Current Pesticide Research: Findings

Research Summary

From 1993 to 1996, the temporal trends of three phenoxy- herbicides (2,4-D, dichlorprop and MCPA) and one phenolic herbicide (bromoxynil) were determined for the South Tobacco Creek watershed. These were examined in the surface water and in the air and rainfall during this period. Additional calculations were also conducted to determine the relative importance of each of the atmospheric pathways of pesticides to the South Tobacco Creek system.

Key Findings
1) Elevated levels of all four herbicides examined were discovered in spring snowmelt runoff, but reached peak levels during local crop application activities.

2) All of the herbicide residues were present in concentrations much below the Canadian water quality guideline values for the protection of freshwater aquatic life.

3) Although surface runoff has traditionally been considered the major route of pesticide entry into surface waters, atmospheric sources are also important. Current-use herbicides such as phenoxyacids were present in surface water at elevated levels during periods when air and rainfall concentrations were at their maximum. During the 1994-96 field seasons, the elevated levels in water, air and precipitation all coincided, even when there was no overland runoff, leading to the conclusion that the elevated levels in the water were not related to overland runoff, but corresponded to elevated levels in the atmosphere.

The level of concentration of phenoxyacid herbicides in South Tobacco Creek water follows the concentration patterns in precipitation over the watershed more closely than air concentration patterns. However, the concentration of bromoxynil in the creek water corresponded to the levels in both the air and precipitation.

4) Elevated levels of herbicides were most evident in spring runoff when no significant runoff occurred late in the previous season. Late fall rains have a tendency to "flush" the system. If they don't occur, then elevated herbicide residues occur during the snow melt runoff. 

5) In all years except 1995, low concentrations of MCPA continued to be detected in the air during periods when there was no local use of this chemical. This presence may be the result of regional use, since local application of MCPA had been completed.

6) Atrazine was detected in the air, rainfall and creek water during the study even though it was not applied within the watershed. This presence was associated with the movement of air masses up from the American Midwest where atrazine is widely applied to cornfields for weed control.
 
 

Field Runoff Sampling at the West Weir.