Pesticide Fate and Occurrence on Urban Hard Surfaces

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Chapter 4

Pesticide Fate and Occurrence on Urban Hard Surfaces Zachary Cryder,*,1 Jaben Richards,1,2 and Jay Gan1 1Department

of Environmental Sciences, University of California, Riverside, 900 University Avenue, Riverside, California 92521, United States 2State of New Mexico Environment Department, Department of Energy Oversight Bureau, 1183 Diamond Drive, Los Alamos, New Mexico 87544, United States *E-mail: [email protected].

Pesticides are applied in urban environments for control of various nuisance species. Urban pest control compounds are typically applied outdoors directly onto impervious surfaces such as concrete or asphalt. These surfaces are often covered with a thin layer of urban dust, a complex mixture of particles and environmental contaminants that emanate from abraded impervious surfaces or that are transported onsite by weather events. It is vital to understand the behavior of urban pesticides on concrete surfaces and in urban dust particles to mitigate the offsite transport of these compounds via surface runoff. A growing body of research indicates that dust particles and concrete matrices can facilitate the transformation of pesticide active ingredients. In addition, these media seem to serve an important role in the transport of pesticides away from their sites of application.

© 2019 American Chemical Society Goh et al.; Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment, and Management ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

Introduction Pesticides are utilized frequently in urban environments for the control of a myriad of indoor and outdoor pest insect species. Urban pest control products tend to target ants, termites, roaches, spiders, and other invertebrate organisms that may compromise structures, damage landscape features, pose a human health risk, or act as a general nuisance. Indoor pesticide uses involve direct pest elimination using consumer spray products, insect bait stations, and veterinary flea medication. Pesticides are applied outdoors for purposes such as landscape preservation and structural pest control. Common outdoor pesticide formulation types include liquid solutions that are applied as sprays, granular powders, or viscous gels. Regardless of the formulation type employed, outdoor use of these biologically active compounds poses a risk for sensitive nontarget organisms, especially other invertebrates, following the offsite transport of pesticide residues. Outdoor pesticide use is particularly concerning as these residues are transported offsite in storm water that is rarely treated prior to assimilation into surface water systems. Furthermore, the high incidence of impervious surfaces in urban environments ensures rapid, efficient transport of contaminated storm water directly into surface water. Impervious surfaces, by definition, prevent water from infiltrating into underlying soils and therefore enable rapid offsite movement of runoff water following irrigation or storm events (1). This is a desirable feature for flood prevention measures in urban environments but raises concerns regarding the quality of runoff water. Under predevelopment conditions, there is significant infiltration of runoff into soil matrices, so transport of anthropogenic compounds to surface water is less of a concern. However, developed urban areas exhibit a high incidence of impervious surfaces, accounting for approximately 50% and 90% of residential and commercial surface areas, respectively (1). The offsite transport of contaminants in urban runoff will only intensify as a result of rampant urban expansion, which is estimated to triple the worldwide urban land area by 2030 (2). In addition, the increasing human population will encourage still greater levels of urbanization. An increase in the coverage of impervious surfaces and an expansion of the human population will combine to increase the existing high rates of urban pesticide use and the resultant wash-off of their residues in urban runoff. Therefore, it is necessary to understand the occurrence, partitioning, and transformation of pesticides on urban hard surfaces. It is also vital to investigate the role played by urban dust on the impervious surfaces, since pest control compounds are often applied directly onto concrete surfaces and the layer of urban dust covering them. Dust particles may be transported by weather events and human activity (e.g., traffic), making it difficult to ascertain the origin of these particles. Similarly, dust particles in urban environments that are treated with, and contaminated by, pesticide residues may be carried offsite by the aforementioned mechanisms. Due to the ease with which urban dust can be transported in urban environments, pesticide occurrence, partitioning, and transformation in this compartment must be examined. Knowledge regarding pesticide behavior on impervious surfaces and in urban dust will allow for the pursuit of more practical and effective runoff mitigation 44 Goh et al.; Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment, and Management ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

strategies. Reducing pesticide concentrations in urban runoff is desirable not only to alleviate toxicity to nontarget organisms but also to improve surface water quality in general. This chapter provides background information on pesticides that are typically applied in urban environments, discusses their fate on concrete surfaces, and examines the role of urban dust particles in their offsite transport. Fipronil, pyrethroids, and neonicotinoids are pesticides that are commonly utilized in urban environments for outdoor pest control purposes, and they are the primary pesticides of interest in this review.

Common Urban Pesticides Fipronil Fipronil (5-amino-1-[2,6-dichloro-4-(trifluoromethyl) phenyl]-4-(trifluoro methyl sulfinyl) pyrazole-3-carbonitrile) is a phenylpyrazole pesticide used by licensed pest management professionals to combat infestations of a multitude of pest insects in a variety of contexts. When the use of many organophosphate and carbamate pesticides was limited in the late 1990s—due to toxicity concerns in humans and aquatic organisms—fipronil, as a newer generation pesticide, became one of the replacement compounds of choice for urban pest control (3). Applications of fipronil include perimeter treatments of structures to manage pests such as ants and termites, underground foundation injections, topical veterinary flea and tick treatments, impervious surface crack and crevice gel treatments, insect control baits, and landscape maintenance (Table 1) (4–7). The Pesticide Use Reporting database provided by the California Department of Pesticide Regulation states that more than 24,000 kg of fipronil active ingredient were applied in California during 2016 (8). Application of fipronil products in California, which is predominantly performed in Southern California where the majority of the population resides, is constrained to urban uses, making the role of impervious surfaces and urban dust a significant consideration. The insecticidal activity of fipronil originates from its inhibitory activity in the central nervous system. Specifically, fipronil interacts with γ-aminobutyric acid (GABA)-gated chloride channels in the cell membranes of neurons, interrupting signal transmission in the nervous system and ultimately causing neural hyperexcitation, paralysis, and mortality (9–12). This mechanism of action makes fipronil highly effective against pest organisms. However, the same biological activity that makes fipronil commercially invaluable also places nontarget invertebrates at risk once it is transported to surface water via runoff. After application in the urban environment, fipronil breaks down to form a series of degradation products. The primary degradation products are fipronil desulfinyl, fipronil sulfide, and fipronil sulfone, which are formed following photolysis, reduction, and oxidation, respectively (13). Fipronil and these degradation products are collectively referred to as fiproles. Fipronil degradates have been shown to exhibit toxicity that is equal to or greater in magnitude than that of the parent compound to nontarget species (14–17). Therefore, it is essential to include these degradates in the evaluation of fipronil fate and transport. 45 Goh et al.; Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment, and Management ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

Table 1. Summary of urban pesticide properties and use Compound

Log KOW

Urban Use (a.i.)

Urban Applications

Fipronil

3.9–4.1 (18)

>24,000 kg in 2016 (8)

Structural pest control, topical flea and tick treatments, insect control baits, landscape maintenance (4–7)

Pyrethroids

5.7–7.6 (31)

>148,000 kg in 2016 (8)

Structural pest control, consumer pesticide products (26)

>35,000 kg of imidacloprid in 2016 (8)

Landscape maintenance, structural pest control, indoor pest control, flea and tick treatment products, included in various manufactured products (38–41)

Neonicotinoids

−0.66–1.26 (37)

Fiproles have been frequently observed in surface water as a result of outdoor urban applications. Fipronil has a log KOW value of 3.9–4.1 and is thus considered a moderately hydrophobic compound (Table 1) (18). This property results in adsorption of fiproles to urban media such as concrete and urban dust, but also allows for release of these compounds into urban runoff water. An investigation of fipronil runoff from concrete surfaces revealed that 81.1–96.7% of the applied fipronil mass was in the aqueous phase of runoff samples (19). This highlights the fact that fipronil, and likely its degradation products, is easily mobilized from urban surfaces during runoff events. Several monitoring studies have further confirmed the offsite transport of fiproles. For example, in a study of urban runoff collected from residential storm drain outfalls in Southern and Northern California, median total fiprole concentrations were 204–441 ng L-1 and 13.8–20.4 ng L-1, respectively (3). In the same study, 90th percentile total fiprole concentrations were 338–1169 ng L-1 and 62.6–65.3 ng L-1 for Southern and Northern California sites, respectively (3). A 2016 study monitoring urban creeks, rivers, and storm drain outfalls conducted by the California Department of Pesticide Regulation found fipronil sulfide, fipronil sulfone, fipronil desulfinyl, and fipronil in 8%, 63%, 65%, and 75% of samples, respectively (20). To understand the significance of this surface water data, it is necessary to also examine fiprole toxicity thresholds. A variety of nontarget aquatic organisms have been shown to be sensitive to the adverse effects posed by exposure to fiproles. Several toxicity thresholds have been determined for fipronil, including LC50 values of 320 ng L−1 for grass shrimp (Palaemonetes pugio), 140 ng L−1 for mysid shrimp (Neomysis americana), and 180–310 ng L−1 for the black fly (Simulium vittutum) (21–24). The most sensitive species studied so far is Chironomus dilutus, which exhibited mean 96 h EC50 values of 32.5 ng L−1 and 7-10 ng L−1 for fipronil and its degradation products, respectively (14). Comparing these toxicity values with the representative surface 46 Goh et al.; Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment, and Management ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

water concentrations given above highlights the need to mitigate runoff of fiproles from urban sites of application. Pyrethroids Synthetic pyrethroid insecticides are extensively used in a wide range of urban applications because they satisfy the majority of the residential pest control needs that were previously fulfilled by the organophosphate insecticides, before their use was restricted (25). Pyrethroids and fipronil thus became the major replacements for the phased-out organophosphates, with the former group seeing more widespread use. Pyrethroids that are used frequently include bifenthrin, cyfluthrin, deltamethrin, esfenvalerate, lambda-cyhalothrin, permethrin, and cypermethrin. The pyrethroids are utilized by professionals primarily for structural pest control and by homeowners for several types of household applications (Table 1) (26). In fact, most retail insecticide products available to the general public contain one or more pyrethroid active ingredients (25, 26). A 2010 survey of available pesticide products in California home improvement stores revealed that 46% of all insecticidal products contained pyrethroids (27). As a group, professional use rates of pyrethroid pesticides is high. During 2016 in California, pest management professionals applied over 148,000 kg of total pyrethroids for urban pest control (8). To put this number in perspective, it is estimated that the pyrethroids account for up to 74% of pesticide use in urban environments (28). It is noteworthy that the California Department of Pesticide Regulation’s Pesticide Use Reporting database only supplies a record of professional pesticide use in the state. Homeowner applications of products containing pyrethroids are not recorded and are difficult to estimate. Due to the fact that pyrethroids are the primary active ingredients in retail pesticide products, it is probable that the actual mass of applied pyrethroids in urban areas is much higher (29). The toxicity of pyrethroids is a result of the interaction of these compounds with voltage-gated sodium channels in the central nervous systems of sensitive organisms (30). Pyrethroids bind to these channels and produce hyperexcitation of the central nervous system, ultimately resulting in death of the organism (30). This mechanism of toxicity makes the pyrethroids effective pesticides but also subjects sensitive aquatic organisms to adverse effects following transport of these compounds from urban environments. Some aquatic organisms are extremely sensitive to pyrethroid exposure, making these insecticides potent environmental toxicants despite their hydrophobicity. The pyrethroids have log KOW values of 5.7–7.6, making them some of the most hydrophobic anthropogenic contaminants (Table 1) (31). This hydrophobicity results in the majority of pyrethroid residues being adsorbed to solid phases like sediment following transport to surface water in urban runoff. However, aqueous concentrations of these compounds in surface water may be still sufficiently high to cause toxicity in organisms residing in the water column. The widespread application of pyrethroid insecticides for outdoor urban purposes has led to their frequent detection in urban runoff and surface water. For instance, one study that monitored urban storm drain outfalls detected 47 Goh et al.; Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment, and Management ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

pyrethroids in 100% of water samples collected (26). In the same study, bifenthrin was detected in 96% of water samples at a median concentration of 5–17 ng L−1 and a maximum concentration of 73 ng L−1 (26). In addition, the authors observed maximum concentrations of 23 ng L−1, 125 ng L−1, and 26 ng L−1 for cyfluthrin, permethrin, and cypermethrin, respectively (26). A study of surface water samples collected from tributaries, drains, and pump stations providing input to the American River as well as samples from the river itself in Northern California showed bifenthrin, cyfluthrin, cypermethrin, and permethrin at maximum aqueous concentrations of 106.4 ng L−1, 20.5 ng L−1, 9.4 ng L−1, and 21.1 ng L−1, respectively (32). A more recent monitoring study investigating the occurrence of pesticides in urban creeks, rivers, and storm drain outfalls in Southern California showed bifenthrin, cyfluthrin, and permethrin in 80%, 40%, and 43% of water samples, respectively (20). The levels of pyrethroids in urban runoff are sufficient to induce toxic effects in sensitive aquatic invertebrates. There is substantial evidence in the literature that pyrethroids are extremely toxic to some sensitive aquatic species found in urban streams. One such species is the amphipod Hyalella azteca, for which multiple sets of toxicity assays have been performed. For bifenthrin, an extensively used pyrethroid, the EC50 for this amphipod was only 3.3 ng L−1 (33). Additional measurements for Hyalella azteca have established 96 h LC50 of 21.1 ng L−1 for permethrin and EC50 of 2.3 ng L−1 for lambda-cyhalothrin (34, 35). These exceedingly low values are exacerbated by evidence of additive pyrethroid toxicity (36). Contaminated surface water contains a complex mixture of pollutants, including a variety of pyrethroid insecticides. The environmental levels of commonly used pyrethroids would indicate that resident invertebrates may experience adverse effects because of exposure to individual pyrethroid compounds. However, the actual response in such organisms is likely greater in magnitude due to additive toxicity, wherein individual pyrethroids contribute to increase the overall toxicity effects.

Neonicotinoids Neonicotinoid insecticides are well known for their use in agricultural operations, primarily as soil and seed treatments for the protection of seedling plants. However, they are also frequently used for the management of urban pests (37). One of the more popular neonicotinoid pesticides, imidacloprid, has many outdoor urban applications, including landscape maintenance, structural pest control, underground termite injections, and flea and tick treatments for domestic animals (Table 1) (38, 39). Imidacloprid is also incorporated into a variety of manufactured products, which may release imidacloprid residues during the process of weathering or degradation. Such materials include polystyrene insulation, adhesives, vinyl siding, outdoor textiles, sealants, and wood-decking (39–41). According to the Pesticide Use Reporting database maintained by the California Department of Pesticide Regulation, more than 35,000 kg of imidacloprid were applied for landscape maintenance and structural pest control by pest management professionals in California in 2016 (8). For perspective, this is greater than the total amount of fipronil use in the same year (8). Other 48 Goh et al.; Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment, and Management ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

neonicotinoids that have seen wide adoption include acetamiprid, clothianidin, dinotefuran, nitenpyram, thiacloprid, and thiamethoxam (37). The mechanism of action by which neonicotinoids exert adverse effects involves their agonistic binding to nicotinic acetylcholine receptors in the central nervous systems of invertebrates, where they compete with the endogenous neurotransmitter acetylcholine for binding sites (37). Toxicity studies with arthropods have indicated that neonicotinoid binding to nicotinic acetylcholine receptors is long-lasting, a property that causes delayed and cumulative effects over time under chronic exposure conditions (42–44). The nature of this neurotoxic activity places sensitive nontarget invertebrates at risk following offsite transport of neonicotinoid insecticides, even when exposed to low concentrations over extended periods of time. Runoff transport of these compounds is facilitated by their hydrophilicity. Acetamiprid, clothianidin, dinotefuran, imidacloprid, nitenpyram, thiacloprid, and thiamethoxam have log KOW values of 0.80, 0.91, −0.55, 0.57, −0.66, 1.26, and −0.13, respectively (Table 1) (37). This property of neonicotinoids suggests that most neonicotinoid residues transported to surface water in runoff will be in the dissolved form, and hence bioavailable, in the water column. Extensive outdoor urban use of neonicotinoids, imidacloprid in particular, has resulted in their detection in surface water following runoff events. A study of pesticide occurrence in California urban waters revealed that imidacloprid was the second most frequently detected compound (45). The same study also showed that the median and maximum concentrations of imidacloprid during the dry season were 50 ng L−1 and 160 ng L−1, respectively, with a corresponding maximum storm season concentration of 670 ng L−1 (45). Another study showed imidacloprid in 73% of water samples collected from urban creeks, rivers, and storm drain outfalls in Southern California (20). It is apparent that application of neonicotinoid insecticides in outdoor urban environments has resulted in their transport to surface water, posing a potential risk to aquatic organisms. Toxicity studies with aquatic invertebrates suggest that concentrations of neonicotinoids in surface water are a cause for concern. The U.S. Environmental Protection Agency has recently established an aquatic life benchmark of 10 ng L−1 for imidacloprid based on chronic exposure experiments with invertebrate species (46). This benchmark value agrees with recent findings that suggest that invertebrate species are very sensitive to imidacloprid exposure. Mayflies, for example, experience sublethal adverse effects such as immobilization when exposed to imidacloprid concentrations as low as 100 ng L−1 (37). Mayfly nymphs are even more sensitive, with an EC10 value of 30 ng L−1 to achieve the toxicity endpoint of immobilization when exposed to imidacloprid (47). The European Union recently established a more stringent benchmark concentration for imidacloprid through consideration of a species sensitivity distribution, and the predicted no-effect concentration generated from this assessment was only 4.8 ng L−1 (48). It is clear that surface water concentrations of imidacloprid are sufficient to elicit adverse effects from the most sensitive aquatic organisms.

49 Goh et al.; Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment, and Management ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

Pesticide Fate and Occurrence on Concrete Fipronil Several studies have examined the behavior of fipronil and its primary degradation products on impervious concrete surfaces in urban environments. For example, a recent study investigating the transformation of urban use pesticides on concrete discovered that these impervious surfaces facilitated rapid abiotic conversion of pesticides to their biologically active degradation products (49). Once applied onto concrete surfaces, fipronil degraded to form primarily fipronil desulfinyl and fipronil sulfone via photolytic and oxidation processes, respectively (49). The three major fipronil degradation products—fipronil desulfinyl, fipronil sulfide, and fipronil sulfone—were all detected in runoff water emanating from the treated concrete surfaces 1 day after application (49). Significantly, transformation products of fipronil were mainly present in the dissolved phase of runoff water, indicating that fiproles washed off from treated concrete surfaces are relatively mobile and bioavailable (18, 49). The authors hypothesized that the extensive transformation of pesticides applied to concrete surfaces was likely caused by exposure to the alkaline environment of the concrete matrix, the presence of metal oxides in the concrete, and outdoor conditions conducive to extensive photolysis (49). Concrete is alkaline due to the curing and hydration processes, which introduce a complex interior pore system coated with Ca(OH)2 (49). Impervious surfaces therefore should be considered as reactive toward certain pesticides (49). A study of the washoff of pesticides from concrete surfaces shed light on the persistence of fiproles within such matrices. The washable fraction of a compound describes how much of the applied pesticide mass is available for runoff transport following a specific contact time. After application on concrete surfaces, the washable fraction of fipronil decreased quickly (half-life = 3.3 days) up to 7 days, which was followed by a much slower decline (half-life = 12.2 days) from 14 to 116 days (50). The initial rapid decrease in washoff potential was likely caused by irreversible retention of fipronil residues in the porous interior of the concrete matrix (50). The slower subsequent release might be attributed to the possibility that a fraction of pesticide residues was located below the concrete surface and became relatively isolated from degradation or volatilization (50). The results imply that fipronil residues remain on concrete for extended periods of time after application, and that fiproles may continually contaminate surface water via runoff (50). Another study focused on the runoff of urban pesticides from concrete surfaces was carried out using a series of 1-h simulated rainfalls of varying intensity (25 or 50 mm h−1) following the treatment of a fipronil suspension concentrate formulation (51). Surface runoff samples were analyzed for fipronil and its degradates (51). The maximum fipronil concentration (143.3 µg L−1) was detected in the initial fraction of runoff water collected from the first rainfall event after a 1-day contact time (51). In addition, concentrations of fipronil in subsequent runoff events were still relatively high, with 7-day and 14-day runoff concentrations of 98.2 µg L−1 and 12.0–31.0 µg L−1, respectively (51). Fipronil sulfone and fipronil desulfinyl were detected in the washoff after 1 day and were 50 Goh et al.; Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment, and Management ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

also detected in all runoff samples collected throughout the course of the study (51). The results of this study provide further evidence that fipronil is available for runoff long after application on concrete surfaces. Moreover, the parent compound is quickly transformed to biologically active degradation products, which should also be considered when developing practical mitigation strategies. Pyrethroids Due to their widespread use by both professional applicators and homeowners, the behavior of pyrethroid insecticides on concrete surfaces has been more extensively studied than fipronil. A study of pesticide degradation on concrete surfaces showed that permethrin was rapidly degraded to the endocrine disrupting compound 3-phenoxybenzoic acid (3-PBA) while in contact with concrete matrices (49). This degradation process is of concern due to the biological effects elicited by 3-PBA in sensitive organisms. It is thus important to not only study the fate of pyrethroid insecticides on concrete but the fate of 3-PBA as well. After 1 day of contact following application of permethrin on concrete, 3-PBA was detected in runoff samples (49). This biologically active degradation product continued to be present at relatively constant concentrations until 89 days (49). These results highlight the complex and reactive nature of concrete surfaces that encourage retention, transformation, and release of pyrethroid insecticides and their metabolites in urban runoff long after initial product application. An in-depth study on the runoff potential of the pyrethroid insecticides bifenthrin and permethrin (cis and trans isomers) considered the concentrations of these compounds in runoff water from concrete surfaces exposed to outdoor hot and dry summer conditions for several months. These pyrethroids were detected in runoff 1 day after treatment at concentrations of 82 µg L−1, 5143 µg L−1, and 5518 µg L−1 for bifenthrin, cis-permethrin, and trans-permethrin, respectively (52). These high initial runoff concentrations were above their solubility limits (31, 52), and it was probable that the aqueous solubility of these compounds was enhanced by surfactants present in the professional pesticide formulations and by fine particles (31, 53). Fine particles, due to their large surface areas, were likely enriched in pyrethroid residues (54). Following their high initial runoff concentrations, bifenthrin and permethrin were detected in runoff samples even after 221 days, although at much lower concentrations (0.15–0.17 µg L−1 and 0.75–1.15 µg L−1, respectively) (52). This finding suggests that concrete may act as a reservoir for applied pesticides, with some of the residues slowly becoming desorbed during rainfall or irrigation-induced runoff (52). Of the cumulative runoff loss, the loss via the 1-day washoff accounted for 83.3%, 93.1%, and 90.8% of bifenthrin, cis-permethrin, and trans-permethrin, respectively (52). To put these numbers in perspective, only 0.88%, 0.30%, and 0.44% of the cumulative pesticide mass lost in runoff for bifenthrin, cis-permethrin, and trans-permethrin, respectively, was due to runoff after the fourth consecutive washoff event (52). To summarize, the vast majority of pyrethroid residues that were washed off during the course of the experiment were released in the first flush event, with much lower mass fractions being released after repeated simulated rainfall events. However, only a small proportion of the mass applied for each pesticide was 51 Goh et al.; Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment, and Management ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

actually washed off over the course of the entire study. Due to the tendency for the majority of residues in runoff loss to occur shortly after the treatment, it would be critical not to apply pesticides if a rain event was in the forecast or an irrigation event was scheduled (52). Such a practice may help to reduce surface water contamination by reducing the concentration of pesticides available for runoff in the first place. Another study on concrete surfaces further examined the occurrence of pyrethroids in runoff (50). The authors sought information regarding the washable fraction of applied insecticides over time, defined as the percent mass washed off relative to the initially applied mass. After application of liquid formulations on concrete, the washable fractions of bifenthrin, permethrin, cyfluthrin, and lambda-cyhalothrin within the first 7 days of the study decreased rapidly, with half-lives of 0.9 day, 1.1 day, 3 days, and 1.1 days, respectively (50). The rapid decrease was likely a result of irreversible sorption of pyrethroid residues to the porous concrete subsurface (50). A second, slower kinetic transfer phase followed, with half-lives of 25.4 days, 15.2 days, 66.6 days, and 6.2 days for bifenthrin, permethrin, cyfluthrin, and lambda-cyhalothrin, respectively (50). This slower phase likely existed due to the long-term, subsurface retention of pyrethroid residues that were protected from abiotic degradation and volatilization (50). The authors also monitored runoff of solid pyrethroid formulations and found that the washoff potential of granular and powder formulations was consistently higher than the washoff of liquid formulations (50). Interestingly, pesticide residues were still detectable in runoff 112 days after treatment, and traces of pyrethroids continued to be present in washoff after 14 repeated washing-drying cycles over 42 days (50). This finding indicates that pyrethroids reside in concrete matrices over long periods of time even if the surfaces are subjected to repeated washoff events. In other words, pyrethroid pesticides applied on concrete surfaces can potentially be transported offsite via runoff for many months after application. A detailed exploration of the mechanisms of pyrethroid degradation on concrete surfaces quantified the contribution of volatilization, binding/ hydrolysis, and photolysis to the overall loss of permethrin, lambda-cyhalothrin, deltamethrin, fenpropathrin, and esfenvalerate on the concrete surface exposed to natural sunlight (55). On concrete surfaces irradiated with sunlight, permethrin was the most stable compound with a half-life of 30 h, as compared to a half-life of 3 h for lambda-cyhalothrin (55). Despite the relative stability of permethrin on irradiated concrete, photolysis was found to be the dominant loss pathway for this compound, accounting for 75% of its total loss over the course of the experiment (55). It is important to note that the properties of the concrete surfaces did not inhibit photolysis, since the first-order degradation rate constants for photolysis on concrete and glass surfaces were similar (55). Binding/hydrolysis was the most significant loss pathway for fenpropathrin, lambda-cyhalothrin, esfenvalerate, and deltamethrin (55). These results provided more evidence that concrete matrices act as reactive surfaces for pesticides, with long-term binding and transformation playing vital roles in their persistence and off-site transport. Since pyrethroids may reside in concrete matrices for a prolonged time and remain available for contaminating runoff, it is important to understand their sorption to these impervious surfaces. One study examined the sorption 52 Goh et al.; Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment, and Management ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

of 14C-permethrin onto concrete and the subsequent desorption into water after different contact times (56). Sorption was rapid and was followed by a two-phase desorption over the 300-h experiment: an initial rapid phase and a prolonged slower phase (56). This was in agreement with the two-phase washoff observed for pyrethroids from concrete surfaces (50). The degree of permethrin desorption over 300 h depended on the contact time prior to the initiation of desorption, and 56.2% was desorbed immediately after spiking; 34.1% and 23.7% of the applied mass was desorbed 1 day and 7 days after spiking, respectively (56). The authors attributed the decreased desorption over time to decomposition of permethrin on the alkaline concrete as well as irreversible sorption; more than 20% of the applied mass remained in the concrete matrix as bound residues at the conclusion of the experiment (56). The findings from these experiments collectively suggest that runoff concentrations of pyrethroids will be the highest right after application and will gradually decrease as time increases after the treatment. However, these contaminants will likely continue to appear in runoff, albeit at decreasing levels, posing a potential risk to sensitive aquatic organisms in the downstream surface water.

Neonicotinoids Relative to fipronil and the synthetic pyrethroids, the fate of neonicotinoids on concrete surfaces has not been extensively studied. However, a recent study did consider the surface runoff of imidacloprid emulsifiable concentrate from concrete surfaces during 1 h rainfall simulations of varying intensity (25 or 50 mm h−1) following contact times of 1.5 h, 1 day, 7 days, and 14 days (50). At 1.5 h after application, approximately 57.3% of the applied imidacloprid mass was washed off from concrete surfaces, corresponding to an event mean concentration of 392.0 µg L−1 (51). Unsurprisingly, the maximum pesticide concentrations were detected in the initial runoff fractions of the first simulated rainfall events, corresponding to a concentration of 3267.8 µg L−1 for imidacloprid (51). However, imidacloprid was not persistent on concrete surfaces and became undetectable in the washoff water after 7 days (51). It is likely that the absence of imidacloprid in the runoff water after 7 days was primarily due to photolytic degradation on the concrete because the insecticide has a photodegradation half-life of less than 1 h in water (51). Alternatively, hydrolysis could have occurred under the alkaline conditions of the concrete matrix (51). The study showed that the initial imidacloprid concentrations in runoff from concrete surfaces may reach toxicologically concerning levels. However, the adverse effect of imidacloprid may dissipate quickly due to its short persistence. More information regarding the occurrence and transformation of imidacloprid and other neonicotinoids on concrete surfaces is required to better understand the contribution of urban impervious surfaces to their occurrence in downstream surface aquatic ecosystems.


Pesticide Fate and Occurrence in Outdoor Urban Dust Fipronil A complete understanding of the fate and occurrence of urban-use pesticides on concrete surfaces requires a complementary consideration of pesticide fate in urban dust. Urban dust refers to the fine particles residing on impervious surfaces in outdoor urban environments. The composition of this mixture of solids is complex and heterogeneous, containing broken asphalt and concrete particles as well as dust, soil, and plant debris transported by wind, atmospheric deposition, and/or rainfall (57). In a study on the occurrence and distribution of pesticides in urban dust during hot summer months, when heavy pesticide application occurs to combat rampant pest pressures, 360 particle samples were collected and analyzed for a suite of urban-use insecticides, including fipronil and its degradation products (57). Insecticides were detected in 99.4% of the dust samples, with 75.8% containing 5 or more of the target compounds (57). A particularly interesting finding was that fiprole dust concentrations were higher than concentrations of fiproles in the sediment of urban streams; this is likely due to the moderate water solubility of fipronil, allowing residues to desorb from dust particles and partition into the overlaying water (57, 58). Another study examining urban dust samples collected from curbside gutters, sidewalks, and street surfaces in Southern California showed presence of fiproles in approximately 50% of the samples (59). The maximum fiprole concentrations ranged from 2.5 to 858.9 µg kg−1 (59). These results show that moderately hydrophobic pesticides like fipronil have the potential to accumulate and become enriched in loose particles on impervious surfaces, indicating that urban dust particles are an important source of pesticide residues in runoff since they are easily transported in irrigation or rain water (57). Another study on urban dust focused on the spatial distribution of pesticideladen dust particles on residential driveways, gutters, and streets (54). Fiproles were detected in 50.6–75.5% of the dust samples throughout the course of the study, and the spatial and temporal patterns of fiprole occurrence suggested that fipronil was rapidly transformed to its degradates (54). The median concentrations of fiproles in dust were 1 to 2 ng g-1, as compared to maximum concentrations of 1069–6188 ng g−1 (54). Fiprole dust concentrations were shown to be significantly higher in driveway samples, near the sites of fipronil application, as compared to those collected at the gutter or street (54). In California, at the time of this study, fipronil products could only be legally applied 0.3 m up and 0.3 m out (the current label only allows legal application 0.15 m up and 0.15 m out) around a structure, suggesting that fipronil was highly susceptible to abiotic transformation prior to movement from sites of application on driveways (54). Another important finding was that fiprole concentrations increased with decreasing particle size fractions of dust (54). The finest dust particles that are the most vulnerable for offsite transport via wind or water are thus the most contaminated with fiproles.


Pyrethroids As is the case with fipronil and its degradation products, urban dust particles have been shown to be important in the environmental fate of pyrethroid insecticides. In a study on pesticide occurrence in urban dust during the summer months in Southern California, 360 dust samples were collected and analyzed for the presence of the commonly used pyrethroids, including fenpropathrin, lambda-cyhalothrin, bifenthrin, cis-permethrin, trans-permethrin, cyfluthrin, cypermethrin, esfenvalerate, and deltamethrin (57). Pyrethroids had the highest detection frequency among the compounds monitored, especially bifenthrin, cyfluthrin, and permethrin (57). For example, bifenthrin was detected in 97.5% of the samples at concentrations up to 6205 ng m−2 (57). This study showed that the majority (88%) of dust particle samples contained pyrethroids at concentrations of approximately 5–500 ng g−1, which is the same mean concentration range of pyrethroids in sediment samples collected from eight urban creeks (57, 58). This implies that pyrethroid-contaminated dust particles are transported offsite into urban surface water systems. Similarly, analysis of runoff samples collected after simulated rainfall on concrete surfaces showed that greater than 80% of pyrethroids present in runoff were associated with particles greater than 0.7 µm (60). The authors concluded that particles contaminated with pesticides likely originated from dust particles that were already present on the concrete surface prior to the pesticide treatment (60). These studies together provide strong evidence that dust particles on urban impervious surfaces such as concrete are an important source for runoff contamination of hydrophobic pesticides such as the pyrethroids, as these compounds have a high affinity for fine solid particles (57). In a recent study, the spatial distribution of pyrethroid residues on pavements around residential urban environments was examined. Dust samples were collected from gutters, sidewalks, and street surfaces from 40 homes in Southern California (59). Pesticides were detected frequently in these samples, with a median total analyte concentration of 85 µg kg−1 (59). Two compounds, bifenthrin and permethrin, accounted for 55% of the total detected pesticides of interest, and 75% of samples contained at least five target compounds (59). Bifenthrin was detected in nearly every sample, at concentrations up to 6408 µg kg-1 (59). Other frequently detected compounds included cis-permethrin, trans-permethrin, cyfluthrin, and cypermethrin (59). A similar study showed that eight pyrethroid compounds were detected in 53.5–94.8% of urban dust samples collected from driveways, gutters, and streets in residential areas in Southern California (54). Median concentrations of these pyrethroids were 1–46 ng g−1, with maximum concentrations of 523–8852 ng g−1 (54). Permethrin and bifenthrin were the most commonly detected pyrethroids, found in 94.8% and 92.7% of samples, respectively (54). An interesting finding was that pyrethroids were distributed relatively uniformly in areas adjacent to residences, suggesting that significant redistribution of dust particles had taken place (54). These studies together demonstrated that pyrethroids are ubiquitously present in urban dust particles, suggesting that this medium likely serves as an important avenue for offsite transport of pyrethroids used in residential areas. 55 Goh et al.; Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment, and Management ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

Neonicotinoids Information regarding the fate of urban-use neonicotinoids, such as imidacloprid in urban dust, is currently lacking. However, studies investigating the sorption of imidacloprid on soil particles may provide some insight into the potential behavior of this compound in dust particles. One study showed that soil organic matter was the most important component affecting the sorption of imidacloprid in soils (61). Another study validated this finding, suggesting that the sorption of imidacloprid on humic acids was much greater than sorption to calcium-clay minerals (62). In addition, a major metabolite of imidacloprid—imidacloprid-urea—competed for soil sorption sites with the parent compound, inhibiting its sorption on clay minerals (62). These studies establish that organic matter is an important factor in regulating the sorption of imidacloprid in soils, as is the case for many other pesticides. Since urban dust may contain both soil particles and plant debris, it is likely that urban dust has a relatively high organic matter content. Therefore, it is possible that imidacloprid and its major metabolite adsorb readily to dust particles following application on concrete surfaces. However, it is unclear how much of the contaminated dust particles may contribute to the offsite transport of imidacloprid in runoff. It is probable that the direct washoff of imidacloprid from concrete surfaces would contribute the most to its runoff loads due to the hydrophilicity of neonicotinoids. It is necessary to experimentally measure the occurrence and distribution of imidacloprid in urban dust particles in order to better assess the importance of loose dust on pavement in its contamination of downstream surface water.

Conclusions This review has provided evidence to reinforce the assertion that impervious surfaces such as concrete, and dust particles residing on such hard surfaces, play important roles in the fate of pesticides used in urban environments. The most common insecticides applied in such environments are fipronil, pyrethroids, and neonicotinoids. The fate and occurrence of most of these compounds on concrete and urban dust has been studied to some extent, with the exception of neonicotinoids. Research thus far has shown that concrete matrices are capable of serving as long-term reservoirs of hydrophobic pesticides and that they are capable of facilitating offsite runoff transport of urban-use pesticides for prolonged periods of time after application. These compounds can also be enriched in urban dust particles, allowing for their transport via wind and/or runoff water. Further research is needed to ascertain the precise contributions of concrete surfaces and urban dust to runoff loads. It is equally important to evaluate the effects of mitigation strategies on the levels of fiproles, pyrethroids, and neonicotinoids in urban runoff.


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Pesticide Fate and Occurrence on Urban Hard Surfaces

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