Polychlorinated biphenyls in a rural watershed in the southern prairie region of Canada

Polychlorinated biphenyl (PCB) concentration patterns were determined in air, precipitation and surface water in a small agricultural watershed (70 km2) in the Canadian prairies, with no known local source of these compounds. Air temperature was related to PCB concentrations for lower chlorinated congeners and air mass back-trajectories indicated that elevated levels in ambient air were observed when air moved into the region from the south. Dissolved phase ΣPCB concentrations ranged from 20 to 6350 pg/liter, air concentrations ranged from 1.4 to 751 pg/m3 and concentrations in precipitation ranged from 88 to 9703 pg/liter. The stagnant layer (two-film) model and the surface renewal models were used to estimate PCB volatilization during both 1994 and 1995. The surface renewal model predicted greater volatilization losses than the two-film model, although the surface renewal model has been shown to under-predict gas exchange in small turbulent streams. This suggests that for a small stream, the two-film model drastically under-predicts gas exchange estimates. There was overall net deposition of PCBs to the South Tobacco Creek when precipitation and dry deposition inputs to the entire watershed were included. Similar levels of PCBs are exported from this watershed by both volatilization and stream discharge.