Temporal Patterns Of Glyphosate Leaching At a Loamy Agricultural Field In Denmark.

Glyphosate is a herbicide used worldwide for weed control in agriculture. Glyphosate is believed to pose a low risk for groundwater reservoirs due to its strong sorption to soil minerals and fast degradation. However, during the last ten years several studies have shown detections of glyphosate in drainage water and groundwater at concentrations exceeding the EU drinking water quality criterion of 0.1 µg L-1. This study presents a unique 12-year time series of monitoring results for glyphosate (applied five times during the period) and its degradation product (AMPA) at a cultivated, highly-structured loamy field (Estrup) located in southern Denmark. The Estrup field is part of the Danish Pesticide Leaching Assessment Program (PLAP), which monitors and evaluates pesticide leaching from five differently textured agricultural fields in Denmark. The study investigates the effect of successive glyphosate applications in combination with the effect of precipitation events, drain water runoff, soil water content at 25 cm soil depth, management, and particle leaching patterns, and compares this with monitored field-scale glyphosate and AMPA leaching to a tile drainage system. Preliminary findings indicate that there is an accumulation of glyphosate and AMPA in the soil after the successive applications of glyphosate, as the level of the peaking concentrations right after applications increases. Furthermore, large precipitation events with subsequent high drain water runoff together with management, especially plowing and harvesting, trigger the leaching of mobile colloids, glyphosate, and AMPA. As a likely consequence, glyphosate has occasionally been detected in groundwater monitoring wells installed at the location down to 3.5-4.5 m depth in concentrations exceeding the EU drinking water quality criterion.