Variability of Herbicide Losses from 13 Fields to Surface Water within a Small Catchment after a Controlled Herbicide Application

Abstract

Diffuse losses from agricultural fields are a major input source for herbicides in surface waters. In this and in a companion paper, we present the results of a comprehensive field study aimed at assessing the overall loss dynamics of three model herbicides (i.e., atrazine, dimethenamid, and metolachlor) from a small agricultural catchment (2.1 km2) and evaluating the relative contributions of various fields having different soil and topographical characteristics. An identical mixture of the three model herbicides as well as an additional pesticide for identification of a given field were applied within 12 h on 13 cornfields (total area approximately 12 ha), thus ensuring that the herbicides were exposed to identical meteorological conditions. After the simultaneous application, the concentrations of the compounds were monitored in the soils and at the outlets of three subcatchments containing between 4 and 5 cornfields each. Particular emphasis was placed on the two rain events that led to the major losses of the herbicides. The rank orders of herbicide dissipation in the soils and of the compound-specific mobilization into runoff were the same in all three subcatchments and were independent of the field characteristics. In contrast, the field properties caused the relative losses from two subcatchments to differ by up to a factor of 56 during the most important event, whereas compound-specific differences of the three neutral herbicides caused the losses to vary only by a factor of 2 during the same event. The enormous spatial variability was mainly caused by factors influencing the fraction of rain that was lost to surface water by fast transport mechanisms. Thus, the key factors determining the spatially variable herbicide losses were the permeability of the soils, the topography, and the location of subsurface drainage systems. These results illustrate the large potential to reduce herbicide losses by avoiding application on risk areas.

Modelling biocide and herbicide concentrations in catchments of the Rhine basin

“…Mechanistically this can be explained by the occurrence of fast transport processes (with high herbicide concentrations) such as surface runoff and fast subsurface flow through drainage systems or macropores (Leu et al, 2004a) during discharge events. Hence the concentration (C, g m −3 ) in the river is described -in a first approximation -as proportional to the discharge Q(t) (m 3 d −1 ) in the case of a recent application on the fields; the load (g d −1 ) increases quadratically with discharge:…”

“…Certain herbicides are present in significant concentration outside of the application period too (see for example, Leu et al, 2004a). Therefore, we added a constant background concentration (C back , g m −3 ) to the substance transfer model.…”

Modelling biocide and herbicide concentrations in catchments of the Rhine basin

“…Mechanistically this can be explained by the occurrence of fast transport processes (with high herbicide concentrations) such as surface runoff and fast subsurface flow through drainage systems or macropores (Leu et al, 2004a) during discharge events. Hence the concentration (C [g m -3 ]) in the river is described -in a first approximation -as …”

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

“…Drainage systems, preferential flow (e.g. in earthworm burrows and cracks) and subsurface storm flow can thereupon cause fast transport to surface water (Gavrilescu, 2005;Müller et al, 2003;Leu et al, 2004b). In contrast, pesticide leaching to groundwater represents a slow subsurface transport mechanism (Holvoet et al, 2007;Flury, 1996).…”