Pesticides in the Nelson River watershed, 2006 to 2011

The Nelson River watershed is the second largest basin in Canada by population (5,492,879 inhabitants in 2011) and has a surface area about 987,000 km² among which 472,000 km² were used for agricultural purposes in 2006. This territory includes 70% of the land dedicated to agriculture in Canada. Due to the vast expanse of cultivated land, more pesticides are used in the Prairies than in any other region in Canada. Aquatic ecosystems are therefore becoming vulnerable to pesticide contamination.

Carrot River

Between 2006 and 2011, water samples from the Carrot River sampling station were analyzed for 47 pesticides, and 17 pesticides were detected.

This table shows the number of samples analysed, the number of samples with pesticide detected, the detection frequency between samples, the minimum, maximum and median concentrations and Canadian Water Quality Guidelines (column headings) for clopyralid, 2,4-D and MCPA (row headings).

* = Limit of detection

Of the 17 pesticides detected, clopyralid is the most prevalent, followed by 2,4-D and MCPA. The Canadian Water Quality Guidelines indicate a threshold concentration of 2600 ng/L for MCPA and 4000 ng/L for 2,4-D; however, there is no threshold concentration for clopyralid thus far. The maximum measured concentrations are much lower than guidelines.

The results show variations in concentrations from one year to the next, particularly for clopyralid and 2,4-D. These variations were partly influenced by the area’s hydrology (e.g., water flow). In general, an increase in flow can cause an increase in concentrations. For example, in 2006, the average flow was 44 m³/s, nearly four times greater than in 2009, when the lowest concentrations of pesticides were noted.

The 2011 seasonal variation results reveal that maximum concentrations were observed in July, i.e. in the summer when pesticide use is more intensive. It should be noted that the concentrations measured are below the guidelines established by the Canadian Council of Ministers of the Environment to protect aquatic life.

Assiniboine River

Between 2006 and 2011, water samples from the Assiniboine River sampling station were analyzed for 46 pesticides, and 19 pesticides were detected.

This table shows the number of samples analysed, the number of samples with pesticide detected, the detection frequency between samples, the minimum, maximum and median concentrations and Canadian Water Quality Guidelines (column headings) for clopyralid, 2,4-D and MCPA (row headings).

* = Limit of detection

Of the 19 pesticides detected, 2,4-D was most prevalent, followed by clopyralid and MCPA. The Canadian Water Quality Guidelines indicate a threshold concentration of 2,600 ng/L for MCPA and 4,000 ng/L for 2,4-D; however, there is no threshold concentration for clopyralid thus far. The maximal measured concentrations are much lower than guidelines.

The results show variations in concentrations from one year to the next, particularly for 2,4-D. These variations were partly influenced by the area’s hydrology (e.g., water flow). In general, an increase in flow can cause an increase in concentrations. For example, in 2011, the average flow was 43 m³/s, that is, six times greater than in 2008, when the lowest concentrations of pesticides were noted.

The 2011 seasonal variation results reveal that measured concentrations increased in summer, when pesticide use is more intensive. It should be noted that the concentrations measured are below the guidelines established by the Canadian Council of Ministers of the Environment to protect aquatic life.

Red River

Between 2006 and 2011, water samples from the Red River sampling station were analyzed for 47 pesticides, and 20 pesticides were detected.

This table shows the number of samples analysed, the number of samples with pesticide detected, the detection frequency between samples, the minimum, maximum and median concentrations and Canadian Water Quality Guidelines (column headings) for 2,4-D, clopyralid, dicamba, MCPA, MCPP, and metolachlor (row headings).

* = Limit of detection

Of the 20 pesticides detected, 2,4-D and clopyralid were the most prevalent, followed by Dicamba and MCPA. The Canadian Water Quality Guidelines indicate a threshold concentration of 2,600 ng/L for MCPA and 10,000 ng/L for dicamba; however, there is no threshold concentration for clopyralid, MCPP or metolachlor thus far. The maximal measured concentrations are much lower than guidelines.

The results show concentration variations from one year to the next, particularly for clopyralid. These variations are partly influenced by the area’s hydrology (e.g., water flow). In general, an increase in flow can cause an increase in concentrations. For example, in 2007, the average flow was 44 m³/s, that is, three times greater than in 2008, when a decrease in pesticide concentration was noted.

The 2011 seasonal variation results reveal that measured concentrations increased in summer, when pesticide use is more intensive. It should be noted that the concentrations measured are below the guidelines established by the Canadian Council of Ministers of the Environment to protect aquatic life.

In summary,

• clopyralid, 2,4-D and MCPA are some of the most frequently detected pesticides and this, in the three aquatic ecosystems sampled;
• Red River shows the greatest number of frequently detected pesticides;
• pesticide concentrations vary from one year to the next, generally in correlation with the flow of water;
• pesticide concentrations vary from one season to the next, while being highest during the summer.