Pathways of pesticide spread in the environment

From field to stream or groundwater

Swedish studies have shown that around 0.01-1% of the total amount of pesticides used in an area can be spread to neighbouring streams via surface run-off and leaching.

Surface run-off. Pesticides can accompany water running off the surface of the soil to the nearest drainage channel or water course. During heavy rain the soil can become saturated and rainwater then runs off the surface, carrying with it soluble substances and soil particles. The shorter the time between spraying and heavy rain, the greater the risk of the pesticide ending up in streams and rivers. In addition, the risk of surface run-off is related to the soil type in the sprayed field. Structurally weak silty and fine sandy soils have a greater risk of surface run-off than clay soils with a stable structure or coarse sands with high permeability. The risk of surface run-off decreases if the banks of streams and ditches adjoining the field are covered in vegetation. International studies have shown that surface run-off alone is often responsible for losses comprising several percent of the total amount of pesticides used in an area.

Leaching. The rain or irrigation water that cannot be stored in the soil is transported downwards in the soil profile and ends up in drainage pipes or is carried onwards to the groundwater. Pesticides dissolved in the soil solution are carried with the moving water. The risk of drainage losses is greatest in soils where the water can percolate easily. These include e.g. sandy soils and fractured clay soils.

Through surface run-off and leaching to drainage pipes, pesticides can be carried out into streams and rivers. They can also by leaching through the soil end up in the  groundwater.