Seasonal transport of herbicides to surface water as measured on six agricultural fields under different corn-cropping regimes (St-Lawrence Lowlands)
The use of pesticides in agriculture may result in the degradation of surface water quality. Since agricultural practices affect the transport of pesticides, there is a need to identify practices which minimize the contribution of the different transport paths to the streams, i.e. runoff and drainage. The aim of this study was to evaluate at the field scale and for one growing season the transport of the herbicides atrazine and metolachlor to surface water under soil, climatic and agricultural conditions representative of those encountered for intensive corn cropping in the St-Lawrence Lowlands (Quebec). Six agricultural fields (Figure 1) were studied in 1995. Previous agricultural practices in 1994 and soil texture are summarized in Tables 1 and 2, respectively. Conventional practices (tillage with moldboard plow and application of herbicides over the entire area of the field) and conservation practices (no-till and banded application of herbicides over the seeded row) were studied. Each field was solely and entirely drained by one subsurface drain. The commercial formulation used in 1995 contained a mass of metolachlor two times higher than that for atrazine. Herbicide concentrations in runoff and drainage waters were monitored during the two first important rainfall events that occurred after herbicide application (Table 3). Sampled runoff corresponded to the water reaching a drainage channel or a stream. Drainage water was also collected for 3.5 - 4.5 months following the initial application. A total of 164 water samples was obtained. After sediment removal, metolachlor, atrazine and its dealkylated metabolite deethylatrazine (DEA) were extracted using a liquid-solid extraction procedure and analyzed by gas chromatography. Only two fields produced runoff and the concentrations of parent-compounds (Figures 2 and 3) were high and varied during rainfall events between 60-500 mg/L (Field 2) or 130-2400 mg/L (Field 6). Concentrations found during the first rainfall event were higher than those encountered during the second event. The DEA/atrazine concentration ratio (DAR) was below or near 0.1, indicating runoff of recently applied atrazine (low degradation). These two fields present similar soil texture, pluviometry and sampling periods after herbicide application. Based on runoff coefficients observed for other agricultural fields (1-30%), it was estimated that the mass losses for herbicides (Table 4) would be higher under conventional tillage(Field 6) as compared to no-till (Field 2). Significant transport of herbicides by drainage was observed during the two rainfall events. The losses of herbicides that occurred after these events and during a dry growing season (little or no drainage flow) were low. The drainage losses (concentration or masses) during the two rainfall events for Field 1 (clay) were very low. This was attributed to the low drainage capacity of the soil, to the low rainfall intensities as well as to the important delay between the initial application and the subsequent rainfalls. For silty clay loam to loam soils, the drainage flow increased in the 6-12 h period following the onset of rainfall, as did the herbicide concentrations. Metolachlor concentrations were slightly higher or close to those for atrazine: this was attributed to its possible more rapid decay and to its stronger tendency to adsorb to the soil. During the rainfall events, four fields exhibited herbicide concentrations from drainage less than 6 mg/L (mostly < 1-2 mg/L). Fields 2 and 6 yielded parent-compound concentrations as high as 40-60 mg/L (Figures 4 and 5). The DAR values found for drainage water of Field 2 (0.1-0.5) were higher than those observed from runoff, indicating significant dealkylation of atrazine had occurred during its transport in the unsaturated zone. Field 6 allowed the monitoring of the DAR over the growing season and an inverse relationship was found between the DAR and atrazine concentration (Figure 6). This was attributed to the larger variation in atrazine concentration during a rainfall as compared to that of DEA. A DAR value near 1 was obtained at 1-2 months after application, indicating important degradation of atrazine. The total mass losses of parent-compounds (two rainfall events) were evaluated (Table 5) except for Fields 2 and 4 which present frequent submerged drains. Banded herbicide application (Field 5) results in consistent lower losses of herbicide masses than those obtained for application over the entire surface (e.g. Field 5 compared to Fields3 and 6). It should be noted that the higher export observed for the entire surface application may be partly attributed to a shorter delay between application and rainfalls (Fields 3 and 6) or to a higher rainfall intensity (Field 6). Although runoff reaching surface waters was limited, it was estimated that the total herbicide losses (Table 4) during the two rainfall events were higher than those from drainage (Table 5). In the perspective of reducing the herbicide loads reaching streams, it appears that remedial actions should focus on this main route of transport. Thus, complementary actions such as vegetated buffer strips to intercept crop land runoff may possibly be useful to limit herbicide transfer to streams in intensive agricultural zones.