Pesticides are Entering the St. Lawrence River through Its Tributaries

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Pesticides are Entering the St. Lawrence River through Its Tributaries

A number of pesticides are present in the water of the St. Lawrence River near Quebec City and at the mouths of some tributaries of Lake Saint-Pierre. This is what scientists at Environment Canada observed in 2003 and 2004 while doing research as part of the project on the Presence of Pesticides in the St. Lawrence River and its Tributaries, in collaboration with the Ministère de l’Environnement du Québec and the Centre d’expertise en analyse environnementale du Québec.

Water sampling in 2003 and 2004 at the mouths of the Yamaska, Saint-François and
Nicolet rivers and in the St. Lawrence River

Herbicide, insecticide, fungicide… are they all pesticides?

A pesticide is a generic term for a substance or chemical product capable of controlling, attracting, repelling or destroying living organisms (microbial, animal or plant) that are considered a nuisance or to prevent their development (Wikipedia 2004). These products are distinguished by their specific action to control plants (herbicides), insects (insecticides) or microscopic fungi (fungicides).

The St. Lawrence River, especially Lake Saint-Pierre, is vulnerable to contamination by pesticides because several tributaries draining farming areas empty into it. This is the case for the Nicolet, Saint-François and Yamaska rivers, situated south of Lake Saint-Pierre, where land is used for large-scale agricultural production. Of the three, the Yamaska River basin has the highest proportion of farmland (52% of its area).

Thirteen different pesticides have been detected at the mouth of the Yamaska River, the highest among all the sampling sites. A total of 54 pesticides were analysed and the results are shown below.

**Pesticide and detection frequency in sampled rivers**
	Percentage of samples containing pesticides
	Yamaska	Nicolet	Saint- François	St. Lawrence
Herbicides
2,4-D	< 10	< 10	< 10	–
2,4-DB	–	11–30	–	–
2,4,5-T	–	–	–	–
Atrazine • Deethyl-atrazine•Deisopropyl-atrazin	> 30 > 30 11–30	> 30 < 10 11–30	> 30 – –	> 30 > 30 < 10
Bentazone	> 30	11–30	–	< 10
Bromoxynil	11–30	–	11–30	–
Butylate	–	–	–	–
Clopyralid	11–30	–	–	–
Cyanazine	–	–	–	11–30
Dicamba	> 30	> 30	11–30	< 10
Dichlorprop	–	–	–	–
Diclofop-methyl	–	–	–	–
Dimethenamid	> 30	–	–	11–30
Dinoseb	–	–	–	–
EPTC	–	–	–	–
Fenoprop	–	–	–	–
MCPA	11–30	11–30	11–30	11–30
MCPB	–	–	–	–
Mecoprop	> 30	11–30	11–30	11–30
Metolachlor	> 30	> 30	> 30	> 30
Metribuzin	11–30	–	–	–
Picloram	–	–	–	–
Simazine	11–30	11–30	11–30	> 30
Triclopyr	–	–	–	–
Trifluralin	–	–	–	–
Fungicides
Chlorothalonil	11–30	–	11–30	–
Myclobutanil	–	–	11–30	–
Insecticides
Azinphos-methyl	–	–	–	–
Bendiocarb	–	–	–	–
Carbaryl • 1-naphthol**	– –	– –	– –	– –
Carbofuran	–	–	–	–
Chlorfenvinphos	–	–	–	–
Chloroxuron	–	–	–	–
Chlorpyrifos	–	–	11–30	–
Diazinon	–	–	–	–
Dichlorvos	–	–	–	–
Dimethoate	11–30	11–30	–	11–30
Disulfoton	–	–	11–30	–
Diuron	–	–	–	–
Fenitrothion	–	–	–	–
Flumetsulam	< 10	–	–	–
Fonofos	–	–	–	–
Imazethapyr	11–30	–	–	–
Linuron	–	–	–	–
Malathion	–	–	–	–
Methidathion	–	–	–	–
Methyl-parathion	–	–	–	–
Mevinphos	–	–	–	–
Nicosulfuron	11–30	–	–	–
Parathion	–	–	–	–
Phorate	–	–	–	–
Phosalone	–	–	–	–
Rimsulfuron	–	–	–	–
Tebuthiuron	–	–	–	–
Terbufos	–	–	–	–
* Atrazine metabolites. ** Carbaryl metabolite.

Of the twenty-two pesticides detected in the St. Lawrence near Quebec City and at the mouths of the three tributaries, atrazine and metolachlor remain the most frequently detected (in 85% and 68% of samples analysed in 2003, respectively) and at the highest concentration.

The degree of water contamination depends on the physical and chemical properties of the pesticide, the quantity applied, the basin hydrology, and the prevailing weather conditions around the time of spraying.

Atrazine concentrations in the Yamaska River exceeded the quality criteria for the protection of aquatic life in 7% of samples, while dicamba exceeded the quality criteria for agricultural irrigation in 78% of samples. In the Saint-François River, chlorpyrifos concentrations exceeded the quality criteria for the protection of aquatic life in 17% of samples and chlorotalonil in 7% of samples. Maximum pesticide concentrations in the aquatic environment are generally measured in the month following product application or during heavy rainfalls (10 mm or more).

While the input of pesticides entering from the sampled tributaries (< 6.5% of flows measured at Quebec City) appears negligible compared to the quantity of pesticides exiting the Great Lakes (see Lake Ontario: The Main Source of Herbicides in the St. Lawrence), the Yamaska River, whose daily inputs of atrazine can reach 27.7 kg, is likely to contribute significantly to the contamination of the water in some stretches of the St. Lawrence, including Lake Saint-Pierre.

In 2000, Lake Saint-Pierre was declared a World Biosphere Reserve by UNESCO shortly after its designation as a Ramsar site in 1998 under the Convention on Wetlands of International Importance. Characterized by a remarkable biological diversity, this unique ecosystem is located in a rural area where farming activities are likely to threaten its ecological integrity. Environment Canada continues to work toward enhancing our knowledge of the presence and the fate of pesticides in tributaries of Lake Saint-Pierre, and at the mouth of the St. Lawrence River at Quebec City.

St. Lawrence Info
Lake Ontario: The Main Source of Herbicides in the St. Lawrence River

Water Quality Monitoring in the Fluvial Section: Toxic Substances

Monitoring the Water Quality in the Fluvial Section: Physico-chemical and Bacteriological Parameters

Ministère de l’Environnement du Québec – Pesticides Management Code

Ministère de l’Environnement du Québec – Répertoire des principaux pesticides utilisés au Québec (in French only)

Centre d’expertise en analyse environnementale du Québec

Did You Know?

The exposure of frogs to mixtures of pesticides, especially atrazine, can compromise their immune response and make them vulnerable to parasitic infection. See the project on the Impact of Agricultural Pesticides on the Health of Amphibian Populations.

Atrazine: A Popular Choice

Atrazine (C₈H₁₄ClN₅) is a systemic herbicide that is used primarily to control grassy and broadleaf weeds in cornfields, rapeseed and lowbush blueberry, and as a general weed killer (uncultivated land and industrial zones). Widely used for a number of years, this product was introduced to Canada in 1960. Environmental concerns have reduced its use by half since the 1980s. Atrazine is still the most frequently detected pesticide in surface waterbodies in summer. At varying concentrations, it can be toxic to fish, freshwater invertebrates and aquatic plants.

*Links to sites external to Environment Canada (EC) are provided as a convenience and their inclusion in no way implies that Environment Canada endorses or accepts any responsibility for the content or use of these sites. As the organizations that maintain these sites may not be subject to the Official Languages Act, information found on these sites may be presented only in the language in which it was written.

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Date Published: 2004-12-06
Updated: 2006-07-19

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