Use of the insecticide lambda-cyhalothrin in agriculture may result in the contamination of water bodies, for example by spray drift. Therefore, the possible exposure of aquatic organisms to this insecticide needs to be evaluated. The exposure of the organisms may be reduced by the strong sorption of the insecticide to organic materials and its susceptibility to hydrolysis at the high pH values in the natural range. In experiments done in May and August, formulated lambda-cyhalothrin was mixed with the water body of enclosures in experimental ditches containing a bottom layer and macrophytes (at different densities) or phytoplankton. Concentrations of lambda-cyhalothrin in the water body and in the sediment layer, and contents in the plant compartment, were measured by gas-liquid chromatography at various times up to 1 week after application. Various water quality parameters were also measured. Concentrations of lambda-cyhalothrin decreased rapidly in the water column: 1 day after application, 24-40% of the dose remained in the water, and by 3 days it had declined to 1.8-6.5%. At the highest plant density, lambda-cyhalothrin residue in the plant compartment reached a maximum of 50% of the dose after 1 day; at intermediate and low plant densities, this maximum was only 3-11% of the dose (after 1-2 days). The percentage of the insecticide in the ditch sediment was 12% or less of the dose and tended to be lower at higher plant densities. Alkaline hydrolysis in the water near the surface of macrophytes and phytoplankton is considered to be the main dissipation process for lambda-cyhalothrin.
The photodegradation of six crop protection products (CPPs) was studied in 16 natural waters collected from across the midwest of the United States under simulated sunlight to determine the significance of indirect photolysis. The rate of degradation of five of the CPPs was faster in irradiated natural waters than in buffer systems, with the effect particularly significant with the relatively photostable compounds propiconazole and prometryn. Degradation rates were correlated with the concentration of one or more photosensitizers, or ratios thereof, by means of a Pearson's correlation and linear regression analysis. It was found that the photodegradation of chlorotoluron, pinoxaden, propiconazole and prometryn were linked to the concentration of nitrate, pointing to a significant role of hydroxyl radical ((.)OH) as a reactive intermediate. Increased concentrations of dissolved organic carbon (DOC) and bicarbonate relative to nitrate were found to decrease the rate of degradation of these compounds, consistent with a quenching role. Chlorothalonil appeared to be rapidly degraded by means of the carbonate radical ((.)CO(3)(-)), whereas the photodegradation of emamectin was particularly complex. Overall, indirect photolysis significantly enhanced the rate of CPP degradation and fate models based on these experiments appear to offer more realism than those that only take into account direct photolysis.
Aquatic exposure assessments for pesticides are generally based on laboratory studies performed in water alone or water sediment systems. Although aquatic macrophytes, which include a variety of bryophytes, macroalgae, and angiosperms, can be a significant component of many aquatic ecosystems, their impact on pesticide fate is generally not included in exposure assessments. To investigate the influence of aquatic plants on the fate and behavior of the pyrethroid insecticide lambda (lambda)-cyhalothrin, two laboratory experiments (to assess adsorption and degradation) and an indoor microcosm study (to assess fate under semirealistic conditions) were conducted. In the laboratory studies, adsorption to macrophytes was extensive and essentially irreversible, and degradation occurred rapidly by cleavage of the ester bond. In the indoor microcosm, which contained water, sediment, and macrophytes from a pond, degradation was also rapid, with DT50 and DT90 values of less than 3 and 19 h, respectively, for dissipation from the water column and of less than 3 and 56 h, respectively, for the whole system. For adsorptive and readily degraded pesticides like lambda-cyhalothrin, we conclude that macrophytes have considerable influence on fate and behavior in surface waters.
Rates of pesticide degradation in aquatic ecosystems often differ between those observed within laboratory studies and field trials. Under field conditions, a number of additional processes may well have a significant role, yet are excluded from standard laboratory studies, for example, metabolism by aquatic plants, phytoplankton, and periphyton. These constituents of natural aquatic ecosystems have been shown to be capable of metabolizing a range of crop protection products. Here we report the rate of degradation of six crop protection products assessed in parallel in three systems, under reproducible, defined laboratory conditions, designed to compare aquatic sediment systems which exclude macrophytes and algae against those in which macrophytes and/or algae are included. All three systems remained as close as possible to the Organisation for Economic Co-operation and Development (OECD) 308 guidelines, assessing degradation of parent compound in the total system in mass balanced studies using ((14) C) labeled compounds. We observed, in all cases where estimated, significant increases in the rate of degradation in both the algae and macrophyte systems when compared to the standard systems. By assessing total system degradation within closed, mass balanced studies, we have shown that rates of degradation are enhanced in water/sediment systems that include macrophytes and algae. The contribution of these communities should therefore be considered if the aquatic fate of pesticides is to be fully understood.
The evaluation of a chemical substance's persistence is key to understanding its environmental fate, exposure concentration, and, ultimately, environmental risk. Traditional biodegradation test methods were developed many years ago for soluble, nonvolatile, single-constituent test substances, which do not represent the wide range of manufactured chemical substances. In addition, the Organisation for Economic Co-operation and Development (OECD) screening and simulation test methods do not fully reflect the environmental conditions into which substances are released and, therefore, estimates of chemical degradation half-lives can be very uncertain and may misrepresent real environmental processes. In this paper, we address the challenges and limitations facing current test methods and the scientific advances that are helping to both understand and provide solutions to them. Some of these advancements include the following: (1) robust methods that provide a deeper understanding of microbial composition, diversity, and abundance to ensure consistency and/or interpret variability between tests; (2) benchmarking tools and reference substances that aid in persistence evaluations through comparison against substances with well-quantified degradation profiles; (3) analytical methods that allow quantification for parent and metabolites at environmentally relevant concentrations, and inform on test substance bioavailability, biochemical pathways, rates of primary versus overall degradation, and rates of metabolite formation and decay; (4) modeling tools that predict the likelihood of microbial biotransformation, as well as biochemical pathways; and (5) modeling approaches that allow for derivation of more generally applicable biotransformation rate constants, by accounting for physical and/or chemical processes and test system design when evaluating test data. We also identify that, while such advancements could improve the certainty and accuracy of persistence assessments, the mechanisms and processes by which they are translated into regulatory practice and development of new OECD test guidelines need improving and accelerating. Where uncertainty remains, holistic weight of evidence approaches may be required to accurately assess the persistence of chemicals. Integr Environ Assess Manag 2022;1-34.
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