SPATIAL AND TEMPORAL VARIATION OF ISOPROTURON RESIDUES AND ASSOCIATEDSORPTION DESORPTION PARAMETERS AT THE FIELD-SCALE/

Spatial and temporal variation of isoproturon (IPU) residues (IPUr), a nd their sorption (K-d) and desorption (K-app) coefficients were monit ored at the field scale in the cultivated horizon of a clay soil follo wing pre- and post-emergent IPU applications to a winter barley crop. The half life in 25 subplots ranged from 31 to 483 days with a median value of 90 days. K-app increased as IPUr declined, ranging from a med ian value of 9.9 L/kg, 27 days after the pre-emergent application (aut umn 1991) to 77.8 L/kg four days prior to the post-emergent applicatio n (spring of 1992). K-d's were more consistent, with median values ran ging from 3.7 to 6.4 L/kg over the same period. However, median K-d's of 7.1 and 8.7 L/kg in May and June 1992 respectively would suggest th at increases may occur during the summer months when temperatures incr ease and the soil dries. Sorption nonideality (K-d/K-app) was observed on each sampling occasion, ranging from approximately 0.5, 27 days af ter the pre-emergent herbicide application, to approximately 0.2 four days prior to the post-emergent application. All parameters were posit ively skewed. Variation of all parameters at the local scale (i.e. wit hin 1 m(2)) was less than that at the field scale, but greater than th at associated with the experimental methods employed indicating that b oth intrinsic and extrinsic factors influenced the distribution of IPU r, K-d and K-app Variability of IPUr and K-d on individual sampling da tes was consistent with coefficients of variation generally ranging fr om 26 to 44% and from 18 to 44% respectively. Variability of K-app was more erratic, increasing to over 200% and 100% following two major ra instorm events in November 1991 and February 1992 repectively. Normali zation of K-d's on a soil organic matter basis failed to reduce the va riability observed. The implications of these results for contaminant transport and fate modelling are discussed. Copyright (C) 1996 Elsevie r Science Ltd