Agrochemical transport in the field scale: the case study of a subsurface drainage system in the Kishon Basin, Israel

Non-point pollutants, such as fertilizers and pesticides, are transported in water and travel via complex hydrologic flowpaths, with each field being a diffuse source of agrochemicals. Although pollutant transport in tile drains has been investigated widely, most studies occurred in temperate zones, with insufficient focus on leaching timing. We investigate the leachate timing and specific pathways from the field to the stream, to better understand the unique transport dynamics in Eastern Mediterranean climates, in areas with extensive subsurface drainage systems. To improve basin management strategies, this study targets the knowledge gap regarding specific pollutant transport and timing, which results in inefficient policies to reduce water pollution.  In our investigation of two crop fields in the Kishon basin, Israel, the systems drain both soil water and high groundwater, providing an opportunity to examine water quality dynamics in multiple pathways. We collected water samples from field runoff, subsurface pipes, and groundwater during summer irrigation and winter storm events. Results show a clear spatial distribution of agrochemicals due to their properties. Higher number of pesticides were found in ponded field water and their concentrations were higher in order of magnitude in compare to tile drainage pipes. We identified pesticides in all samples that had not been applied to the field within the last 1.5 years.  Leaching timing was demonstrated with higher pesticide concentration appearing in water collected from drainage pipes during irrigation and pesticides concentration decreasing after irrigation ended. The concentration changes were observed within 12-15 hours after opening or closing irrigation. Tracking the pools, the high concentration in the top soil creates a pesticides reservoir transported with the onset of irrigation and rain.  The leaching timing was demonstrated as well by lab results and measuring in-situ EC and pH.  After a four-day storm, EC declined drastically, demonstrating the input of relatively low nutrient content water to the high concentrations present in the high water table. Later in the winter, surface runoff in the main draining trench from the field to the stream contained high concentrations of phosphate and sediments and low concentrations of nitrate and chloride, compared to surface runoff in the field. Since nitrate and chloride are highly soluble, the reduced concentration in the draining trench to the stream demonstrates that water percolates downwards through the soil column with solutes rather than propagating with surface runoff, and particulate-bound species and sediments travel via surface runoff to the stream. Demonstrating pollutant transport pathways and timing, we provide a window into the propagation mechanisms of pollutants at a small scale, to support decision making and improve basin management.