Hydraulic shortcuts increase the connectivity of arable land areas to surface waters

Surface runoff represents a major pathway for pesticide transport from agricultural areas to surface waters. The influence of artificial structures (e.g. roads, hedges, and ditches) on surface runoff connectivity has been shown in various studies. In Switzerland, so-called hydraulic shortcuts (e.g. inlet and maintenance shafts of road or field storm drainage systems) have been shown to influence surface runoff connectivity and related pesticide transport. Their occurrence and their influence on surface runoff and pesticide connectivity have, however, not been studied systematically. To address that deficit, we randomly selected 20 study areas (average size of 3.5 km2) throughout the Swiss plateau, representing arable cropping systems. We assessed shortcut occurrence in these study areas using three mapping methods, namely field mapping, drainage plans, and high-resolution aerial images. Surface runoff connectivity in the study areas was analysed using a 2×2 m digital elevation model and a multiple-flow algorithm. Parameter uncertainty affecting this analysis was addressed by a Monte Carlo simulation. With our approach, agricultural areas were divided into areas that are either directly, indirectly (i.e. via hydraulic shortcuts), or not at all connected to surface waters. Finally, the results of this connectivity analysis were scaled up to the national level, using a regression model based on topographic descriptors, and were then compared to an existing national connectivity model. Inlet shafts of the road storm drainage system were identified as the main shortcuts. On average, we found 0.84 inlet shafts and a total of 2.0 shafts per hectare of agricultural land. In the study catchments, between 43 % and 74 % of the agricultural area is connected to surface waters via hydraulic shortcuts. On the national level, this fraction is similar and lies between 47 % and 60 %. Considering our empirical observations led to shifts in estimated fractions of connected areas compared to the previous connectivity model. The differences were most pronounced in flat areas of river valleys. These numbers suggest that transport through hydraulic shortcuts is an important pesticide flow path in a landscape where many engineered structures exist to drain excess water from fields and roads. However, this transport process is currently not considered in Swiss pesticide legislation and authorization. Therefore, current regulations may fall short in addressing the full extent of the pesticide problem. However, independent measurements of water flow and pesticide transport to quantify the contribution of shortcuts and validating the model results are lacking. Overall, the findings highlight the relevance of better understanding the connectivity between fields and receiving waters and the underlying factors and physical structures in the landscape.

Role of Concentrated Flow Pathways on the Movement of Pesticides through Agricultural Fields and Riparian Buffer Zones

HighlightsLand management and hydrologic connectivity cause concentrated flow pathways (CFPs) to serve various functions.Pesticide concentrations diminished along flow pathways from row-cropped fields through functional riparian zones.CFPs facilitated pesticide transport into pasture/hay fields from upgradient corn fields.Subsurface transport was likely a more important transport pathway relative to surface runoff for imidacloprid.Abstract. Riparian buffers, which are an important component of watershed management strategies, can effectively mitigate nutrients and pesticides in agricultural runoff. However, concentrated flow pathways (CFPs) can undermine the performance of buffers by allowing contaminant-laden runoff to bypass the mitigation potential offered by the buffer soils and vegetation. To determine the extent to which CFPs increase pesticide transport from agricultural fields to nearby streams, soil samples (0-2 cm depth) were collected along both CFPs and overland flow (OLF) pathways from the field to the stream for nine fields in a Long-Term Agroecosystem Research (LTAR) site in the ridge and valley physiographic region of Pennsylvania. Soil samples were analyzed for atrazine, metolachlor, and imidacloprid, with two dominant patterns emerging. In corn fields, pesticide concentrations were higher in OLF than CFP samples, suggesting that pesticides were mitigated during transport through each corn field. In contrast, hay and pasture fields, which had not been treated with any of the three pesticides of interest, had lower pesticide concentrations in the OLF samples than the CFP samples. Because the CFPs from these fields originated in upgradient unsampled corn fields, these results suggest that the CFPs were a conduit for pesticides applied in the corn fields and were simply flowing through the hay and pasture fields. Similarly, CFPs in riparian buffers and grass pathways located between the row-cropped fields and the stream tended to have lower concentrations than the upland field (OLF-F) but higher concentrations than the buffer OLF, suggesting a potential for increasing overland flow effectiveness in riparian zones by interrupting CFPs leading to the stream. This study highlights the importance of the land management factors and hydrologic connectivity that cause CFPs to serve different functions (mitigation or enhancement) as runoff is conveyed from agricultural fields to a riparian buffer, and ultimately to an adjacent stream. Further, the results highlight the need for design and maintenance solutions addressing the erosion and sediment control issues that commonly undermine agricultural buffer effectiveness. Keywords: Buffers, Concentrated flow, Contaminant fate and transport, Hydrology, Land management, Pesticides, Overland flow, Water quality.

Hydraulic Shortcuts Increase the Connectivity of Arable Land Areas to Surface Waters

Surface runoff represents a major pathway for pesticide transport from agricultural areas to surface waters. The influence of man-made structures (e.g. roads, hedges, ditches) on surface runoff connectivity has been shown in various studies. In Switzerland, so-called hydraulic shortcuts (e.g. inlets and maintenance manholes of road or field storm drainage systems) have been shown to influence surface runoff connectivity and related pesticide transport. Their occurrence, and their influence on surface runoff and pesticide connectivity have however not been studied systematically. To address that deficit, we randomly selected 20 study areas (average size = 3.5 km2) throughout the Swiss plateau, representing arable cropping systems. We assessed shortcut occurrence in these study areas using three mapping methods: field mapping, drainage plans, and high-resolution aerial images. Surface runoff connectivity in the study areas was analysed using a 2 × 2 m digital elevation model and a multiple-flow algorithm. Parameter uncertainty affecting this analysis was addressed by a Monte Carlo simulation. With our approach, agricultural areas were divided into areas that are either directly connected to surface waters, indirectly (i.e. via hydraulic shortcuts), or not connected at all. Finally, the results of this connectivity analysis were scaled up to the national level using a regression model based on topographic descriptors. Inlets of the road storm drainage system were identified as the main shortcuts. On average, we found 0.84 inlets and a total of 2.0 manholes per hectare of agricultural land. In the study catchments between 43 and 74 % of the agricultural area is connected to surface waters via hydraulic shortcuts. On the national level, this fraction is similar (54 %). These numbers suggest that transport through hydraulic shortcuts is an important pesticide flow path in a landscape where many engineered structures exist to drain excess water from fields and roads. However, this transport process is currently not considered in Swiss pesticide legislation and authorisation. Therefore, current regulations may fall short to address the full extent of the pesticide problem. Overall, the findings highlight the relevance of better understanding the connectivity between fields and receiving waters and the underlying factors and physical structures in the landscape.

What pesticide legislation forgot about: Pesticide transport through hydraulic shortcuts

<p>Pesticides from agricultural origin may harm surface water quality and pose a risk for aquatic organisms. In Europe, the regulations on agricultural pesticide usage are currently focusing on “classical” pesticide transport pathways, such as surface runoff, spray drift into surface waters, or tile drainage flow. Recent studies have shown that in certain cases also so-called <em>hydraulic shortcuts</em> (e.g. road storm drains, or manholes of the tile drainage systems) can be of major importance for pesticide transport into surface waters. However, until now research has widely neglected this transport pathway.</p><p>In this study, we investigated the relevance of hydraulic shortcuts for the pesticide transport from arable land to surface waters in Switzerland. We selected twenty small catchments throughout the Swiss midlands as study areas by performing a weighted random selection on a nation-wide hydrological catchment stratification dataset. On average, they have an area of 3.5 km<sup>2</sup> with a fraction of 44 % of arable land. In the agricultural areas of these catchments, we mapped hydraulic shortcuts using different data sources: Field surveys, high-resolution aerial images captured by a fixed-wing drone as well as plans of the road storm drains and the tile drainage systems. Subsequently, we modelled the hydrological connectivity of arable areas to surface waters using a digital elevation model and a D-infinity flow direction algorithm. Within this model, we distinguished between areas with a direct and indirect (i.e. via shortcuts) surface water connectivity.</p><p>Our model results show that major fractions of the arable areas with surface water connectivity are not connected directly, but via hydraulic shortcuts: The fraction of indirectly connected areas ranges between 18 % and 90 %, with a median of 52 % for the 20 catchments. In order to check the model robustness we performed sensitivity analyses for different model parameters, such as sink filling depth, maximal flow length, or parameters addressing the influence of roads, forests, and hedges. In certain cases, changes of those model parameters have a strong influence on the absolute extent of directly and indirectly connected areas. However, their fractions compared to the total connected area were insensitive to changes in the model parameters.</p><p>In addition, we will present the results of a model predicting the fraction of arable land connected to shortcuts within a catchment, depending on auxiliary quantities (e.g. length of roads of a certain type, land use, slope). Using this model, we can estimate the arable land fraction per catchment on a national scale.</p>

Effectiveness of Vegetative Filter Strips for Mitigation in Higher-Tier Pesticide Exposure Assessments: Mechanistic Analysis with VFSMOD

<p>Pesticides high-tier, long-term environmental risk assessments (ERA) are based on the combination of complicated mechanistic models to evaluate regulatory compliance. The modeling framework often involves large sets of input factors (model parameters, initial and boundary conditions, and other model structure options). How can we identify the relative importance of human, chemical, physical and biological drivers on the assessment results? Is there a case for “the right answers for the right reasons”? For the case of pesticide mitigation practices like vegetative filter strips (VFS) for runoff mitigation, what are the important factors controlling or limiting their efficiency under different field settings? We evaluate the combination of the current ERA frameworks (US EPA PWC and EU FOCUS SWAN) in combination with VFSMOD, an established and commonly used numerical model for the analysis of runoff, sediment, and pesticide transport in VFS. We present a systematic study of the importance of different field conditions that have been proposed in the past as limiting the efficiency of VFS in realistic settings: flow concentration (channelization) through the filter, timing of pesticide application compared to other drivers, assumptions about the degradation and remobilization of pesticide trapped in the filter between runoff events, seasonal presence of a shallow water table near the receiving water body. We identify instances in which the importance commonly assigned to these factors is not supported by the mechanistic analysis, where other factors different than those proposed largely control the results of the assessments.</p>

Effect of Remobilization of Pesticide Residues in Vegetative Filter Strips for Mitigation in Higher-Tier Pesticide Exposure Assessments with VFSMOD

<p>Vegetative filter strips (VFS) are commonly implemented in the field to mitigate runoff pesticide inputs into surface waters and protect aquatic ecosystems. The efficiency of this mitigation practice can be evaluated within the current regulatory high-tier, long-term environmental risk assessments (ERA) in combination with VFSMOD, an established and commonly used numerical model for the analysis of runoff, sediment, and pesticide transport in VFS. For every rainfall/runoff event in the long-term time series, VFSMOD takes the PRZM calculated edge-of-the-field surface runoff, eroded sediment yield, and dissolved and particle-bound pesticide load.  It then calculates infiltration, sedimentation and pesticide trapping in the VFS during the event, and the outflow into the downslope aquatic body for further calculations and risk analysis. Importantly, at the end of each event, VFSMOD calculates the amount of pesticide residue retained in the filter (sediment-bound and infiltrated in the liquid phase), its degradation until the next event in the series, and the fraction of pesticide residue that is remobilized and added to the next runoff event. In earlier VFSMOD versions, full remobilization of the pesticide residue sorbed to sediment and that dissolved in the soil surface mixing layer (typically the top 0.5-5 cm) was calculated conservatively. Recent VFSMOD ERA applications for very highly-sorbed (i.e. pyrethroids) or persistent pesticides indicate that the full remobilization scheme might be too conservative in some cases. In this work, we evaluate new alternative partial remobilization schemes in VFSMOD, i.e. no remobilization of adsorbed residues, but full remobilization of dissolved residues in the mixing layer, or alternatively just a fraction of the mixing layer by diffusive exchange with the runoff. We evaluate the effects of the alternative remobilization schemes on observed total VFS pesticide reductions from available field data. In addition, employing global sensitivity analysis, we assess the relative importance of the alternative remobilization model structures in the context of the expected field variability of other known drivers of VFS efficiency (hydrology, soils, vegetation, pesticide chemical characteristics). The study provides science-based recommendations for future high-tier pesticide ERA with VFS mitigation.</p>

Counterfactual hydrological pesticide transport modelling: Can we detect long-term in-stream pesticide trends due to mitigation?

<p>In many countries, agroecological schemes are implemented in order to reduce water quality impairment from agricultural pesticide use. However, demonstrating the success or failure of these schemes is challenging because other influencing factors can confound their effects. For instance, in-stream pesticide concentrations have been found to vary greatly due to the interannual variability in weather conditions (e.g., the timing, intensity, and duration of precipitation events) and pesticide application practices (e.g., the variability in timing and spatial application of differing pesticides that have different chemical properties).</p><p>Our current work aims to investigate the necessary conditions to detect significant trends in pesticide concentrations in the context of the Swiss National Action Plan (NAP), which aims to halve the pesticide risk from agricultural activities within Swiss river networks by 2027. We use a modelling approach to explore possibilities and limitations of the existing monitoring scheme for separating long-term effects of the NAP from interannual variability due to weather conditions. For that purpose, we use an existing model for simulating pesticide transport at the catchment scale. After calibration, we simulated 10 years of herbicide concentrations with and without (i.e., the counterfactual) an assumed 50% reduction of the pesticide applied and evaluated the resulting concentration levels.</p><p>Our results indicate that the interannual variability due to weather conditions can exceed even a 50% change in pesticide application. This implies that the concentration levels themselves are insufficient to demonstrate the effectiveness of the NAP within a reasonable time horizon of a decade. This is because the lowering of in-stream pesticide concentrations may be due to the timing and intensity of precipitation relative to the application of pesticides and not from the effectiveness of pesticide mitigation measures. Therefore, we explore ways to account for the weather effects on the pesticide concentration levels. Furthermore, we found that comparing the pesticide concentrations in years that have both above average precipitation during pesticide application periods and contain precipitation events that occur shortly after pesticide application can lead to more robust statements about the effectiveness of the mitigation measures. Preliminary double mass analyses of cumulative rainfall during the application period versus cumulative maximum concentrations suggest that significant trends can be identified with 11 years of data (6 years before NAP implementation and 5 years into it). We are currently exploring how sensitive our results are to pesticide properties, such as sorption and degradation half-lives.</p>

Reactive transport modelling to assess pesticide dissipation at the sediment-water interface

<p>Rivers that are hydrologically connected to an agro-ecosystem act as a source or sink of pollutants transported by surface runoff and subsurface water. The Sediment-Water Interface (SWI) of rivers is a critical boundary for river dynamics where hydrological and biogeochemical processes tightly control pesticide dissipation. Transport processes govern pesticide transit time and distribution across the SWI depending on the water flow and hyporheic exchanges. Simultaneously, reactive processes such as sorption and biodegradation are responsible for retardation or actual degradation of pesticides within the porous sediment. However, knowledge on the interplay of these processes at the SWI remain sparse mostly because a physically-based generalized framework to model transport and reactivity at fluid-porous interfaces is still lacking. Here, we combine model development and laboratory experiments to investigate the effects of representative hydrological conditions on pesticide transport at the SWI.</p><p>An innovative discrete flow-transport model accounting for sorption was developed to consider the pure fluid layer (via Navier-Stokes model) and the porous medium (via Darcy-Brinkmann model). Advanced and appropriate numerical techniques are implemented to solve the coupled models (Navier-Stokes and Darcy-Brinkmann) without any interface conditions or empirical transfer functions. Conservative (NaCl) and non-conservative (Foron Blue 291 – sorptive) tracer experiments were performed within a 15 cm long and 10 cm deep recirculated river model (3 < equivalent length < 30 km) to characterize pesticide transport at the SWI. Configurations with the contaminant in the sediment (sediment as contaminant source) or contaminations from the overlying water (sediment as a sink) were tested. Simulated tracer concentrations fitted well to measured concentrations over a range of laminar flows representative of low Strahler order rivers (Re < 700, bulk velocities 10 <  U < 100 mm.s<sup>-1</sup>). In all flow conditions, the first few mm of sediment constituted the most dynamic layer, which was controlled by advective processes. In contrast, the tracer in deeper sediment layers undergone diffusive transport with lower exchange rates. Sorption was also observed to significantly increase residence time within the sediment and to slow down the progression of the tracer plume into the sediment. The times required to reach the bottom of the river model rose up to 12 times as compared with the non-sorptive tracer, indicating limited hyporheic exchanges with increasing sorption.</p><p>To account for biodegradation at the SWI, the model is further extended to include degradation kinetics and stable isotope fractionation of organic micropollutants. Changes of stable isotope ratios of the remaining, non-degraded pool of pollutants over time or across the sediment layer is used as a proxy of <em>in situ</em> biodegradation. Biodegradation is interpreted as a function of oxygen zonation within the sediment. This model is eventually tested against tracer experiments with caffeine, which is used here as a fast degrading anthropogenic micropollutant. Patterns of micropollutant dissipation at the SWI arising from these developments will be further extrapolated at river reaches within an agricultural catchment (Souffel catchment, France). Altogether, this study will help understanding how rivers influence pesticide transport, storage and degradation at the catchment scale.</p>