Seasonal variation of DDT and its metabolites in the St Lawrence River (Canada) and four of its tributaries

Water samples were collected in the St. Lawrence River and at the mouths of four major tributaries, the Richelieu, the Yamaska, the St. Francois and the Nicolet, between August 1990 and November 1991. Analyses were done for p,p'-DDT, o,p'-DDT, p,p'-DDE, p,p'-DDD and o,p'-DDD. In the St. Lawrence River, the highest mean ((x) over bar = 3.02 ng/l) was measured in April, during the period of spring thaw, and concentrations subsequently decreased. The seasonal patterns in the tributaries vary. In April, the two most likely sources of DDT in the St. Lawrence River art the release of atmospherically-derived DDT from snowpack, and inputs from the Great Lakes. In the tributaries, the increase in concentration in April could be attributed to the melting of snowpack, and also to the runoff of contaminated soil particles from the watersheds. Ratios of p,p'-DDT/p,p'-DDE + p,p'-DDD in the tributaries decreased from April to September, indicating an annual cycle of time and temperature dependence on degradation. The same trend was not detected in the St. Lawrence River, which may be attributed to the recent use of dicofol containing DDT as an impurity in the Great Lakes Basin which would 'unbalance' the ratios of p,p'-DDT/transformed DDT. The difference between the ratios of p,p'-DDT/Sigma DDT in marine mammals and in the river suggests that metabolic transformation of p,p'-DDT is occurring faster than the degradation in the aquatic environment and that there are still continuous inputs of p,p'-DDT in the St. Lawrence River Basin.