Spatial variability of herbicide mobilisation and transport at catchment scale: insights from a field experiment

Abstract

During rain events, herbicides can be transported from their point of application to surface waters, where they may harm aquatic organisms. Since the spatial pattern of mobilisation and transport is heterogeneous, the contributions of different fields to the herbicide load in the stream may vary considerably within one catchment. Therefore, the prediction of contributing areas could help to target mitigation measures efficiently to those locations where they reduce herbicide pollution the most. <br><br> Such spatial predictions require sufficient insight into the underlying transport processes. To improve the understanding of the process chain of herbicide mobilisation on the field and the subsequent transport through the catchment to the stream, we performed a controlled herbicide application on corn fields in a small agricultural catchment (ca. 1 km<sup>2</sup>) with intensive crop production in the Swiss Plateau. Water samples were collected at different locations in the catchment (overland flow, tile drains and open channel) for two months after application in 2009, with a high temporal resolution during rain events. We also analysed soil samples from the experimental fields and measured discharge, groundwater level, soil moisture and the occurrence of overland flow at several locations. Several rain events with varying intensities and magnitudes occurred during the study period. Overland flow and erosion were frequently observed in the entire catchment. Infiltration excess and saturation excess overland flow were both observed. However, the main herbicide loss event was dominated by infiltration excess. <br><br> Despite the frequent and wide-spread occurrence of overland flow, most of this water did not reach the channel directly, but was retained in small depressions in the catchment. From there, it reached the stream via macropores and tile drains. Manholes of the drainage system and storm drains for road and farmyard runoff acted as additional shortcuts to the stream. <br><br> Although fast flow processes such as overland and macropore flow reduce the influence of the herbicide's chemical properties on transport due to short travel times, sorption properties influenced the herbicide transfer from ponding overland flow to tile drains (macropore flow). However, no influence of sorption was observed during the mobilisation of the herbicides from soil to overland flow. These observations on the role of herbicide properties contradict previous findings to some degree. Furthermore, they demonstrate that valuable insight can be gained by making spatially detailed observations along the flow paths

A model-based assessment of the potential use of compound-specific stable isotope analysis in river monitoring of diffuse pesticide pollution

“…Inflow of water and pesticides from upstream parts was not considered in this model. The hillslope is convex with an average gradient of 5 %, which is comparable to the average slope in previously studied agricultural catchments (Doppler et al, 2012;Leu et al, 2004a). A river bank was incorporated as a vertical drop of 2 m over the last 5 m in the x direction, which increased the average slope to 5.8 %.…”

“…Surface runoff in scenario 2 occurred as infiltration excess overland flow, which can be an important mechanism of pesticide transport from agricultural land (Doppler et al, 2012). The infiltration excess overland flow was generated by increasing the coupling length from 0.1 m to 0.8 m, which decreased the coupling between overland flow domain and subsurface domain.…”

Implications of hydrologic connectivity between hillslopes and riparian zones on streamflow composition

“…Thus, long-term, seasonal or event-based monitoring of river water quality (i.e., solutes used as environmental tracers) provides valuable information about the source zones of catchment streamflow (e.g., quickflow, baseflow) and groundwater-surface water interaction processes (e.g., Cook and Herczeg, 2000;Leibundgut and Seibert, 2011). In association with agricultural land use, various studies found a rapid and significant increase of nutrient export rates when potential source areas become hydrologically connected to the river system during rainfall events (e.g., Doppler et al, 2012;Ocampo et al, 2006;Stieglitz et al, 2003).…”

Modelling biocide and herbicide concentrations in catchments of the Rhine basin

“…The application of S-metolachlor and terbuthylazine in Ossingen, for example, took place just before an intensive precipitation event. Through direct shortcuts, such as manholes of drainage systems and storm drains, the transfer to the river was accelerated and very high concentrations were measured (Doppler et al, 2012).…”

“…Such studies are rare and we used the following studies situated in the north-eastern part of Switzerland. The sampling campaigns from Gomides Freitas et al (2008) and Doppler et al (2012) measured herbicide concentrations at the small-scale agricultural catchments Summerau and Ossingen, respectively, after a controlled herbicide application. Wittmer et al (2010) monitored the mass and dates of herbicide applications in a slightly larger catchment, Mönchaltorf (25 km 2 ), with mixed land use.…”