Investigation of Pesticides in Groundwater at Three Irrigated Sites near Outlook, Saskatchewan

Abstract During 1987 and 1988, the pesticides 2,4-D, diclofop-methyl, dicamba, MCPA, triallate and bromoxynil were detected in shallow groundwaters beneath three irrigated research sites in the Outlook Irrigation District of Saskatchewan. All of the pesticide detections were below the limits set by the Canadian Drinking Water Quality Guidelines. Meteorological and hydrogeological conditions are generally not conducive to pesticide leaching to the water table in this region. However, it is suspected that the pesticides applied at the surface were able to migrate to the water table through the numerous vertical fractures that exist in the tills and silts underlying the fields. When combined with irrigation that is applied at these sites, these two factors create a mechanism for the rapid movement of pesticide to a greater depth of leaching than would occur in unfractured soils and with typical precipitation. Further, it is suspected that storage of pesticide in the soil matrix and subsequent desorption during later intense recharge or irrigation events is responsible for observed pesticide detections weeks or months after a pesticide was applied.