginning of Winter season (RNBW), introduced before the winter of each hydrological year. This value should represent all biogeochemical transformations of nitrogen and agricultural practices from previous crop. This variable can explain until 80% of the total nitrate flux exported yearly. Hence, SIDRA-N model requires only two input variables: the drainage discharge and the RNBW. A set of parameters was introduced to regulate nitrate fluxes and discharge transiting through compartments to the drain outlet.

Calibration and validation (C/V) procedures are fundamental to the assessment of the performance and the robustness of water quality models. In this study, the split sample test for the model calibration and validation (C/V) was carried out using data set from Rampillon study site (355 ha, data for 6 years), located East of Paris, in France. The C/V step was performed using high frequency observations (hourly time-step) of nitrate concentrations and drainage discharge. The results showed performance criteria of KGE greater than 0.5 and RMSE less than 5 mgN/l. These results confirm the very good quality of simulations. Finally, a seasonal model calibration was implemented to observe the yearly parameter variability and ensure the model stability and consistency.

How to cite: Tournebize, J., Chelil, S., Henine, H., and Chaumont, C.: Improving the estimates of nitrate concentrations at subsurface drained agricultural catchment scale using a new conceptual water quality model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14540, https://doi.org/10.5194/egusphere-egu21-14540, 2021.

Aquatic ecosystem recovery from anthropogenic degradation can be hampered by internal feedbacks that stabilize undesirable states. The challenges of managing and predicting alternative states in lakes are well known, but state shifts in rivers and their attendant effects on ecosystem function remain understudied despite strong recent evidence that such shifts can and do occur. Using three decades of measurements of key state variables such as turbidity, nutrient concentrations, Corbicula fluminea clam densities, and chlorophyll a, including hourly dissolved oxygen, we investigated a sudden shift from phytoplankton to macrophyte dominance in the middle Loire River (France), and its associated effects on the rivers metabolic regime. We show, instead, that despite large and synchronous shifts across all state variables, changes in gross primary production and ecosystem respiration were modest (25% and 14% declines, respectively) and that these shifts lagged the ecosystem state changes by a decade or more. The shift to a macrophyte-dominated state reduced the sensitivity of primary production to abiotic drivers, altered element cycling efficiency, flipped the net carbon balance from positive to negative, and, crucially, weakened the temporal coupling between production and respiration. This weakened coupling, detected using Granger causality, increased the temporal autocorrelation of net ecosystem production, yielding a robust early warning indicator of both state- and metabolic-shifts that may provide valuable guidance for river restoration.

How to cite: Diamond, J., Moatar, F., Cohen, M., Poirel, A., Martinet, C., Maire, A., and Pinay, G.: Metabolic regime shifts and ecosystem state changes are decoupled in a large river, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10666, https://doi.org/10.5194/egusphere-egu21-10666, 2021.

Export of nutrients from watersheds is largely regulated by the capacity of terrestrial and aquatic ecosystems to use resources in the face of processes that promote hydrologic export and release to the atmosphere. For nitrogen (N) and phosphorus (P) soil and plant uptake systems can be particularly strong sinks, taking up useable forms of N and P. Despite this efficient use, over long time scales ecosystems may be subject to substantial N and P losses that varies across time and space. Understanding the trends and seasonality of these nutrient losses on different ecosystems becomes important for the long-term maintenance of terrestrial nutrient limitation as well as for patterns of resource export to recipient aquatic ecosystems. To improve the understanding of nutrient losses from different land covers in natural boreal catchments we used long-term data (2008-2020) from the interdisciplinary, multi-scale Krycklan Catchment Study (KCS) in northern Sweden. We focused on 13 intensively monitored catchments with areas ranging from 12 ha to over 6780 ha and with different percentage of land cover characteristics; primarily forest (almost 100%), wetlands (nearly 50%) and lakes. For both P and N the main focus was on the dissolved inorganic phosphorus (DIP) and dissolved inorganic nitrogen (DIN). We evaluated the trends in stream nutrient concentration using Mann-Kendall tests to determine the Theil-Sen estimate slopes and a Seasonal Mann Kendall to evaluate the seasonality of the trends. Our results show a steady decline of DIP in all catchments and a decline in most catchments for DIN (11 out of 13). Although all catchments have a negative DIP trend we could not relate the magnitude of the slope to specific land cover or catchment size. Contrary to what we expected, negative trends of DIN during summer were inversely related to forest coverage, meaning that catchments with higher coverage of forest displayed a slower DIN decrease. While negative trends were evident at annual scales for both inorganic nutrients, more detailed assessment revealed time windows when most of this long‐term change occurred. Here, seasonal Mann Kendall tests revealed almost opposite seasonality for both inorganic nutrients, with significant DIP decline during the autumn, winter and spring and strong DIN declines during summer. We suspect these seasonal differences are linked to different processes that are being affected differently by changing seasonal characteristics, including warmer and shorter snow cover periods during winters and warmer and longer summers, respectively. Finally, in light of ongoing increasing trends of dissolved organic carbon (DOC), DIP:DOC and DIN:DOC molar ratios are also steadily increasing over time in most catchments. As a result nutrient balances in the river waters are becoming even more carbon rich and N-P poor, offsetting the balance mainly in growing season for DOC:DIN ratio and in spring and autumn for DOC:DIP ratio.

How to cite: Mosquera, V., Hasselquist, E., Sponseller, R., and Laudon, H.: Oligotrophication of boreal headwater rivers: persistent and widespread decline of inorganic nitrogen and phosphorus., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9646, https://doi.org/10.5194/egusphere-egu21-9646, 2021.

The ‘Dominant Source Layer’ (DSL) is defined as the riparian zone (RZ) depth stratum that contributes the most to water and solute fluxes to streams. For any given period of time, the DSL position is inferred from the relationship between RZ groundwater table and stream runoff by assuming that lateral fluxes at any given RZ soil depth are proportional to the groundwater table – stream runoff curve. In forest headwaters, the DSL approach can be used to explain timing and amount of water and solute transferred from RZs to streams.

Here, we used the DSL conceptual framework to investigate the potential impact of future climate changes on the long-term mobilization of water and solutes in a subhumid Mediterranean headwater catchment. We used the rainfall-runoff model PERSiST and synthetic temperature, precipitation, and inter-event length scenarios to simulate reference (1981–2000) and future (2081–2100) stream runoff in the catchment. Simulated stream runoff was then used to estimate RZ groundwater tables, and thus, the DSL position, based on the characteristic RZ groundwater table – stream runoff relationship previously established for this catchment. Our simulations indicate that future changes in temperature and precipitation will lead to reductions in stream runoff and water exports, and that the DSL will move down by as much as ca. 30 cm. As a result, shallow organic-rich layers in the RZ may only be hydrologically activated during sporadic large rainfall events predicted for the most extreme inter-event length scenarios. To better understand the transfer of solutes during these large rainfall events, we examined the RZ groundwater table – stream runoff relationship during five large storms for which we had empirical data. We found that this relationship varied among individual storm events depending on antecedent hydroclimatic conditions. Specifically, antecedent drier conditions led to steeper slopes in the RZ groundwater table – stream runoff relationship, which resulted in relatively larger concentrations of dissolved organic carbon (DOC) and nitrate (NO₃^-) in the stream. A steeper slope of the relationship implies that more RZ layers will be hydrologically connected to the stream, increasing the ‘thickness’ of the DSL and thus the chances for relatively more DOC and NO₃^- to be mobilized.

Overall, we highlight the importance of identifying the layers in the RZ vertical profile that are hydrologically active (i.e., DSLs) and the factors contributing to their temporal dynamics both at long- and short-term scales, to better predict the transfer of water and solutes from catchment soils to forest headwater streams

How to cite: Ledesma, J. L. J., Lupon, A., Ruiz-Pérez, G., Poblador, S., Futter, M. N., Martí, E., and Bernal, S.: The ‘Dominant Source Layer’ approach to infer long- and short-term water and solute mobilization in a subhumid Mediterranean catchment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2762, https://doi.org/10.5194/egusphere-egu21-2762, 2021.

Dissolved organic carbon (DOC) constitutes the biggest portion of carbon that is exported from soils. During the last decades, widespread increases in DOC concentrations of surface waters have been observed, affecting ecosystem functioning and drinking water treatment. However, the hydrological controls on DOC mobilization are still not completely understood.

We sampled two different topographical positions within a headwater catchment in the Bavarian Forest National Park: at a steep hillslope (880 m.a.s.l.) and in a flat and wide riparian zone (770 m.a.s.l.). By using piezometers, pore water samplers (peepers) and in-stream spectrometric devices we measured DOC concentrations as well as DOC absorbance (A₂₅₄/A₃₆₅ and SUVA₂₅₄) and fluorescence characteristics (fluorescence and freshness indices) in soil water, shallow ground water and stream water in order to gain insights into the DOC source areas during base-flow and during precipitation events.

High DOC concentrations (up to 80 mg L^-1) were found in soil water from cascading sequences of small ponds in the flat downstream part of the catchment that fill up temporarily. The increase of in-stream DOC concentrations during events was accompanied by changing DOC characteristics at both locations, for example increasing freshness index values. As the freshness index values were approaching the values found in the DOC-rich ponds in the riparian zone, these ponds seem to be important DOC sources during events. Our preliminary results point to a change of flow pathways during events.

How to cite: Blaurock, K., Garthen, P., Gilfedder, B. S., Fleckenstein, J. H., Peiffer, S., and Hopp, L.: Elucidating sources and pathways of dissolved organic carbon in a small, forested catchment – A qualitative assessment of stream, soil and shallow groundwater, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15785, https://doi.org/10.5194/egusphere-egu21-15785, 2021.

Glyphosate is the most widely used herbicide active ingredient in the world, with 1.35 Tg used globally in 2017. Despite a strong affinity for binding to soil and rapid microbial degradation, in recent studies glyphosate has been detected in agricultural runoff at significant concentrations. This unexplained phenomenon necessitates further study into the mechanism of glyphosate transport from agricultural fields. This study was an investigation into the internal hydrology at a 4.9 ha agricultural catchment and the hydrological processes driving glyphosate transport at this site. Chloride is introduced to this standalone watershed via a point source of sodium chloride road de-icer salt at the top edge of the catchment. The goal of this project was to employ chloride as a tracer to unlock how water moves in the catchment. Since 2015, we have been undertaking annual extensive field sampling campaigns to monitor runoff for glyphosate at an outlet weir. In this project, we used archived samples from the 2018, 2019 and 2020 field campaigns and analyzed over 700 samples for electrical conductivity, over 400 samples for chloride and over 500 samples for glyphosate. During storms, chloride concentration and electrical conductivity decreased as the baseflow component carrying dissolved ions was diluted by the fast response overland flow. During the peak flow of a storm, chloride makes up a consistent fraction of electrical conductivity as the entire catchment contributes to flow at the outlet. We also found that the ratio of glyphosate concentration to electrical conductivity increased linearly with flow rate. The rate of increase (ie., the slope of glyphosate to conductivity ratio versus flow rate) decreases between sequential storms as glyphosate adsorbs and microbially degrades, and from this we extrapolated an empirical degradation half-life of less than 10 days. Cumulatively, the observations of chloride and electrical conductivity suggest that the catchment behaves as a series of connected reservoirs, each with a slow-moving subsurface component and a fast-response overland component. By exploiting the existence of a road salt chloride tracer and soil electrical conductivity in a variable source area, we were able to unlock the hydrological processes at play in areas where surface runoff is generated.

How to cite: Payne, E., Pacenka, S., Richards, B., Brindt, N., Schatz, A., and Steenhuis, T.: Employing Legacy Road Salt and Soil Electrical Conductivity as Tracers to Unlock Glyphosate Transport Processes in a Variable Source Area, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5587, https://doi.org/10.5194/egusphere-egu21-5587, 2021.

The extraction and use of rare earth elements, platinum group elements and other trace metals is growing exponentially around the world. The occurrence of these trace elements in anthropogenic waste streams is increasing correspondingly. Yet, conclusive data on trace element concentrations in urban runoff and wastewater is scarce as these elements are typically not part of governmental surveillance programs and barely environmentally regulated. The human imprints on natural trace element fluxes and their potential environmental impacts therefore remain poorly quantified. We are working to quantify natural and anthropogenic trace element fluxes in the Great Lakes basin. The Great Lakes basin provides a globally unique setting to investigate human imprints on large-scale elemental cycling because it houses >60 million people, contains >20% of the world’s freshwater, and is divided into serially connected sub-basins that facilitate environmental system analyses at various scales.

First, we established baseline estimates of current (natural) trace element fluxes in the Great Lakes by aggregating hydrometric and water quality data in simplified black-box mass-balances and dynamic reactor models. These models were informed by >100,000 hydrometric and >50,000 water quality measurements collected across the Great Lakes between 1980-2020 and were calibrated to existing long-term water level and water chemistry records. The bulk of the incorporated data stems from Canadian and US federal and provincial and state monitoring programs, including publicly available datasets from NOAA, EPA, ECCC, Ontario and Michigan state, municipalities, and local conservation authorities. Mass-balance could be achieved up to 94% for conservative elements (Cl, Na), while our dynamic models reveal significantly different source/sink behavior across the upper and lower lakes for more reactive elements. We are currently expanding our models with new ultra-trace level analyses of recent freshwater samples from cruise expeditions, major tributary rivers, and precipitation, as well as sediment records.

Second, we considered municipal and industrial wastewater as a proxy for human activity. We collected and analyzed wastewater effluent and digested sludge samples from >40 US and Canadian wastewater treatment facilities (WWTF) and estimated, for >20 trace elements, average discharge rates into the Great Lakes basin. We compared average wastewater-effluent loads with large-scale natural biogeochemical fluxes in the Great Lakes, allowing us to rank the analyzed trace elements as well as individual lakes and tributaries by their apparent human imprint. Our results show anomalously high loading rates for select rare earth elements and precious metals in several tributary systems. Geospatial attributes of the sampled sewersheds (demographics, land use, industrial activity) serve as independent variables in our ongoing effort to source-track these anomalous loads and establish human imprints on catchment tributaries further upstream.

How to cite: Pinter, F. J., Bentley, C., and Vriens, B.: Assessing Human Imprints on Trace Element Fluxes in the Great Lakes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6472, https://doi.org/10.5194/egusphere-egu21-6472, 2021.