Pesticide use has been credited as one of the major contributors to modern intensive agricultural production. The discovery of pesticides that could control insects, plant diseases and weeds in agricultural and horticultural crops and pastures dramatically changed plant production systems throughout the world. The use of pesticides has been emphasised by the growing world-wide demand for food and fibre. Economic return to the farming community from the use of pesticides varies between $ 3 and 4 for every $ 1 spent.

Increased use of pesticides in agriculture/horticulture is causing great concern with regards to surface water and groundwater contamination. Some pesticides are now recognised as posing a threat to groundwater quality. Pesticide residues have been detected in groundwater from a number of countries under conditions where agricultural use could be identified as the major source of contamination. This has received considerable public and scientific interest, primarily due to concern about the potential effects of these pesticides on human health and ecosystem functioning. Based on this evidence a range of pesticide compounds have now been banned.

Since most concern is associated with the presence of pesticides in the soil environment, it is essential that the processes affecting the fate of pesticides in the soil, especially in the unsaturated zone be understood. The main transformation processes affecting the efficiency, and the ultimate fate of pesticides in the soil include retention (sorption-desorption), degradation (biological and chemical) and transport into the soil, atmosphere and surface waters.

Some of major issues with the study of pesticide interactions in soils include the cost of pesticide analysis and the lack of analytical facilities. Isotopic labelled pesticide compounds are commonly used to examine the fate of pesticides in soils. Following a brief overview of the sources of pesticides in soil and aquatic environments, this paper provides a synthesis of various published data on the transformation and transport of pesticides in soils. Selected case studies involving laboratory, glasshouse and field experiments have been used to examine the fate of ¹⁴C labeled pesticides in the unsaturated zone of variable charge soils. These case studies include sorption and desorption of pesticides by runoff sediments, sorption and desorption of pesticides as affected by soil depth, and the impact of sorption on degradation and leaching of pesticides.

Keywords: isotopic techniques, ¹⁴C labeled pesticide, pesticide transformation, environment.