However, degradation product concentrations and detection frequencies were lower in the present study, likely attributable to the fact that samples were collected from homes treated with a single application of fipronil. Together, results from this and other studies indicated that dust particles exposed to fipronil may retain fipronil and its degradation products for many months after application. This suggests that urban dust may serve as a source of fiproles, especially fipronil and fipronil desulfinyl, in urban runoff long after the conclusion of pest treatment activity, barring removal or offsite transport of the dust prior to the occurrence of a runoff event. Fipronil sulfone levels gradually increased from 1 d to 153 d, with mean concentrations ranging from 1.43-209 ng g-1. Fipronil has an aerobic soil half-life of 188 d , which supports the finding that mean fipronil concentrations were similar over the 153 d period considered in this study. In addition, the gradual formation and increasing soil concentrations of fipronil degradates was consistent with the relatively slow degradation rate of the parent compound. Fipronil and fipronil sulfone were detected with the greatest frequency and at the highest maximum concentrations. Fipronil desulfinyl and fipronil sulfide were detected less frequently and at substantially lower maximum concentrations. Fiproles were measured in soil samples at detection frequencies similar to those measured in dust , but maximum soil concentrations were much lower than maximum dust concentrations. It is possible that soil concentrations were low relative to dust concentrations because soil samples were collected to a depth while dust particles partially originated from wind erosion of the surficial soil. Fiproles have been shown to be enriched in fine particles characteristic of urban dust ,ebb and flow trays suggesting that residues initially present in the surrounding soil may have contributed to contamination of loose dust particles on impervious surfaces.
Results summarized herein reveal that soil treated with fipronil-based pesticide formulations remains contaminated by fiproles for a significant amount of time following treatment and is a source of fipronil degradation products. These data collectively imply that soil has the potential to contribute fipronil and its degradation products to their loads in urban runoff. However, this contribution likely depends upon the entrance of soil particles into runoff, either by inundation of soil with a large runoff volume after a prolonged rainfall, an irrigation event, or by prior transport of soil particles onto urban impervious surfaces. Mean concrete concentrations of fiproles were at their highest 1 d after application and decreased subsequently by 57-89% at the 30 d sampling point. Fipronil was rapidly transformed after application such that its degradation products were detected at high mean concentrations 1 d after application. This finding was consistent with results of a recent study focused on the degradation of pesticides on urban hard surfaces, where it was observed that fipronil was rapidly transformed to its biologically active degradation products on concrete in bench and field experiments. Mean concentrations then remained relatively stable for the duration of the sampling campaign, with 30 d concentrations being similar to those at 79 d, 110 d, and 153 d. Detection frequencies of fiproles in concrete ranged from 27 to 92%, with maximum concentrations of 3.19-25.4 µg m-2.The most prevalent degradation product was fipronil sulfone , while fipronil desulfinyl was detected at a higher maximum concentration than the other degradates, second only to the parent compound. An investigation of the contribution of fine particles to the runoff loads of pyrethroid pesticides also revealed high concentrations of bifenthrin and permethrin on concrete following application of professional pesticide formulations. Concrete data further showed that fiproles were present in the concrete at detectable concentrations for several months after initial application of fipronil for pest treatment. This suggests that concrete may act as a long-term source of these compounds in urban runoff.
Several linear regression analyses were performed to assess the presence of statistically significant linear relationships between fiprole concentrations in different urban solid matrices and concentrations in runoff water.A statistically significant relationship would indicate that a given component may be an important source for fiproles in runoff. It was observed that statistically significant relationships existed between the runoff and concrete concentrations of fipronil desulfinyl, fipronil sulfide, fipronil, and fipronil sulfone. A previous study similarly uncovered a highly significant linear relationship between runoff concentrations of pyrethroids and their concentrations on concrete surfaces measured using a surface wipe method. In this study, significant relationships were also found between the runoff and dust concentrations of fipronil desulfinyl and fipronil. Recent studies have also implicated dust particles in the offsite transport of hydrophobic organic contaminants, but the present study was the first to directly evaluate the connection between dust and runoff loads of fiproles. The significance of the concrete-runoff and dust-runoff relationships for fiproles together suggested that dust on impervious urban surfaces and residues on concrete are important sources of fiproles in runoff. Statistical analysis, however, did not show soil as a significant source for fiproles in runoff water. As discussed above, even though soil was not a direct source, it is possible that soil particles in the surface layer may be transported via wind and other mechanisms onto impervious surfaces, indirectly contributing to the contamination of runoff water by fiproles. Soil particles likely represent a major component of urban dust; other components may include concrete fragments generated from weathering and plant debris. Taken together, the most important finding of this analysis was that the effectiveness of mitigation efforts would be improved by focusing on reduction of dust particles on impervious surfaces and prevention of pesticide contact with concrete surfaces such as driveways. Moreover, the established regression equations may be used to predict fiprole loads in runoff using levels in urban dust and residues on impervious surfaces, before a runoff event occurs.
Contamination of surface water by fiproles poses a threat to many benthic invertebrate species. Fiproles may therefore exert a significant effect on the benthic community structures of urban streams.However, even though runoff from a given residential area enters downstream surface water as a point source,4×8 flood tray surface runoff from individual homes in a neighborhood resemble nonpoint sources and is technically challenging to control. Identification of concrete surfaces and urban dust as the major sources of fiprole contamination of surface runoff at the site of pesticide treatment highlights their importance in the effort to reduce fiprole residues around a homesite, especially on impervious surfaces. The combination of rampant urbanization, rapid population growth, and global climate change has resulted in an extraordinary reduction in the potable and non-potable water supply worldwide. The deficiency of clean water supplies has led several nations, including the United States, to encourage a reduction in water use and pursue a myriad of water treatment and recycling initiatives. Water scarcity is exacerbated by pollution of surface water and ground water resources by anthropogenic contaminants such as pesticides. Indoor and outdoor use of insecticides in urban areas has been shown to cause contamination of urban surface water sources. Urban-use insecticides are incompletely removed at wastewater treatment plants before the release of effluent into surface streams, and runoff after rain and irrigation events further exacerbates surface water contamination. Fipronil and the synthetic pyrethroids are insecticides utilized at high rates in urban environments for professional and homeowner control of structural pest species such as ants, termites, spiders, and roaches, as well as for elimination of fleas and ticks in veterinary medications. Fipronil and its primary degradation products, fipronil desulfinyl, fipronil sulfide, and fipronil sulfone are moderately hydrophobic compounds while the pyrethroids are highly hydrophobic with log Kow =5.7-7.6. Numerous studies have shown occurrence of both insecticide classes in urban surface water at toxicologically relevant concentrations as well as in the sediment where residues may persist long after deposition. Furthermore, fipronil’s major degradation products exhibit toxicity equal to or greater than that of the parent compound. There is also evidence of additive pyrethroid toxicity in sensitive organisms. Fiproles and pyrethroids are easily transported in surface runoff and are present in WWTP effluents , aggravating the risk of toxicity to non-target aquatic species. In arid or semi-arid regions such as California, some urban streams are predominantly fed with urban runoff drainage and WWTP effluents.
Constructed wetlands are one potential solution to the shortcomings of WWTPs and the general lack of storm water treatment. They have been shown to remove nitrogen and phosphorous species, metals, antibiotic resistance genes, and various organic compounds. Existing data suggest that CWs are effective in reducing concentrations of fiproles and pyrethroids. However, field data on the performance of urban wetlands are limited, and in-depth information on the role of various wetland compartments is scarce. In this study, samples were collected from the Prado Wetlands, a 182 ha constructed treatment wetland system located in Southern California containing open water and vegetated cells, from June 2018-January 2019 and analyzed for fiproles and pyrethroids. The primary objectives were to determine the removal of these trace contaminants by the surface flow wetland, to understand the underlying processes most responsible for contaminant removal, and to estimate potential alleviations in invertebrate toxicity. It was hypothesized that sediment sorption and biodegradation would play a major role in the removal of fiproles and pyrethroids, resulting in reduced aquatic toxicity. Results from this study may be used to optimize the design of CWs and related water treatment systems to improve the quality of recycled water and to attenuate ecotoxicological and human health risks from potable and non-potable applications.Fipronil , fipronil desulfinyl , fipronil sulfide , and fipronil sulfone were obtained from the United States Environmental Protection Agency’s National Pesticide Standard Repository. Bifenthrin and deuterated bifenthrin were purchased from Toronto Research Chemicals. Cyfluthrin was purchased from Santa Cruz Biotechnology. Ethiprole was obtained from the Shanghai Pesticide Research Institute. Decachlorobiphenyl was purchased from AccuStandard.Solvents and other chemicals used were of pesticide or GC-MS grade. This study was undertaken at the Prado Wetlands in Corona, CA. This 182 ha complex of 45 surface flow wetland ponds was constructed in the 1990s and was initially established to remove NO3 – from the Santa Ana River. Up to 50% of the Santa Ana River flow, which consists primarily of treated wastewater during non-storm seasons, is diverted into the wetland system for treatment. Additional details regarding the Prado Wetlands are provided in the Supporting Information. The ponds selected for use in this study were cells S5 and S6 , which together constitute a 4.45 ha vegetated CW complete with inlet and outlet weir boxes. In the context of this study, this vegetated CW will be referred to as the Prado Constructed Wetland. The hydraulic retention time of the PCW was estimated to be 1.29 d based on the results of a pilot-scale rhodamine WT tracer experiment conducted at the Prado Wetlands. Samples and measurements were taken at the inlet weir box , the interface between ponds S5 and S6 following the connection pipe , and the outlet weir box. Water, sediment, and plant samples were collected from the PCW monthly during the period of June 2018-January 2019, with the exception of September 2018 when there were ongoing maintenance activities. Triplicate 1 L water samples were collected in amber glass bottles at the inlet, midpoint, and outlet of the PCW. Inlet and outlet samples were collected from the water flowing into the corresponding weir boxes, while midpoint samples were collected by placing bottles below the surface of the water against the direction of flow. Sample bottles were transported to the laboratory on ice and stored at 4 °C until extraction. Before extraction, water samples were passed through 0.7 µm filters to separate the TSS from the water. Filtered TSS samples were then dried in preparation for extraction. Wetland water samples were extracted using a method in Gan et al. , with modifications. Briefly, 30 mL of NaCl was combined with each water sample and liquid liquid extraction was performed with 60 mL aliquots of dichloromethane. Each extract was drained through a funnel containing anhydrous Na2SO4 to remove residual water, evaporated with a Büchi RE121 Rotovapor , and solvent exchanged into 9:1 hexane:acetone. Samples were then evaporated to approximately 0.5 mL under a gentle nitrogen stream and reconstituted in 1.0 mL hexane for analysis.