Summary statistics for carbamate pesticides in individual years are shown in Table 1 while site-specific statistics are shown in Table 2. Overall summary statistics for the 2007–2010 sampling period are shown in Table 3. The occurrence of the most prevalent carbamates and their corresponding frequencies of detection are illustrated in Figs. 2 and 3, respectively.
Aldicarb, although withdrawn from the Canadian market in 1996, was included as an analyte in the study to investigate the possibility of legacy sources or current illegal application. Used under the trademark Temik 10G, aldicarb was an insecticide widely used in agriculture in Canada, but can still be detected in water samples, especially groundwater (CCME 2009). Although the United States Environmental Protection Agency (USEPA) has implemented a ban on distribution of aldicarb by 2017, as recently as 2011 it was still used extensively in orchard and grape applications in the United States (USGS 2014). In a total of 272 samples over the course of the current study, aldicarb was not detected.
In 2010, carbofuran was targeted for phase out by 2012 by Health Canada’s Pest Management Regulatory Agency (PMRA); however, timelines have not yet been confirmed but would be determined according to normal practice (Health Canada 2010). The most recent usage data (Table 4) estimated just over 650 kg of carbofuran was used as an insecticide on crops in Ontario in 2008 (McGee 2010). Both the 2003 and 2008 reports excluded some types of agricultural use, such as greenhouse spraying, seed treatments and livestock sprays; therefore temporal comparisons between 2003 and 2008 usage data are subject to uncertainty. In the 2003 survey, total use of carbofuran was estimated at 1778 kg (McGee et al. 2004). Overall, carbofuran was detected in roughly 15 % of the total samples over the course of the study (39 of 272 samples; 14.3 %). In the last year of sampling for the study (2010), carbofuran was detected in only one sample. The United States Geological Survey (USGS) has not reported usage data for carbofuran in the United States since 2009. Of the 39 samples in which carbofuran was detected over the course of this study, one-third (13) were in Richardson’s Creek, which discharges a watershed characterized by greenhouse operations, nurseries and orchards. The Canadian Water Quality Guideline for carbofuran for the protection of aquatic life (1800 ng/L) was never exceeded in the study.
Carbaryl has been in production in excess of 50 years and global usage since that time exceeds that of all other carbamates combined. Over the course of the survey, it was among the most frequently detected compounds, being detected in roughly 64 % of the samples (Table 3). The broad spectrum use of carbaryl was reflected in its frequent detection across the entire geographical range of sampling locations. In the United States, the primary application of carbaryl continues to be orchards and grapes (USGS 2014). Carbaryl was also detected in Indian Creek, which was the urban control stream. There was a decline in carbaryl seen in 2009 and 2010 that was likely in response to an Ontario-wide provincial ban on the sale and use of pesticides for cosmetic (non-essential) purposes (Todd and Struger 2014). Carbaryl exceeded the Canadian Water Quality Guideline for the protection of aquatic life (200 ng/L) in 3 of 272 samples; these occurred at 2 Mile Creek, Prudhomme (Vineland) Creek and Indian Creek.
Occurrences and distributions of oxamyl and methomyl were similar, presumably due to similarities in their applications. Both are used as insecticides and nematocides on row crops, ornamentals and fruits and vegetables. In the United States, these compounds are used primarily on fruits and vegetables, with some use of methomyl on corn (USGS 2014). Both oxamyl and methomyl were detected at roughly 10 %–20 % frequency at the 2 Mile Creek and 4 Mile Creek sites (Table 2). The highest frequency of detections over the course of study was at Richardson’s Creek (64 and 71 % detection frequencies for methomyl and oxamyl, respectively), with its high density of greenhouse operations. These data indicate both compounds could have found application for control of insects and nematodes on ornamentals. Limited usage data for Ontario is available for both compounds (Table 4); these data exclude greenhouse applications.
Interestingly, there has been a significant shift in application and corresponding reduction in usage of metalaxyl in the United States from primarily orchards and grapes, cotton and vegetables and fruit in the 1990s to primarily soybeans over the period 2008–2011 (USGS 2014). Data from individual years in the current study show metalaxyl was detected in at least 50 % of the samples; overall, this compound was detected at a frequency of 75 % (204 of 272 total samples) over the course of the study (Table 3). Mean annual concentrations of metalaxyl for the period of 2007–2010 showed a trend toward increasing concentrations for the first 3 years (3.04, 18.8 and 27.4 ng/L for 2007, 2008 and 2009, respectively). Metalaxyl is used as a fungicide for a wide range of applications, including row crops, vines, vegetables, ornamentals and turf. In this respect, its usage reflects the entire range of agricultural land use. Turf applications may be reflected in detection in over 40 % of the samples from Indian Creek (Table 2), which is representative of a typical urban environment. The frequency of detection in Indian Creek was second only to carbaryl (68 %, Table 2). Metalaxyl was also detected in the Niagara River at both Fort Erie and Niagara-on-the-Lake, suggesting measurable inputs and loadings into the Great Lakes.
Pirimicarb is used for control of aphids on vegetable, cereal and orchard crops. Detections were prevalent in mixed-use agricultural watersheds; overall, pirimicarb was detected in 86 of 208 total samples (41 %, Table 3). The 208 samples represent 3 years of data, compared to 4 years of data for the other compounds, as pirimicarb was not added to the analytical suite until the completion of the first year of study. No recent usage data for pirimicarb is available. The USGS reports very low usage of pirimicarb in the United States, primarily on pasture and hay (USGS 2014). Pirimicarb was also detected in the Niagara River and along with metalaxyl were the only two compounds detected in the Great Lakes connecting channel.
The usage statistics, frequency of detection and ranges of concentrations measured for carbamates were similar to those we observed for other classes of pesticides in southern Ontario, including the sulfonyl urea herbicides (Struger et al. 2011). There is a lack of applicable Canadian guidelines for the pesticide suite covered in this study; a comparison of maximum concentrations against USEPA aquatic life benchmarks resulted in no exceedances of acute benchmarks for invertebrates, and only one exceedance (899 ng/L for methomyl in 2008) of the chronic benchmark for invertebrates (USEPA 2015).
There were also some interesting differences in trends in concentrations of some compounds with respect to flow conditions. In the case of carbaryl, the highest concentrations observed in 2007, 2008 and 2010 occurred during high flow conditions that indicated watershed runoff to be the primary vector of entry into the watercourses. These observations could be rationalized given the high usage of carbaryl and its extremely broad range of applications. Conversely, over the same time period the highest concentrations of metalaxyl were correlated with base flow conditions. We attribute these observations to the potential for drift and/or overspray inputs directly to watercourses during application. The results of this study show that carbamate pesticides and metalaxyl continue to be important components of integrated pest management strategies in southern Ontario. While metalaxyl was the most-frequently detected compound, carbaryl was also detected at a rate that indicates a broad range of use five decades after its introduction. Despite its ubiquity, maximum concentrations of carbaryl rarely exceeded the CCME guideline value. The scope, frequency and temporal nature of the information presented in this study provide regulators with an understanding of the occurrence and distribution of select pesticides in surface waters, which in turn has ramifications for registration and subsequent use.