The presence and cycling of pesticides in the ecosphere

Abstract

Widespread pesticide application over the last generation has been one of the principal causes for the immense improvement witnessed in the fields of food and fiber production and in human health. This improvement, however, has also been a cause of alarm to many (especially scientists) because of the accumulated evidence of the possible long term hazards of pesticides to man's living environment due to contamination and their presence in a large part of the ecosphere. When pesticides are applied to plants and soil, small portions may drift in the atmosphere and contaminate very remote areas. Part of the pesticide may be photochemically degraded before it reaches the plants and soils. Part of the pesticide reaching the plant soil system will be lost to the atmosphere by evaporation and codistillation, part will be photochemically degraded, and the rest will enter plants and soil and will be either degraded there or persist almost indefinitely. Persistence will depend on the chemical structure of the pesticide itself and on a multitude of interacting environmental factors. The fates of pesticides entering the plant tissue may be, first, immediate metabolic degradation, second, they may become a part of the soil system after plant residue incorporation and burning, and third, they may enter the food chain and contaminate remote areas. Burning may also contaminate the atmosphere. The pesticide which has found its way into the soil by direct application, by plant incorporation and burning, and by accidental atmospheric drift may be partly taken up by plant roots or be degraded biochemically by soil microorganisms or even chemically. Another part may be lost through volatilization and through transport by water and wind. At any rate, biodegradation in the soil is the major cause of pesticide disappearance from the ecosphere and follows similar pathways whether it is effected by microorganisms, plants, or animals. Water transport of pesticides is chiefly through surface runoff and it may cause contamination of lower lying areas and of water bodies. Water contamination may also occur from accidental spills, industrial waste discharges, purposeful direct application, sewage discharges, air drift, etc. Once a pesticide enters a water body it may be volatilized, remain in the water in solution or suspension, or precipitate as sediment.