Pesticides can be transported to ground water more rapidly through preferential flowpaths than would be predicted from their physico‐chemical properties. The leaching rates of the herbicides 2,4‐D, bromoxynil, clopyralid, dicamba, diclofop, MCPA, and mecoprop were compared in this study on plots filled after harvest (conventional till, CT) and those that were not (fall tillage operation omitted, NT). The soil‐incorporated herbicides triallate and trifluralin were applied to the CT plot only. Herbicide was applied immediately prior to a leaching irrigation for salt removal, which represents a “worst‐case” scenario for pesticide leaching. Direct evidence of preferential flow was obtained when the herbicides, with the exception of triallate and trifluralin, were detected in the first water reaching the tile drains. Although the nonincorporated herbicides were transported preferentially at the same rate, the amounts transported depended on the solubility and adsorption coefficient (Koc) of the herbicide. Only 0.01% of the application of the least soluble herbicide, diclofop, was transported, compared with 0.46% of the most soluble herbicide, dicamba. Preferential flow was only slightly reduced by the tillage pass. The amounts of herbicide transported to the tile drain, however, were substantially reduced on the CT plot. The tillage effect was greatest for the more soluble and less strongly absorbed herbicides. There was no clear relationship between amounts transported in the year after application and reported persistence but herbicides with the longer half‐lives persisted in relatively greater amounts than the other herbicides.

Pesticides and nutrients can be transported from treated agricultural land in irrigation runoff and thus can affect the quality of receiving waters. A 3-yr study was carried out to assess possible detrimental effects on the downstream water quality of the South Saskatchewan River due to herbicide and plant nutrient inputs via drainage water from an irrigation district. Automated water samplers and flow monitors were used to intensively sample the drainage water and to monitor daily flows in two major drainage ditches, which drained approximately 40% of the flood-irrigated land within the irrigation district. Over three years, there were no detectable inputs of ethalfluralin into the river and those of trifluralin were less than 0.002% of the amount applied to flood-irrigated fields. Inputs of MCPA, bromoxynil, dicamba and mecoprop were 0.06% or less of the amounts applied, whereas that for clopyralid was 0.31%. The relatively higher input (1.4%) of 2,4-D to the river was probably due its presence in the irrigation water. Corresponding inputs of P (as total P) and N (as nitrate plus ammonia) were 2.2 and 1.9% of applied fertilizer, respectively. Due to dilution of the drainage water in the river, maximum daily herbicide (with the exception of 2,4-D) and nutrient loadings to the river would not have resulted in significant concentration increases in the river water. There was no consistent remedial effect on herbicides entering the river due to passage of the drainage water through a natural wetland. In contrast, a considerable portion of the nutrients entering the river originated from the wetland.

Pesticide leaching is a potential problem on irrigated land in Saskatchewan. This study examined a worst case scenario for leaching of the herbicide, clopyralid. Herbicide concentrations were measured in water samples from suction lysimeters and tile drain effluent during and after a leaching irrigation to remove salts. Clopyralid was found throughout the soil profile within 20 d of application and was detected in the first water flowing in the tile drains. Approximately 1.5% of the applied clopyralid was lost in the tile drain effluent. The presence of higher concentrations of clopyralid in tile drain water samples at 2 m than in lysimeter water samples taken from 1.5 and 1.8 m was attributed to preferential transport. Another indication of preferential flow was the disparity between measured soil hydraulic characteristics and the timing of the arrival of water in the tile drains. Good agreement between observed and simulated clopyralid distributions was obtained using a bimodal flow scenario with the LEACHM model. It assumed that 40% of the applied water moved through preferential flowpaths with saturated hydraulic conductivities two orders of magnitude greater than the bulk soil. Chemical moving with this water did not penetrate the soil matrix and was not accessed by suction lysimeters. The remaining 60% of applied water moved through the soil matrix according to measured hydraulic characteristics. The modeling approach described herbicide concentrations in the soil matrix and preferential flow from a large field integrated by tile drainage but would not be applicable in soil column studies or small plots.

This paper focuses on sub-theme one: An Enabling Policy Environment for Water, Food and Energy Security. It discusses water food and energy (WFE) security and their interrelations as the background for policy discourse and introduces the WFE nexus and its quantification by modelling as one of the tools for developing a broader approach to resources management.

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