Persistence and Movement of Atrazine and Isoproturon in Agricultural Soil.

Physically based distributed models to predict persistence and movement of organic chemicals in soils have achieved only limited success because our understanding of many of the processes which govern chemical behaviour in soils is incomplete. The objectives of this study were to provide a better quantitative understanding of (1) sorption phenomena, (2) effect of soil physical properties on solute movement, (3) influence of soil solution chemistry on chemical behaviour and (4) importance of spatial and temporal heterogeneity of soil physico-chemical properties and agricultural practices on the persistence and movement of organic chemicals. The objectives were achieved using (i) laboratory batch equilibrium studies with atrazine and isoproturon on clay soil, (ii) studies on leaching of atrazine and isoproturon in a large (1.1 m x 0.8 m id.), undisturbed and heavily cracked clay soil core under controlled hydrological conditions and (iii) studies on the persistence and leaching of isoproturon in the field where a heavy textured clay soil was subject to different agricultural practices over the growing season of a winter barley crop. Sorption Kd's derived from batch 'equilibrium' studies for both atrazine and isoproturon by < 2 mm clay soil were approximately 3.5 litres / kg which was consistent with those determined for field studies. The similarity of Koc's for the < 2 mm clay soil and < 2 mm clay soil oxidised with hydrogen peroxide suggested that the sorption of isoproturon was strongly influenced by soil organic matter. By contrast, Koc's for atrazine sorption by oxidised soil were three times greater than those for < 2 mm soil indicating that the soil mineral components might have affected sorption of this herbicide. No significant differences between the sorption of either herbicide by < 2 mm clay soil and (i) < 250 microm clay soil, (ii) clay soil mixed with wheat straw or ash at ratios similar to those observed on plots 6 and 10 at the field site or (iii) < 2 mm clay soil in the presence of dissolved organic matter as opposed to organic free water, were observed. Field studies show that isoproturon was more persistent in the cultivated horizon of land where straw and stubble had been burnt than in soil where straw was incorporated. In the field experiment different mole drainage systems were found to affect the efficiency with which water was removed from the upper horizons but this had no effect on the total amount of isoproturon lost by leaching. This suggests that other processes including biodegradation, volatilisation or immobilisation of leaching chemicals in subsoils must be responsible for the difference in the isoproturon concentrations observed in the cultivated horizons where straw had been burnt or incorporated. Temporal variation of sorbed isoproturon residues, and associated sorption/desorption coefficients, were controlled by rainfall whilst spatial variation was found to be more strongly dependent on natural heterogeneity of soil physico-chemical properties than on agricultural practices. However, variation was both irregular and complex and could not be predicted in a simple way. This, taken with the complexity of the sorption/desorption mechanisms and the variability of water and solute flow pathways at the local scale suggests that the development of succesful physically based distributed models for the field scale remains unlikely. Whilst such models are unquestionably beneficial in contributing to our understanding of the behaviour of organic chemicals at the local scale, field scale management models can probably be more successful where relatively simpler conceptual approaches are adopted