Based on this finding, and on the conclusions drawn from the data presented in Figures 3.1-3.3, it is likely that removal of these compounds was not markedly influenced by seasonal fluctuations in temperature or weather patterns and was more dependent on availability of sediment bindings sites, degradation in the sediment compartment, and, in the case of fipronil, rate of plant uptake. Sediment microorganisms responsible for biotic degradation likely appeared to be unaffected by seasonal temperature variations because temperatures are relatively high year-round in Southern California. In addition, the emergent macrophytes present in the PCW experience rampant growth due to the constant availability of nutrients in the Santa Ana River water. Therefore, aquatic macrophytes were always available for plant uptake and subsequent transformation of fipronil. In addition to concentration removal, another important metric for ascertaining the efficacy of the PCW is the mass flux of fiproles and pyrethroids. In particular, mass influx, mass efflux,nft hydroponic and change in mass flux were calculated from the chemical concentrations and water flow rates.
Fipronil, bifenthrin, and cyfluthrin were imported into the PCW at the highest rates, with mean mass influxes of 115-12700 mg d- 1 , 2.37-701 mg d-1, and 100-6090 mg d-1, respectively. Fipronil desulfinyl ,fipronil sulfide , and fipronil sulfone exhibited much lower import rates into the wetland. Changes in mass flux, which represents the net import or export of chemicals to or from the PCW, were also the highest for fipronil , bifenthrin , and cyfluthrin. The majority of changes in mass flux for these three compounds were statistically significant. In contrast, only one of the changes in mass flux values was statistically significant for fipronil desulfinyl , fipronil sulfide , and fipronil sulfone since the difference between the mass influxes and effluxes for these compounds was much smaller. Negative changes in mass flux values were found for fipronil and cyfluthrin in the months of November and December 2018, though only two of these four measurements were statistically significant. However, the export of these compounds during this time corresponded to higher outflow than inflow for the PCW , which likely resulted in resuspension of sediment particles as evidenced by the negative sedimentation rates observed during these two months. The flow of water through the entire Prado wetlands is regulated to optimize water quality and quantity, which leads to occasional net outflow from certain wetland cells, as was the case for the PCW in November and December 2018. Therefore, although net mass export of fipronil and cyfluthrin occurred during this time, it was compensated for by the high volume of water exiting the PCW, as indicated by the positive concentration removal values.
Although no outflow, mass efflux, or change in mass flux values could be calculated for January 2019, it is important to mention that the rainfall that occurred during this month resulted in spikes of chemical mass influx and possibly additional resuspension of contaminated sediment particles. However, positive removal values again showed that dilution prevented an increase in outlet concentrations. To further highlight the importance of adsorption in the removal of fiproles and pyrethroids by the PCW, the relative presence of each compound on TSS obtained from water samples was calculated. With two exceptions, the % of each chemical on TSS at the wetland inlet was statistically similar to the outlet value. In August 2018, an average of 87.7% of the bifenthrin in the whole water sample was associated with TSS at the inlet, as compared to 100% at the outlet. In contrast, values decreased from 92.1% at the inlet to 71.9% at the outlet for cyfluthrin in December 2018. In addition, inlet, midpoint, and outlet values were anomalously low for fipronil and cyfluthrin in July 2018. It is likely that some cyfluthrin was associated with dissolved organic matter and included in the aqueous phase concentration. The lower values for fiproles may be attributed to their moderate hydrophobicity as compared to the pyrethroids. The overall results suggested that the fractions of these compounds on TSS were similar throughout the PCW and were also similar over time, with inlet and outlet values ranging from 60-100%.
Combined with the evidence for the importance of sediment binding in the removal of pyrethroids and fiproles, it may be concluded that adsorption to suspended particles and subsequent sedimentation was likely a dominant process governing the fate and transport of these contaminants in CW systems. Linear regression was carried out to identify additional factors contributing to the removal of fiproles and pyrethroids in the PCW. Two dependent variables, concentration-based removal and change in mass flux, and three independent variables, sedimentation rate, water pH, and water temperature, were considered. Statistically significant linear relationships were observed between fipronil removal and water pH , between change in fipronil mass flux and sedimentation rate ,nft system and between change in cyfluthrin mass flux and sedimentation rate. These results demonstrate that water pH and water temperature had minimal impact on the removal of fiproles and pyrethroids in the PCW, with the exception of the influence of water pH on the concentration-based removal of fipronil. This finding may emphasize the importance of pH in determining the ionization state of fipronil and hence its adsorption onto sediment particles under field conditions. In addition, the effect of sedimentation rate on the changes in mass flux for fipronil and cyfluthrin—the analytes detected at the highest concentrations and mass influxes— further supports the notion that settling of insecticide-laden particles played a major role in the removal of fiproles and pyrethroids. The evidence provided by this study therefore highlights that settling of contaminated particles and partition into the wetland sediment are crucial in achieving removal of these urban-use insecticides, which is in agreement with findings from an agricultural drainage wetland. Calculated TUs based on the concentrations measured at the PCW inlet and outlet are given in Table 3.4. Sublethal and lethal toxicity values for the amphipod Hyalella azteca were used to determine the change in potential pyrethroid toxicity between inlet and outlet measurements since previous research has demonstrated this organism’s sensitivity to pyrethroids. Mean sublethal bifenthrin TUs decreased from 0.704-19.4 at the inlet to 0-4.91 at the outlet, while mean lethal TUs decreased from 0.302-8.30 to 0- 2.10 at the inlet and outlet, respectively. All decreases were statistically significant except for the month of November 2018 when inlet TUs were relatively low with high variability and outlet TUs were 0 since no bifenthrin was detected. Cyfluthrin mean sublethal TUs were 26.0-240 at the inlet, and decreased to 7.27-68.0 at the outlet. The corresponding mean lethal TUs were 21.5-198 and 6.01-56.1 at the inlet and outlet, respectively. All decreases in cyfluthrin TUs were statistically significant. The midge Chironomus dilutus was selected for the calculation of fiprole TUs since it has been shown to be extremely sensitive to these chemicals. Mean sublethal TUs for fipronil sulfide decreased from 0-0.414 at the inlet to 0-0.00131 at the outlet in a statistically significant manner. Similarly, mean lethal fipronil sulfide TUs underwent statistically significant decreases from 0-0.0592 at the inlet to 0-1.88 x 10-4 at the outlet. Fipronil mean sublethal TUs decreased from 0.984- 11.4 at the inlet to 0.416-2.72 at the outlet, while mean lethal TUs decreased from 0.392- 4.53 at the inlet to 0.166-1.08 at the outlet.
All inlet-outlet comparisons for fipronil were statistically significant except for TUs corresponding to the month of January 2019 when variability in concentrations at the inlet was high. Mean sublethal fipronil sulfone TUs were 0-4.22 at the inlet, decreasing to 0-1.03 at the outlet. Mean lethal TUs for fipronil sulfone were 0-0.312 and 0-0.0761 at the inlet and outlet, respectively. Statistically significant differences for fipronil sulfone were only observed in the months of October 2018, December 2018, and January 2019, but TU values for the other months were all <1 at the inlet and 0 at the outlet. These results showed that removal of fiproles and pyrethroids by the PCW resulted in toxicity reductions for all urban-use insecticides, and the reductions were statistically significant in most instances. The TU values reported in this study represent hypothetical worst-case single chemical exposure scenarios for the most sensitive aquatic invertebrates. Furthermore, the TU values were derived from whole water concentrations and did not take into account bio-available concentrations. For pyrethroids, studies have shown that bio-availability in whole water and sediment is inhibited by DOM or organic matter. Therefore, it is likely that the TU values in this study overestimated the actual toxicity and would serve as a conservative assessment. The influence of bio-availability on fiproles should be less significant given their moderate hydrophobicity. Moreover, the TUs calculated from PCW data do not represent the Prado Wetlands as a whole, since it is composed of many interconnected ponds operating in series. The effluent from the PCW undergoes dilution as it recombines with additional treated water emanating from adjacent wetland cells, is subjected to further treatment, and is ultimately deposited into Chino Creek. As a result, the TU values for the entire treatment chain would very likely be further reduced. Constructed wetlands are a promising option for the treatment of water containing hydrophobic organic contaminants such as fiproles and pyrethroids. There is a great deal of evidence in the literature demonstrating the efficacy of CWs in the removal of nitrogen species, phosphorous species, metals, antibiotic resistance genes, total suspended solids , biological oxygen demand, and chemical oxygen demand. CWs of various designs have been studied, and it is clear that treatment is typically facilitated by some combination of abiotic degradation, microbial degradation, sorption, and phytoremediation, depending on the treatment endpoint. Existing data regarding select organic contaminants have demonstrated that CWs are effective in their removal , but detailed mechanistic information is largely lacking for many important classes of organic pollutants. There is evidence that sediment sorption and subsequent degradation is vital for removal of HOCs , and plant uptake appears to play a role as well. However, differences in HOC removal efficiency and the mechanisms responsible among CW types and configurations is not well understood, which prevents comprehension of the optimal conditions for their treatment. Unit process CWs are designed to facilitate treatment of a particular endpoint or set of endpoints by encouraging specific physical, chemical, and/or biological processes. Unit processes include open water cells, macrophyte-dominated wetland cells, and bivalve filtration wetland cells. These unit processes encourage degradation of their target contaminants via the following mechanisms: direct photolysis, indirect photolysis, sorption, and biotransformation in open water cells; denitrification, metal sulfide precipitation, plant-derived carbon biotransformation, and peat sorption in macrophyte-dominated wetland cells; and pathogen ingestion, particle-associated contaminant ingestion, and inactivation or transformation of ingested contaminants in bivalve filtration wetland cells. Some water treatment initiatives seek to leverage the unit process concept by installing distinct unit process cells in series to remove several classes of contaminants, allowing for optimization of CW size and isolation of necessary physical and chemical conditions. Previous work has demonstrated that unit process open water cells are effective for the removal of pharmaceuticals via photolysis and microbial biodegradation. It is unknown if UPOW cells are similarly effective for removal of HOCs such as fiproles and pyrethroids. To investigate the efficacy of UPOW cells in the removal of HOCs and begin to understand the mechanisms underlying said removal, water and sediment samples were collected from the Prado Wetlands from June 2018-January 2019 and analyzed for fipronil desulfinyl, fipronil sulfide, fipronil, and fipronil sulfone as well as the pyrethroids bifenthrin and cyfluthrin. The primary objectives of this study were to examine the spatial and temporal trends of fiproles and pyrethroids in the water and sediment compartments of a UPOW wetland cell, to calculate the percent removal of each analyte by the wetland as well as their mass fluxes through it, and to measure changes in invertebrate toxic units caused by UPOW treatment. It was hypothesized that sedimentation of contaminated particles and sediment binding would play a major role in the removal of fiproles and pyrethroids by the UPOW cell, causing significant reductions in sensitive aquatic invertebrate toxicity.