Published September 6, 2018

Journal of Environmental Quality TECHNICAL REPORTS GROUNDWATER QUALITY

Auxin and Pesticide Mixtures in Groundwater of a Canadian Prairie Province

Sirajum Munira, Annemieke Farenhorst, Kamala Sapkota, Denise Nilsson, and Claudia Sheedy*

bout 83% of Canada’s cropland is located in the prairies Abstract spanning the provinces of Alberta, Saskatchewan, and Groundwater samples were collected from piezometers and water Manitoba (Statistics Canada, 2016). herbicides are table wells in both dryland and irrigated agricultural regions of Awidely used in Canadian Prairie agriculture to control broadleaf Alberta, Canada, to examine the occurrence of pesticide mixtures. weeds in cereal crops and have been the focus of studies that previ- Fourteen current-use pesticides and two historical compounds were detected over a 3-yr sampling period. Pesticide mixtures ously examined the occurrence of pesticides in Canadian Prairie were detected in ?3% of the groundwater samples, and the groundwater (Miller et al., 1995a, 1995b; Hill et al., 1996). The frequency of detection increased from spring (1.5%) to summer auxin herbicides (2,4-dichlorophenoxyacetic acid [2,4-D], bro- (3.8%) and fall (4.8%). Pesticide mixtures always consisted of at moxynil, , diclofop, 2-methyl-4-chlorophenoxyacetic acid least one of two auxin herbicides: 2,4-dichlorophenoxyacetic [MCPA], and ) examined in the latter studies were all acid (2,4-D) or 2-methyl-4-chlorophenoxyacetic acid (MCPA). 19% of all samples contained a single pesticide, with auxin herbicides detected, with bromoxynil sometimes present at levels exceeding 2,4-D (7.3%), MCPA (4.4%), and (3.9%) being most its guideline for Canadian Drinking Water Quality (Miller et al., prevalent. We detected 2,4-D predominantly in the fall (72% 1995a, 1995b). When the source of drinking water consisted of of 2,4-D detections) and less in spring and summer (28%). We surface water reservoirs integrated in Canadian Prairie agricultural detected MCPA mostly in summer (85% of MCPA detections) and landscapes, auxin herbicides were detected in both the source and less in spring and fall (15%). Clopyralid was more evenly detected across spring (30%), summer (25%), and fall (45%). Since the treated drinking water (Donald et al., 2007). auxin herbicides above are typically applied in summer, results In Alberta, >14 million kg of pesticide active ingredients suggest that each may have different mobility and are annually applied on ?10 million ha of cropland, with her- persistence characteristics in prairie soils. Guidelines for Canadian bicides accounting for 82% of such applications (AEP, 2015). Drinking Water Quality have been set for a range of individual About 90% of Alberta’s rural population relies on groundwa- pesticides, but not for pesticide mixtures. If Canada is to establish such guidelines, this study demonstrates that auxin herbicides ter for their potable water supply (AWA, 2015). Guidelines for should be prioritized. In addition, only 7 of the 16 compounds Canadian Drinking Water Quality have been set for a range of detected in this study have established maximum acceptable individual pesticides, but not for pesticide mixtures. However, concentrations (MACs), excluding clopyralid, which was detected it is well known that groundwater samples can contain more in all three sampling years. than one pesticide: a study conducted across the United States (?5000 samples collected) revealed that >50% of sampled shal- Core Ideas low groundwater wells contained pesticide mixtures (Gilliom, 2007). In Portugal, groundwater collected from 18 wells in an • Sixteen pesticides were detected in groundwater, most frequently agricultural region showed that samples collected in spring were auxin herbicides. more likely to contain pesticides mixtures, whereas samples col- • Detection of pesticide mixtures increased from spring (1.5%) to fall (4.8%). lected in summer and fall mostly contained an individual pesti- • Samples with pesticide mixtures always contained at least one cide (Silva et al., 2012). auxin herbicide. While the European Drinking Water Directive is using a cau- • In establishing water quality guidelines for mixtures, Canada tionary approach and has set the maximum total concentration must prioritize . of pesticides in water at 500 ng L−1, the Canadian Drinking Water Quality guidelines are based on the potential for an individual pesticide to cause adverse health effects. Should Canada estab- lish guidelines for pesticide mixtures, it is important to know Copyright © American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved. S. Munira, A. Farenhorst, and K. Sapkota, Dep. of Soil Science, Faculty of J. Environ. Qual. 47:1462–1467 (2018) Agricultural & Food Sciences, Univ. of Manitoba, 362 Ellis Building, Winnipeg, MB, doi:10.2134/jeq2018.05.0202 Canada, R3T 2N2; D. Nilsson and C. Sheedy, Agriculture and Agri-Food Canada, This is an open access article distributed under the terms of the CC BY-NC-ND Lethbridge Research and Development Centre, 5403 1st Ave. South, Lethbridge, license (http://creativecommons.org/licenses/by-nc-nd/4.0/). AB, Canada, T1J 4B1. Assigned to Associate Editor Qingli Ma. Supplemental material is available online for this article. Received 23 May 2018. Abbreviations: b-HCH, b-hexachlorocyclohexane; MAC, maximum Accepted 7 Aug. 2018. acceptable concentration; MCPA, 2-methyl-4-chlorophenoxyacetic acid; 2,4-D, *Corresponding author ([email protected]). 2,4-dichlorophenoxyacetic acid.

1462 the types of pesticide mixtures present in Canadian groundwa- and 2010 and situated in an area with fine- to coarse-textured ter. Current use pesticides such as auxins and chloroacetamide soils. Twenty-one wells and 11 piezometers were sampled in this herbicides have reported half-lives (days to 50% degradation) of region ranging in depth from 4.8 to 19.5 m. The mean depth of 0.5 to 5 yr in groundwater (Cavalier et al., 1991). Some histori- groundwater wells was 7 ± 0.8 m, and that of piezometers was cal pesticides that were used in agricultural production decades 15 ± 3.5 m. Groundwater samples were collected in the spring, ago have been detected in drinking water wells long after these summer, and fall periods of 2013, 2014, and 2015, except that pesticides were banned (Burow et al., 2008). In this study, we central Alberta was not sampled in spring 2013. The spring sam- screened groundwater samples from Alberta for 42 active ingre- pling period from late April to early June corresponded to the dients of pesticide products currently registered for agricultural period when preplant and preemergent herbicides are typically use in Canada, and for 63 compounds that were active ingredi- applied in Alberta. The summer sampling in August reflected ents, metabolites, or byproducts of pesticide products no longer the timing of preharvest applications in Alberta, with other in- registered or currently not registered (Supplemental Table S1). crop applications having been completed in June and July. The The objective of the study was to examine the detection fre- fall sampling period from late September to late October corre- quency, types, and concentrations of pesticide mixtures occur- sponded to the preharvest period to initial soil freeze up during ring in samples collected from piezometers and wells in both which preharvest and burn-off herbicides are applied. dryland and irrigated agricultural regions of Alberta. Sample Analysis Materials and Methods The analytical suite included 42 active ingredients that are in Site Locations and Sampling Times pesticide products registered for agricultural use in Canada and 63 other compounds that were active ingredients, metabolites, A total of 436 groundwater samples were collected from or byproducts in pesticide products no longer registered or cur- piezometers and wells in two representative agricultural regions rently not registered (Supplemental Table S1). These compounds of Alberta. The region in southern Alberta is the Battersea drain- were herbicides (44.8%), insecticides (42.9%), fungicides (7.6%), age basin near Lethbridge (49°52¢ N, 112°46¢ W; population and others such as nematicides and algicides (4.7%) and included ? 93,000), located in the moist mixed grassland ecoregion and all pesticides previously detected in surface drinking water and dominated by dark brown Chernozemic soils. The southern groundwater in Alberta such as 2,4-D, MCPA, bromoxynil, clopy- Alberta region has a mean (1981–2010) annual precipitation of ralid, and dicamba (Hill et al., 1996; Donald et al., 2007). 380 mm and a mean annual temperature of 5.9 ± 1.1°C (ECCC, Water samples were analyzed within 3 (minimum) to 7 d 2018). This region includes? 6% of the crop land in Alberta that (maximum), including collection in the field, transportation to receives irrigation ranging from 300 to 450 mm yr−1 and contrib- the laboratory, extraction, and analysis. Water samples were col- uting to >19% of the net worth of primary agricultural produc- lected in an amber glass bottle (1 L) using a polyethylene bailing tion in the province (AAF, 2015a). The region in central Alberta tube and transported to the laboratory in ice-packed coolers for near Red Deer (52°28¢ N, 113°44¢ W; population ? 100,000) storage in the fridge (4°C) prior to extractions. Water samples is located in the aspen parkland ecoregion, dominated by black were filtered through glass wool, acidified with concentrated Chernozemic soils and dryland farming (no irrigation). The H SO (pH 2), and extracted by liquid–liquid partitioning using region has a mean (1981–2010) annual precipitation of 486 mm 2 4 CH Cl . After phase separation, CH Cl extracts were dried and a mean annual temperature of 3.7 ± 1.1°C (ECCC, 2018). 2 2 2 2 with acidified Na SO , methylated using CH Cl , and added to Cereal, oilseed, and forage crops dominate crop production in 2 4 2 2 hexane (40 mL). Nitrogen gas was used to remove the remain- the two regions, with potatoes (Solanum tuberosum L.) also ing CH Cl and adjust the final volume to 10 mL. Esterified being grown on irrigated land in southern Alberta. Typical crops 2 2 extracts were analyzed (2-mL injections) using a HP 6890 C gas grown in the regions are canola (Brassica napus L.), spring wheat chromatograph with a 5973-mass selective detector operating in (Triticum aestivum L.), barley (Hordeum vulgare L.), and for- selected ion monitoring mode. The column was DB-1701 30 m ages, and such crops accounted for 85% of the crop acreages in ´ 0.25 mm ´ 0.25 mm with the temperature programming being the province at the time of this study (AAF, 2015b). Six of the 55°C for 2 min, ramp of 30°C min−1 to 130°C, followed by ramp 43 current-use pesticides included in this study are within the of 6°C min−1 to 250°C and ramp of 30°C min−1 to 280°C, then top 10 most commonly applied for pesticides in Alberta (AEP, holding for 10 min at 280°C for a total run time of 35.5 min. 2015), namely herbicides 2,4-D, , bromoxynil, flu- One target ion and at least two qualifier ions were monitored to roxypyr, MCPA, and triallate. identify compounds at expected retention times. The lower limit The piezometers and wells selected for this study captured of quantification was 25 ng L−1 for most compounds, and detec- a range of hydrogeological scenarios that are part of a long- tions below this limit were considered no detects (0 ng L−1). No term groundwater quality program focused on nutrient, major pesticides were detected in the laboratory blanks. ion, and trace element concentrations in shallow groundwater (Lorenz et al., 2014). In southern Alberta, the wells and piezom- Results and Discussion eters sampled (installed between 1993 and 2011) were situated in a 192-km2 area with fine- to coarse-textured soils. Fifteen wells Sixteen of the 105 compounds analyzed for were detected and 16 piezometers were sampled, ranging in depth from 1.8 to including 14 current-use pesticides and two historical com- 20 m. The mean depth of groundwater wells was 5 ± 2.2 m (± pounds (Table 1). We analyzed 35 current-use pesticides in SD), and that of piezometers was 12 ± 5.1 m. In central Alberta, Alberta, and 14 pesticides were detected (Table 1). Thus, 40% the wells and piezometers sampled were installed between 2002 of the current-use pesticides were detected despite central and

Journal of Environmental Quality 1463 Table 1. Summary of compounds (n = 105) detected in water samples (n = 436) derived from wells (n = 38) and piezometers (n = 28) in Alberta. No. of samples with Year Well or piezometer Compound Concentration MAC† GUS‡ detections detected depth m ———————— n g L −1 ———————— Southern Alberta 2,4-D 19 (3,12,4)§ 1,2,3¶ 3.0–20.0 (11.0, 50%)# 26–154 (60, 68%)# 100,000 1.69 MCPA 13 (4,0,9) 1,2,3 2.3–20.0 (7.0, 64%) 26–1293 (185, 184%) 100,000 2.94 Propyzamide 9 (0,4,5) 2,3 1.8–9.2 (6.4, 43%) 26–299 (92, 93%) – 1.34 Clopyralid 3 (0,1,2) 1,2,3 2.3–2.3†† 1153–7121†† – 4.51 Ethion 3 2 7.0–20.0 47–113 – 0.00 1 2 10 1569 10,000 2.00 1 3 8.3 42 – 2.42 b-HCH 1 1 2.3 38 – – Difenoconazole 1 3 8.0 35 – 0.90 Dicamba 1 1 7.8 27 120,000 1.75 cis-Permethrin 1 1 8.4 26 – – Central Alberta 2,4-D 20 (0,1,19) 2,3 4.8–19.2 (9.4, 50%) 25–361 (66, 135%) 100,000 1.69 Clopyralid 17 (6,6,5) 1,2,3 5.3–15.3 (7.35, 37%) 25–225 (74, 75%) – 4.51 MCPA 15 (0,2,13) 2,3 5.9–15.3 (7.8, 32%) 32–342 (89, 89%) 100,000 2.94 Chlorpyrifos 2 3 6.0–7.0 24–209 90,000 3.63 Fluroxypyr 2 3 6.8–7.0 38–42 – 2.42 Imazethapyr 1 1 7.0 211 – 6.19 Diazinon 1 2 7.0 166 20,000 1.14 EPTC 1 3 5.3 44 – 1.34 Bromoxynil 1 3 7.0 29 5,000 0.03 † MAC, maximum acceptable concentration, Guidelines for Canadian Drinking Water Quality (Health Canada, 2017). ‡ GUS, groundwater ubiquity score with numbers derived from Lewis et al. (2016). § Number of detection in 2013, 2014, and 2015, respectively. ¶ Year detected: 1 = 2013, 2 = 2014, and 3 = 2015. # Numbers refer to minimum-maximum (mean, CV). †† Numbers refer to minimum–maximum. southern Alberta having a semiarid climate and being dominated imadazolinone herbicide imazethapyr and the organophosphate by organic C-rich Chernozemic soils (Pennock et al., 2011). insecticides chlorpyrifos and diazinon (Table 1). Of the current- Two historical compounds were also detected (Table 1). Thus, use pesticides detected, the auxin herbicides 2,4-D, MCPA and only 15% of the 105 compounds analyzed for were detected. clopyralid were the most frequently detected compounds, in Other groundwater studies detected 20 to 82% of the pesticide both southern and central Alberta (Table 1). In a study monitor- compounds screened for, with between 20 and 83 compounds ing 47 pesticides in three rivers in the Prairies from 2006–2011, included in these studies (ECCC, 2005; Woudneh et al., 2009; 2,4-D, MCPA and clopyralid were also the top three pesticides Silva et al., 2012; Toccalino et al., 2014). 89 compounds were not detected (ECCC, 2015). Historical compounds were only detected, including 28 current-use pesticides in Canada of which detected in southern Alberta and included the organophosphate 21 pesticides are recommended for use in Alberta crop produc- insecticide ethion (deregistered in 2003) and b-hexachlorocyclo- tion (Alberta Crop Protection Guide, 2015) (Supplemental hexane (b-HCH), a byproduct of the organochlorine insecticide Table S1). Of the other 61 compounds that were not detected, 29 lindane (deregistered in 2005) (Supplemental Table S1). compounds were not currently registered for use in Canada, and The 2,4-D concentrations detected in wells and piezometers 32 are historical compounds that were deregistered or discontin- (3.0–20.0 m) were always £361 ng L−1. We detected MCPA ued, some as early as 1985 (Supplemental Table S1). Thus, these once at a relatively large concentration (1293 ng L−1) in ground- results suggest that historical compounds may not have entered, water from one piezometer (8.3 m), but other wells and piezom- or were otherwise not persisted in Alberta’s groundwater. eters (2.3–20.0 m) showed MCPA concentrations £342 ng L−1. Current-use pesticides detected in southern Alberta included The detected individual herbicide concentrations were always five auxin herbicides (2,4-D, MCPA, clopyralid, dicamba, flu- well below the maximum acceptable concentration (MAC) roxypyr), the benzamide herbicide propyzamide, the triazine set for MCPA (100,000 ng L−1) and 2,4-D (also 100,000 ng herbicide simazine, the cis-isomer of the pyrethroid insecticide L−1) under the Canadian Drinking Water Quality Guidelines permethrin, and the triazole fungicide difenoconazole (Table 1). (Table 1). Clopyralid was detected at relatively large concentra- Current-use pesticides detected in central Alberta included tions (1153–7121 ng L−1) in one shallow well (2.3 m) in south- four auxin herbicides (2,4-D, MCPA, clopyralid, fluroxypyr), ern Alberta, but four other shallow wells (1.8–4.8 m) in the the hydroxybenzonitrile herbicide bromoxynil, the thiocarba- same region did not show clopyralid detection, suggesting that mate herbicide S-ethyl dipropylthiocarbamate (EPTC), the the observed relatively large concentrations of clopyralid in the 1464 Journal of Environmental Quality 2.3-m well may have resulted from a localized source of contami- frequently than in spring (1 of 136 total spring samples, 0.7%), nation. The 2.3-m shallow well consistently showed clopyralid and not at all in fall. When 2,4-D or MCPA were detected in detection in 2014 and 2015, but no clopyralid was detected in spring, they were only detected in samples collected in early June. this well in 2013. Other pesticides were occasionally detected It is remarkable that 2,4-D was mainly detected in fall, whereas in the 2.3-m shallow well (MCPA, propyzamide, the historical MCPA was rarely detected in that period because the structure compound b-HCH) but at concentrations ranging from 38 to of 2,4-D and MCPA acids only differ at Position 2 of their ben- 142 ng L−1. Deeper wells and piezometers (5.3–15.3 m) showed zene ring, where 2,4-D has a methyl group and MCPA has a Cl. clopyralid concentrations £225 ng L−1. Clopyralid has been reg- Although both herbicides are sold in amine and ester product istered for agricultural use since 1984 (Health Canada, 2018), formulations, ester-formulated products are more likely applied but there are no specific guideline for this herbicide under the because Alberta water is high in bicarbonates that react with Canadian Drinking Water Quality Guidelines. amine-formulated products and deactivate 2,4-D and MCPA Across the three sampling years, the number of groundwater (AAF, 2003). Esters of 2,4-D and MCPA are rapidly hydrolyzed samples collected varied by season and region. The total number in soil to their acids (Smith and Hayden, 1980). The water solu- of samples from 2013 to 2015 was 77 samples in spring, 72 bility of both MCPA and 2,4-D acids is relatively high (>20,000 samples in summer, and 58 samples in fall for southern Alberta, mg L−1 at 20°C) (Lewis et al., 2016). and 59 samples in spring, 84 samples in summer, and 86 samples Both 2,4-D and MCPA are registered for use in spring, in fall for central Alberta. Nineteen percent of the 436 samples summer, and fall and can be used interchangeably. The annual analyzed in this study contained at least one pesticide, and 3.21% sales of MCPA in Alberta typically exceeds those of 2,4-D, with of the samples contained pesticide mixtures (more than one pes- an estimated 920,000 and 640,000 kg of active ingredients sold ticide). We found that 1.47% of the spring samples, 3.82% of in 2013, respectively (AEP, 2015). No data are available on the the summer samples, and 4.79% of the fall samples contained mass of 2,4-D or MCPA applied in spring, summer, and fall in pesticide mixtures, suggesting that the likelihood for more Alberta or other provinces in Canada. However, we obtained than one pesticide moving into groundwater increases over the information about the timing of herbicide applications accord- growing season. Pesticide mixtures (two to three pesticides per ing to a weed survey that was conducted in 2010 and included sample) were only detected in wells and piezometers that were 241 fields representative of major crops grown in Alberta (J. £10 m deep (Table 2). Three samples contained three pesticides, Leeson [Agriculture and Agri-Food Canada], personal com- whereas 11 samples contained two pesticides. munication, 2018). The weed survey data show that MCPA and Pesticide mixtures always contained either 2,4-D or MCPA, 2,4-D are predominantly applied in the summer rather than the and in two cases, both 2,4-D and MCPA (Table 2). Samples spring or fall. Of the 241 fields, 27% (MCPA) and 13% (2,4-D) with pesticide mixtures containing 2,4-D were only detected in received applications in the summer, and 3% received applica- the fall except for one sample where it was present with MCPA tions of 2,4-D in the spring. No applications were reported for in the summer (Table 2). The seasonal detections of 2,4-D as a MCPA in the spring, or for MCPA and 2,4-D in the fall. Thus, single pesticide also showed that 2,4-D was more frequently given this information, we believe that the occurrence in ground- detected in the fall (22 of 144 fall samples, 15.3%) than in the water samples of MCPA in the summer and of 2,4-D in the fall is summer (9 of 156 summer samples, 5.8%) or the spring (1 of 136 a consequence of in-crop applications of these herbicides in the spring samples, 0.7%). In contrast, samples with pesticide mix- summer. It is likely that MCPA moves more readily to ground- tures containing MCPA were mainly detected in summer and water than 2,4-D but that 2,4-D is more persistent than MCPA occasionally in spring but not in the fall except for one sample, and may continue to move to groundwater over the growing where it occurred with 2,4-D (Table 2). The seasonal detection season, with groundwater contaminations peaking in the fall. of MCPA as a single pesticide also showed that MCPA was Previous studies have compared the movement and persistence detected in summer (16 of 156 summer samples, 10.3%) more of 2,4-D versus MCPA for a range of soils (Helling and Turner,

Table 2. Summary of pesticides mixtures detected in Alberta from 2013–2015. Pesticides Total concentration Well or piezometer depth Season: region ng L−1 m 2,4-D, ethion, simazine 1755 10.0 Fall: southern Alberta 2,4-D, ethion 267 7.0 Fall: southern Alberta 2,4-D, diazinon 249 7.0 Fall: central Alberta 2,4-D, clopyralid 121 5.3 Fall: central Alberta 2,4-D, clopyralid 100 7.5 Fall: central Alberta 2,4-D, MCPA 68 8.0 Fall: southern Alberta MCPA, clopyralid, propyzamide 7330 2.3 Summer: southern Alberta MCPA, fluroxypyr 1335 8.3 Summer: southern Alberta MCPA, fluroxypyr 380 6.8 Summer: central Alberta MCPA, fluroxypyr, bromoxynil 244 7.0 Summer: central Alberta MCPA, 2,4-D 172 9.5 Summer: southern Alberta MCPA, clopyralid 108 7.5 Summer: central Alberta MCPA, β-HCH 79 2.3 Spring: southern Alberta MCPA, dicamba 58 7.8 Spring: southern Alberta

Journal of Environmental Quality 1465 1968; Hiller et al., 2008; Fu et al., 2009). These studies indicate and was detected every year. Only 3 of the 436 samples con- that both 2,4-D and MCPA are relatively mobile in most soils, tained pesticide mixtures and exceeded the maximum allowable but that MCPA typically has a weaker sorption or greater mobil- total pesticide concentration (500 ng L−1) set by the European ity than 2,4-D (Helling and Turner, 1968; Hiller et al., 2008). Drinking Water Directive. Despite this, in establishing water In another study, both herbicides degraded rapidly under warm quality guidelines for mixtures, Canada should prioritize mix- temperatures (27°C), but the half-life of 2,4-D was about twice tures of auxins because ?70% of the pesticide mixtures detected as long as that of MCPA (Fu et al., 2009). In addition, in a review contained two auxin herbicides. For this development, we also of the environmental fate of 2,4-D, it was reported that 2,4-D recommend additional groundwater monitoring studies, as there can be moderately persistent in soil, with a reported half-life of are limited data available for Canada, including in the Canadian 59 d (Walters, 1999). Prairies, which has >0.5 million km2 of agricultural land onto Both MCPA and 2,4-D are often formulated in herbicide which auxin herbicides are frequently applied. The study findings products with two or three other active ingredients (MCPA are unique in that, for the first time, it is clearly demonstrated and 2,4-D are not formulated together). Such other active that the frequency of MCPA detections occur in the summer, ingredients can include bromoxynil, clopyralid, dicamba, and whereas those of 2,4-D occurred in the fall, and that the most fluroxypyr. When present as pesticide mixtures, MCPA was likely reason for this is a differential fate in soils. Thus, future detected in combination with bromoxynil, clopyralid, dicamba, groundwater studies in the Canadian Prairies should consider or fluroxypyr, as well as with the herbicide propyzamide (which is taking a seasonal sampling approach similar to the current study. not formulated with MCPA) and with the historical compound b-HCH. In contrast, when present as pesticide mixtures, 2,4-D Conclusion was detected in combination with clopyralid but not with bro- This study detected 14 current-use pesticides and two histori- moxynil, dicamba, or fluroxypyr. In addition, 2,4-D was detected cal compounds in groundwater. Pesticide mixtures were detected in combination with simazine (a herbicide recommended for in ?3% of the 436 samples collected and only in piezometers use in shelterbelts and small fruit crop production in Alberta, and wells that were £10 m deep. The frequency of detection including in the fall; AAFC, 2013), diazinon (an insecticide of pesticide mixtures increased in the order of spring (1.5%) < recommended for use on green forage in Alberta, including in summer (3.8%) < fall (4.8%). Pesticide mixtures always con- the fall; Alberta Crop Protection Guide, 2015), and the histori- tained either 2,4-D or MCPA, and often other auxin herbicides cal insecticide ethion (deregistered in 2003; Pest Management such as clopyralid and fluroxypyr. About 19% of the 436 samples Regulatory Agency, 2003). contained a single pesticide, with 2,4-D (7.3%), MCPA (4.4%), The weed survey data show that clopyralid is less com- and clopyralid (3.9%) being most frequently detected. These monly used than MCPA and 2,4-D, and this agrees with the auxin herbicides were often not detected in shallower wells and top 10 pesticide sales data available for Alberta (AEP, 2015). piezometers (<5 m) and were sometimes detected in deeper wells Only 7.5% of the 241 fields reported applications of clopy- and piezometers (>10 m), and overall there was no evidence that ralid, with all reported applications being in the summer. The sampling depth was an indicator of the likelihood of pesticide detection of clopyralid with MCPA in the summer (Table 2) occurrence. There was some evidence that pesticide contamina- is not unexpected because it has very high leaching potential tion was possibly induced by local applications (e.g., the 1153 in soils (groundwater ubiquity score = 4.51, Table 1) (Lewis and 7121 ng L−1 of clopyralid detected in one shallow well but et al., 2016). The detection of clopyralid with 2,4-D in the fall not in other shallow wells of that region). The concentrations of (Table 2) is also possible because clopyralid is relatively persistent compounds detected never exceeded their respective individual in field soils with DT90 (days to 90% degradation) values rang- MACs. If Canada is to establish guidelines for pesticide mix- ing from 12 to 79 d. Unlike MCPA and 2,4-D, the frequency of tures, this study concludes that auxin herbicides such as 2,4-D, clopyralid detection was relatively even across seasons, with 33% MCPA, and clopyralid should be priority pesticides. of the total detections occurring in spring, 24% in summer, and 43% in fall. The consistent detection of clopyralid across seasons Supplemental Material could be the result of groundwater contamination in previous The online supplemental material shows pesticides and metab- years because clopyralid is relatively stable in water (Lewis et al., olites analyzed for this study with their recovery percentages. 2016). It could also be a result of localized applications of clopy- ralid in any of the seasons, or from the carryover of clopyralid in Acknowledgments the soil from previous years and subsequent movement of clopy- Alberta Innovates provided research funding for this study. The Natural ralid to groundwater in the year of sampling. Sciences and Engineering Research Council of Canada provided support Table 1 lists the individual MACs established for pesticides for the training of K. Sapkota and S. Munira. Alberta Agriculture and Forestry assisted with the collection of samples, and staff from the under the Canadian Drinking Water Quality Guidelines. Ninety Oldman Watershed Council, Alberta Health Services, and Agriculture percent of the rural population in Alberta relies on groundwater and Agri-Food Canada provided additional support. In particular, Dr. for their potable water supply, and although our findings sug- Ross McQueen, Dr. Rob Gulden, Tara Vucurevich, Monique Dawson, gest that such drinking water consumption could lead to pesti- Jessa Drury, and Aiden Martindale are also gratefully acknowledged for cide exposure, the concentrations observed never exceeded their their contribution to this study. respective individual MACs. However, only 7 of the 16 com- pounds detected have MACs. The establishment of a MAC for References clopyralid is particularly important because this herbicide was AAF. 2003. Water quality for mixing herbicides. Alberta Agric. For. www.agri- culture.alberta.ca (accessed 13 July 2018). among the top three pesticides detected in Alberta groundwater 1466 Journal of Environmental Quality AAF. 2015a. Alberta crop production statistics. Alberta Agric. For. https:// Helling, C.S., and B.C. Turner. 1968. 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