What is Bulk Atmospheric Deposition?

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Urban Atmospheric Deposition of Selected Pesticides in Winnipeg, Canada Lindsey Andronak and AnnemiekeFarenhorst Department of Soil Science, University of Manitoba, Winnipeg, MB, Canada R3T 2N2 • What is Bulk Atmospheric Deposition? • • Pesticides in the atmosphere are deposited to land and water surfaces by two processes: • • Wet deposition such as pesticide residues dissolved in rainfall • • Dry deposition such as pesticide residues sorbed to particulate matter • • Wet and Dry deposition combined is referred to as bulk deposition • • Atmospheric bulk deposition can be an important source of non-point source pollution of pesticides in the urban environment, potentially contaminating gardens, sandboxes, swimming pools, and other urban features. . • • Sources of pesticides entering the atmosphere include both agricultural (pre-emergent and post-emergent application to crops) and urban areas (residential cosmetic use, municipal pest control). • Objective • • Monitor the bulk deposition of 72 chemicals throughout the summer at two locations in Winnipeg. • • Correlate the deposition amounts to the amount of rainfall. • Methods • • Bulk deposition samples were collected using a one meter by one meter galvanized metal pan sampler that emptied into 23 L glass carboys shaded by plywood to minimize pesticide photodegradation (Figure 1). • • The weekly bulk deposition samples were collected for 18 weeks from May 25 to September 21, 2010 adjacent to a meteorological station that monitored precipitation levels. • • The research sites were site 1 (Whyte Ridge) and Site 2 (St. Vital). Site 1 is approximately 7 km (4.2 mi) west of Site 2. • Figure 1. Bulk Atmospheric Deposition (rainfall + particulate matter) Sampler • with Meteorological Station (upper left-hand corner) • Results • • Nineteen individual pesticides or their metabolites were detected during the course of this study. These pesticides or metabolites were: • • 15 herbicides – 2,4-D, aminomethylphosphonic acid, atrazine, benzaton, chlopyralid, dicamba, fluroxypyr, gyphosate, MCPA, MCPP, metalochlor, propiconazole, 2,4-dichlorophenol, 2,4-DP and desethyl atrazine • • 3 insecticides (all organophosphates) –clopyrifos, diazinon and malathion • • 1 fungicide - chlorothalonil • • No legacy pesticides (α-BHC, γ-BHC) were detected at either site. • • Total seasonal bulk deposition amounts at each site were: • • Site 1 (Whyte Ridge) – 239 μg m-2 • • Site 2 (St. Vital) – 373 μgm-2 • • The most frequently detected pesticides were (1) malathion, (2) glyphosate and (3) 2,4-D, both at site 1 and 2 (Tables 1 & 2). The seven other pesticides in the top 10 list were also similar for both sites, albeit their ranking depended on the site (Tables 1 & 2). Table 2. Top 10 pesticides detected at Site 2, based on total seasonal deposition Table 1. Top 10 pesticides detected at Site 1, based on total seasonal deposition total pesticide concentration (excluding malathion1) (μg m-2) • • Total weekly deposition rates were adequately significantly related to weekly total rainfall (Figure 2), but only after removing malathion from the data set. Including malathion, the relation was not highly significant (r² = 0.58), suggesting that spray drip from fogging trucks as part of mosquito control programs contributed to malathion amounts deposited in the bulk atmospheric deposition sampler (Figure 1). Dry deposition of pesticides was noted for a range of pesticides because there was not rainfall in the week of July 13 at Site 2, however malathion, MCPA, MCPP, dicamba, bromoxynil, glyphosate, 2,4-D and atrazine were all detected in the sampler. • Discussion and Conclusion • • Atmospheric deposition is a source of non-point source contamination of pesticides in urban areas. • • Pesticides in the atmosphere of urban centres may originate from both urban and rural sources. • • Increasing the number of study sites in future studies will strengthen knowledge of the source, persistence and transport of pesticides in the atmosphere which is important for a full evaluation pesticide deposition in urban environments. • • Concurrent studies run in both the urban centres as well as the surrounding agricultural environment will help to determine if there are differences between these two environments in the types and amounts of pesticides deposited by wet and dry deposition. rainfall (mm) Figure 2. Weekly total pesticide concentration by weekly rainfall amounts. 1 Malathion concentrations removed due to direct spraying of the pesticide by the City of Winnipeg, and not necessarily atmospheric deposition. Acknowledgements The authors would like to acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) for awarding Andronak with an Undergraduate Student Research Award such that this study could be conducted. In addition, operation funds for this research were provided as part of a NSERC-Discovery grant awarded to Farenhorst. The authors also thank Paul Messing, Terri Ramm, Bo Pan, Karin Rose and David Humphries for technical support throughout this study. • Malathion had the highest weekly deposition rates which was 88 μg m-2 week-1 in the week ending on June 22 at Site 1 and 108 μg m-2 week-1 in the week ending on August 31 at Site 2 (Tables 1 & 2). • 2,4-D had the greatest detection frequency at both Site 1 (88 %) and Site 2 (100%) (Tables 1 & 2). • Site 2 generally had greater weekly deposition rates (approximately 1.2 times, r² = 0.317) when compared with Site 1. A second field season has recently been completed and results will be analyzed and compared with the 2010 study. For more information contact umandron@cc.umanitoba.ca