The data presented clearly demonstrate the efficacy of CWs for treatment of water containing current pollutants that either resist treatment in WWTPs or are directly discharged into surface water via runoff. There are several types and variations of CWs that have been utilized to facilitate treatment of a wide array of contaminants in an abundance of applications. The two major types of treatment wetlands are surface flow CWs and subsurface flow CWs. An example of a surface flow CW is shown in Figure 1.3. Surface flow CWs may contain floating plants, submerged plants, emergent plants, or some combination of the three, and possess areas of open water. Subsurface flow CWs may employ horizontal flow or vertical flow; the former usually consists of a vegetated gravel bed with water flowing horizontally below the bed surface from inlet to outlet while the latter involves water treatment as it percolates through the plant root zone of a vegetated sand or gravel bed. Hybrid CWs are typically composed of a surface flow CW and subsurface flow CW operated in series. Recently, unit process CWs have been implemented in an effort to isolate the ideal conditions necessary to treat particular pollutants in independent ponds, with the goal being to operate distinct ponds in series to treat multiple contaminant classes within the same treatment chain. Examples of unit process CWs are open water cells, macrophyte-dominated wetland cells, and bivalve filtration wetland cells. An example of a unit process open water CW is depicted in Figure 1.4. CW treatment is usually the result of a combination of abiotic degradation,flood tray microbial degradation, sorption, and phytoremediation, depending on the specific contaminant and the design of the CW cell. 

There has been some research focused on the treatment of fiproles and pyrethroids by CWs in urban and agricultural settings. A recent study of a treatment wetland receiving treated effluent from a WWTP found that the CW removed 44% of fipronil and 47% of total fiproles. Another study of an agricultural drainage wetland observed removal rates of 52-94% for pyrethroids. It is clear that CWs have the potential to remove urban insecticides such as fiproles and pyrethroids from surface water, but the precise mechanisms and ideal conditions for their treatment remain unclear. Wetlands are among the most biologically productive ecosystems onEarth, allowing them to transform many ubiquitous pollutants at relatively low cost. For these reasons, CWs present an ideal option for the treatment of fiproles and pyrethroids in surface water. Stormwater and wastewater effluent are contaminating urban streams with toxicologically relevant concentrations of urban-use insecticides. It is essential to study novel water treatment strategies to improve the quality of surface water that has been polluted by storm water and wastewater discharge. Alternatively, identifying sources of urban insecticides and modifying their application practices may serve to reduce the mass loadings of these compounds in surface runoff. If water quality is not improved, sensitive aquatic organisms will continue to encounter adverse effects, potentially causing detrimental effects on ecosystem services and local food webs. Furthermore, water reuse initiatives may uncover as-yet-unknown human health consequences presented by use of recycled water containing insecticides. More information is needed to develop strategies to reduce insecticide concentrations being deposited into surface water via storm water and wastewater treatment.

It is imperative to understand the urban sources of insecticide contamination to reduce runoff transport of these contaminants by altering application practices. Furthermore, CWs hold a great deal of promise for reducing urban insecticide concentrations in storm water, wastewater, and surface water. It is vital to assess the removal efficacy of fiproles and pyrethroids passing through CW treatment systems. Pursuing these goals will assist in the development of improved mitigation and treatment practices for water contaminated with urban pollutants. Urban-use insecticides are primarily applied to eliminate structural pests such as ants, termites, roaches, and spiders. Extensive outdoor urban pesticide use is a cause for concern since surface runoff of these biologically active compounds into urban waterways following rainfall or irrigation has the potential to exert adverse effects in non-target aquatic organisms. The high incidence of impervious surfaces in urban environments, which may account for 50-90% of residential and commercial surface areas, prevents infiltration of water into soil and facilitates runoff and offsite transport of pollutants such as pesticide residues to urban streams. Perpetual urban expansion is projected to triple the global urban land area between the early 2000s and 2030 , exacerbating the issue of urban pesticide runoff and contamination to surface water. Fipronil is a moderately hydrophobic phenylpyrazole insecticide applied for a multitude of urban pest control purposes by licensed applicators. Applications include perimeter and underground injection treatments to manage ants and termites, veterinary flea and tick treatments, insect control baits, and landscape maintenance. After application, fipronil degrades primarily into fipronil desulfinyl, fipronil sulfide, and fipronil sulfone following photolysis, reduction, and oxidation, respectively. According to the Pesticide Use Reporting database, over 24,000 kg a.i. of fipronil were applied in 2016 in California, where use is confined to urban areas. Consequently, fipronil and its degradation products are frequently detected in surface water, such as in regions like California.

In a study of runoff discharge collected from residential storm drain outfalls in Southern and Northern California, median total fiprole concentrations were found to be 204-441 ng L-1 and 13.8-20.4 ng L-1, respectively, and 90th percentile total fiprole concentrations were 338-1169 ng L-1 and 62.6-65.3 ng L-1, respectively. In a recent study surveying urban creeks, rivers, and storm drain outfalls, fipronil sulfide, fipronil sulfone, fipronil desulfinyl,ebb and flow tray and fipronil were detected in 8%, 63%, 65%, and 75% of samples, respectively. The presence of fiproles in surface water is significant, since these compounds have been shown to exert toxic effects in a variety of non-target aquatic organisms with an LC50 of 140-320 ng L-1 for Palaemonetes pugio, Neomysis americana, and Simulium vittutum, and an EC50 of 32.5 ng L-1 and 7-10 ng L-1 for Chironomus dilutus. Therefore, in regions such as California, fiproles are ubiquitously present at toxicologically relevant levels in urban surface water ecosystems. However, little is presently known about the principal sources of fiproles in urban surface runoff, which hampers watershed-scale risk assessment as well as the development of effective strategies for mitigation. In the context of this study, runoff sources are defined as environmental matrices that contribute fiprole residues to surface runoff following known pesticide application. Primary objectives of this study were to characterize the affinity of fiproles for common urban matrices, to investigate persistence of fiproles in urban compartments, and to identify potential sources of fiproles in urban runoff. Bench sorption experiments were conducted for fiproles in urban dust, soil, and concrete. In addition, runoff water, urban dust, soil, and concrete wipe samples were collected from multiple fipronil-treated homes in Southern California from July-December 2016. This study represents the first systematic investigation of potential runoff sources of fiproles in urban residential environments. Results may be used to direct mitigation efforts of these compounds and to guide future pollution prevention initiatives for similar contaminants in urban watersheds. Fipronil , fipronil desulfinyl , fipronil sulfide , and fipronil sulfone were obtained from the United States Environmental Protection Agency’s National Pesticide Standard Repository.Isotopically labeled fipronil was purchased from Cambridge Isotope Laboratories. Solvents and other chemicals were of pesticide or GC-MS grade.

Small concrete cubes used in the sorption isotherm experiment were made in the laboratory via a process described elsewhere. Sorption isotherms were constructed over a period of five days by mixing concrete cube, urban dust, or the sandy loam soil samples in aqueous solutions simultaneously spiked with all four fiproles at 5, 20, 50, 100, 200, or 500 μg L-1. NaN3 was amended at 200 mg L-1 in the solution to suppress microbial activity and CaCl2 was added at 100 mg L-1 to adjust the solution’s ionic strength. Samples were prepared by adding 10 mL of the solution to a 40 mL amber glass vial containing one concrete cube, 2 g of dust , or 2 g of soil and mixing on a horizontal shaker at 120 rpm for 5 d. No statistically significant fiprole losses occurred over the course of the incubation. Sample vials were centrifuged at 1500 rpm for 30 min to separate the aqueous and solid phases. Aqueous phases were collected and extracted with 10 mL hexane by mixing on a horizontal shaker at 200 rpm for 30 min a total of two times. Solid phase samples were mixed with 1 g NaCl and 4 g anhydrous Na2SO4 and then extracted with 10 mL of 8:2 acetone:hexane by mixing at 200 rpm for 30 min a total of two times. The solvent extract of the aqueous or solid phases was evaporated to dryness under a gentle stream of nitrogen at 40 °C, and the condensed extracts from solid phase samples were subject to clean up using Florisil cartridges according to the procedure listed in the supporting information. Cleaned extracts were evaporated to dryness at 40 °C under a stream of nitrogen, and all extracts were reconstituted in 1.0 mL hexane before analysis. Five homes in Riverside, CA, received standard perimeter spray treatments of a professional fipronil formulation diluted from a suspension concentrate per the label instructions in July 2016, and the treatment was similar to the conventional treatment described in Greenberg et al.. Pre-treatment runoff concentrations of fiproles revealed low-level background contamination of all four compounds. However, these concentrations were two to three orders of magnitude lower than 1 d runoff concentrations and were similar to 153 d runoff concentrations. Therefore, this background contamination likely did not impact the conclusions presented in this study. Fiproles in runoff, soil, urban dust, and concrete were monitored at five time points during July-December 2016. Weather during this period was very warm and dry until light rainfall occurred in December prior to the collection of the final set of samples. The results reported herein were likely not impacted by this rainfall since the final samples still contained fiproles at concentrations similar to those measured at the previous time point. Runoff samples, one from each home at each time point , were collected by building a temporary water berm approximately 6 m away from the home’s garage door. Berm dimensions and composition were described in detail in Greenberg et al.. Each driveway was rinsed with a hose to generate a volume of runoff sufficient for the collection of a 1 L water sample in an amber glass bottle. Sample bottles were transported to the laboratory on ice within 3 h and stored at 4 °C until extraction. Extraction of runoff water samples was adapted from the methods in Gan et al.. Briefly, water samples were combined with 30 mL NaCl and liquid-liquid extraction was performed with 60 mL dichloromethane a total of three times. Extracts were then evaporated using a Büchi RE121 Rotovapor , solvent exchanged into 9:1 hexane:acetone , and cleaned up by loading into a Florisil cartridge preconditioned with hexane and eluting with 9:1 hexane:acetone. Cleaned extracts were evaporated to dryness under a gentle stream of nitrogen at 40 °C and reconstituted in 1.0 mL hexane. At each house and sampling time point, the following urban solid samples were simultaneously collected: one soil sample from the home perimeter, two dust samples from paved surfaces, and two concrete wipe samples from concrete walkways near the driveway. Soil samples from the 0-3 cm depth were collected into 40 mL amber glass vials. Total organic carbon content for the home soil samples was determined to be 3.80% using the aforementioned method. Urban dust samples were collected using a method similar to Richards et al.. Briefly, dust was sampled using a handheld vacuum fitted with a metal housing and mesh containing a Whatman GF/A glass fiber filter paper. The area to be vacuumed was marked off using a 0.5 m2 frame. If additional dust was needed to obtain a sufficiently large sample, the frame was moved to an adjacent region of the concrete surface for vacuuming. Filter papers were subsequently removed from the vacuum and stored in 40 mL amber glass vials. The total organic carbon content of the urban dust samples was measured to be 6.54% utilizing the previously described method.