Widespread occurrence and spatial distribution of glyphosate, atrazine, and neonicotinoids pesticides in the St. Lawrence and tributary rivers☆
Graphical abstract
Introduction
The use of pesticides in agriculture has intensified in the past decades. Although the new generation of pesticides is deemed less bioaccumulative, the released quantities represent a potential threat to groundwaters, surface waters, and sediments (Aguilar et al., 2017; Chrétien et al., 2017; Hladik et al., 2018; Morrissey et al., 2015; Qu et al., 2017; Struger et al., 2017). Since surface and groundwaters are often used as sources of drinking water, pesticides may also be harmful to human populations (Klarich et al., 2017). The quality of the world's major freshwater hydrosystems is therefore under close scrutiny (Loos et al., 2009; Loos et al., 2017).
The St. Lawrence River (SLR) is one of the major hydrosystems in North America, draining a 1.3 million km2 watershed which includes the Laurentian Great Lakes (Pham et al., 2000). The freshwater inputs of the St. Lawrence provide a source of drinking water production for more than half of the population of the province of Quebec (Canada). Increasing anthropogenic pressures contributed to deteriorating the water quality of the St. Lawrence during the 20th century, but the situation has improved since the 1970s thanks to regulations and efficient sanitation (Giroux, Hébert and Berryman, 2016). For instance, decreasing trends of organochlorine pesticides in beluga whales from the St. Lawrence estuary were reported over a 1982–1994 period (Muir et al., 1996).
While legacy contaminants may gradually subside, there has been an increased mobilization of contaminants of emerging concern in intensive agriculture watersheds in southern Quebec and elsewhere. Neonicotinoid insecticides have been recently reported in environmental waters in Canada (Main et al., 2014; Giroux, Hébert and Berryman, 2016; Chrétien et al., 2017; Struger et al., 2017; Montiel-León et al., 2018). Thiamethoxam is classified as class C sales chemical (1–10 tons) in Quebec province (MDDELCC, 2016), although the report does not include the quantities from pre-treated seeds. Corn seeds and about half of soya seeds are treated with neonicotinoids since 2011 in Quebec (Giroux, 2015). In Ontario, large areas of agricultural fields are also planted with seeds treated with thiamethoxam, clothianidin, and imidacloprid (Ontario, 2015). Thiamethoxam and clothianidin were also frequently applied in the European Union, and their use was initially restricted in 2013 due to their toxicity to natural pollinators. A new regulation entered into force in 2018 allowing their use only in permanent greenhouses. In addition, five neonicotinoid compounds were on the first E.U. watch list (2015/495) and they are also included in the second watch list (2018/840) for E.U.-wide monitoring (acetamiprid, clothianidin, imidacloprid, thiacloprid, and thiamethoxam). Atrazine and glyphosate are the most used pesticide active ingredients in the U.S. (USEPA, 2017). Atrazine is also a high sales herbicide in Ontario and Quebec provinces, albeit in decreased usage in some applications for which glyphosate is now preferred (Farm and Food Care Ontario, 2015; MDDELCC, 2016). In Quebec, atrazine is especially applied in cultures of corn and soya; the treated areas in 2012 represented more than 320,000 acres (Giroux, 2015). Atrazine has been classified as a possible carcinogen to humans (Health Canada, 2013), with potential endocrine disruption (USEPA, 2007). In its 2016 report on pesticides sold in Quebec, the Minister of Sustainable Development, the Environment and the Fight against Climate Change classified glyphosate potassium salt as class F substance with annual sales higher than 1,000 tons of active ingredient (MDDELCC, 2016). Glyphosate usage is diverse and includes agricultural uses (e.g., weed control, pre-harvest crop desiccation) and non-agricultural uses where a large-spectrum weed control is important (e.g., farmyards, parks and railway tracks). Despite their potential environmental health effects, there is only limited information available about the distribution and dynamics of emerging pesticides at large spatial scale in major hydrosystems such as the SLR. Documenting the occurrence of emerging micropollutants and old ones under continued use may lead to a re-assessment of current water quality and health thresholds (Sauvé and Desrosiers, 2014).
In this study, we set out to determine the occurrence and levels of selected pesticides in a 200-km reach of the SLR and its major tributaries. Sampling was conducted in summer 2017 onboard the Lampsilis research vessel. One central hypothesis was that variations in pesticide concentrations will reflect changes in land use and hydrology at the scale of this large hydrosystem, where different agricultural activities may have an impact on ecosystems. How this risk is divided between the major water bodies is, therefore, an important research question to address. Land use vastly differs in St. Lawrence lowlands between south and north shores (i.e., agricultural vs. forest use). Additionally, the water masses that enter the fluvial St. Lawrence tend to show little to no mixing as far as Lake St. Pierre, with the brown waters from the Ottawa River near the north portion, and the blue-green waters from the Great Lakes in the center and south portions. To evaluate the spatial distribution of pesticides, samples were collected from a series of cross-sections within the St. Lawrence, and from bridges at the mouth of tributaries. These sampling efforts resulted in the collection of 68 surface water samples that were quantitatively analyzed for glyphosate and atrazine herbicides, 8 systemic insecticides, and some of their degradation products. This work aimed to address two critical knowledge gaps regarding: i) the current quality status of Quebec surface waters with regard to glyphosate, atrazine, and neonicotinoids; and ii) their spatial distribution within the SLR and its tributaries. The study provides much-needed data on the occurrence and fate of pesticides of high current concern in a complex hydrographic network.
Section snippets
Target compounds
Glyphosate, aminomethylphosphonic acid (AMPA), atrazine, desethylatrazine (DEA), acetamiprid, clothianidin, desnitro-imidacloprid, dinotefuran, fipronil, imidacloprid, nitenpyram, thiacloprid, and thiamethoxam were the compounds targeted in the present study. Further details on native compounds, isotope-labeled internal standards, and other chemicals and materials are provided in the Supporting Information (SI).
Description of the sampling area
The hydrological features of the surveyed area are well described (Hudon, 2004; Pham
Occurrence data and concentration levels
Overall, 99% of the surface water samples (n = 68) were found to be positive to at least one of the targeted pesticides. About two-thirds of the samples were positive to at least one neonicotinoid. Compound-specific descriptive statistics are summarized in Table 1. Out of the 14 quantitatively targeted compounds, 7 were found above the mLODs (Table 1). The most often detected pesticides were glyphosate (84% of the samples), atrazine (82%), thiamethoxam (59%), DEA (47%), and clothianidin (46%).
Conclusions
Glyphosate and atrazine were the most frequently detected compounds in surface water samples from a 200-km reach of the St. Lawrence River and its tributaries. However, their concentrations remained well below the Canadian water quality guidelines for the protection of aquatic life. Nearly one-third of the surface water samples had summed neonicotinoid concentrations above the criterion of 8.3 ng L−1. Limited transversal mixing of the different water masses flowing within the SLR allowed to
Acknowledgments
The authors gratefully acknowledge the crew from the Lampsilis research vessel, Prof. Gilbert Cabana at Université du Québec à Trois-Rivières (UQTR), and all those who participated in the sampling campaign. We thank the Natural Sciences and Engineering Research Council of Canada (NSERC), the Fonds de Recherche du Québec - Nature et Technologies (FRQNT), the Réseau Québec Maritime (RQM), the Government of Québec, and the Canada Foundation for Innovation for their financial support. CONACYT
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This paper has been recommended for acceptance by Dr. Da Chen.