SEASONAL PATTERNS OF ATRAZINE LOADING FOR THE ST-LAWRENCE-RIVER (CANADA) AND ITS TRIBUTARIES

The annual variability in atrazine concentrations and loadings have be en studied for the St Lawrence River (Canada) and four of its tributar ies. Hydrological parameters appear to be the principal controls on th e loadings of atrazine in the St Lawrence River system throughout the year, except during the post-application period. In terms of the annua l variability of loadings, prior to the application period, the spring freshet coincided with a major pulse of atrazine. A post-application peak load of atrazine was detected in the main river as well as in its tributaries. During the fall, atrazine loadings increased again. The results show that atrazine inputs to the system can be detected long a fter application. Average loadings in 1990 and 1991 were 7.7 and 4.4 k g/d in the St Lawrences River, and 0.4 and 0.2 kg/d in the tributaries studied. The seasonal variation of atrazine concentrations in surface waters of the tributaries was consistent with the temporal distributi on reported in the literature, viz. peak concentrations during the pos t-application period. There was also an increase in concentrations in the main river channel during the fall, reflecting the fact that the m ain river also receives inputs from the Great Lakes and not only from proximate agricultural sources. The annual variability in atrazine con centration and loading in the tributaries was directly related to the proximity of agricultural sources. Average concentrations in 1990 and 1991 were 10.4 and 3.4 ng/l in the St Lawrence River, and 31.1 and 27. 9 ng/l in the tributaries. The persistence of atrazine is believed to be a key element controlling its dynamics in these aquatic systems. Th e results demonstrate the widespread dispersion of atrazine in this ri ver basin, with similar responses and seasonal trends being exhibited throughout.