Using Benchmarking To Strengthen the Assessment of Persistence

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Using Benchmarking To Strengthen the Assessment of Persistence Michael S. McLachlan,*,† Hongyan Zou,†,§ and Todd Gouin‡ †

Department of Environmental Science and Analytical Chemistry (ACES), Stockholm University, SE-106 91 Stockholm, Sweden Safety and Environmental Assurance Centre, Unilever, Colworth Science Park, Sharnbrook, Bedfordshire, U.K., MK44 1LQ

‡

ABSTRACT: Chemical persistence is a key property for assessing chemical risk and chemical hazard. Current methods for evaluating persistence are based on laboratory tests. The relationship between the laboratory based estimates and persistence in the environment is often unclear, in which case the current methods for evaluating persistence can be questioned. Chemical benchmarking opens new possibilities to measure persistence in the field. In this paper we explore how the benchmarking approach can be applied in both the laboratory and the field to deepen our understanding of chemical persistence in the environment and create a firmer scientific basis for laboratory to field extrapolation of persistence test results.

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INTRODUCTION Persistence is a metric of the degradability of a chemical in the environment. It is often quantified as the half-life for the removal of the chemical from a specified environment by degradation. Degradation in this context usually means transformation to a chemical structure that is different from the original molecule. It can be assessed for specific environmental media (e.g., the gas phase in the atmosphere), environmental compartments (e.g., a lake), or for the environment as a whole.1,2 The persistence of a contaminant has a strong influence on concentrations in the environment; at a given rate of emission, chemicals which are rapidly degraded will have lower concentrations. Thus, persistence is intimately linked to chemical risk. The persistence of a chemical contaminant is also an important determinant of its exposure hazard (i.e., those components of a chemical’s hazard profile that have to do with exposure). If a chemical is degraded slowly in the environment, it will take a long time to reduce its levels in the environment should unacceptable risks be identified and exposure reduction required. Persistent chemicals can therefore lead to a legacy related to their use, and this legacy potential leads to an inability to effectively manage exposure to the chemical. Hence the assessment of persistence is a key component of both chemical risk and hazard assessment. Chemical degradation occurs via a variety of different mechanisms, each of which is dependent on a range of environmental variables.3 Contaminants can be subject to hydrolysis, redox reactions, or photodegradation, which depend on variables such as pH, temperature, and solar irradiation at the point where the reaction is occurring, and the presence of coreactants.4−8 Chemicals can also be degraded by biodegradation, which is dependent on factors such as the presence and nature of degrading microorganisms in the environment, and temperature.3,9,10 The diverse environmental variables influenc© XXXX American Chemical Society

ing chemical degradation can result in substantial variance in space and time. This poses particular challenges when assessing chemical persistence.

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ASSESSING PERSISTENCE Guidelines for chemical hazard assessment in many national and international regulations set criteria values for persistence that can be evaluated with laboratory tests. For example, Annex XIII of the European chemicals regulation REACH defines criteria for identifying chemicals that are considered persistent (P) or very persistent (vP).11 The REACH guidance suggests first screening using the ready biodegradability test, a simple laboratory test in which water is spiked with the test chemical, inoculated with microorganisms and incubated. If the chemical passes this test then no further testing is required; otherwise more complex laboratory tests (e.g., simulation tests) are mandated.11 In order to assess environmental risk, the degree of persistence is required for quantifying exposure.12 To evaluate exposure, quantitative estimates of degradation half-lives are required for all degradation mechanism/environmental medium combinations that significantly influence the exposure of the target organisms. More sophisticated tests that provide quantitative measures of persistence are thus required. Exposure modeling can be used to prioritize the environmental media for which accurate degradation rates are needed.1,13 Common to both chemical hazard and exposure assessment is the reliance on standardized laboratory experiments that measure degradation of the chemical under controlled conditions. Different experimental designs are used depending on the degradation mechanism and the environmental medium of interest. They range from a simple hydrolysis experiment conducted in buffered sterile water (OECD 111)14 to complex Published: November 30, 2016 A

DOI: 10.1021/acs.est.6b03786 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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measurements of chemical persistence in the natural environment. Here we suggest that the benchmarking concept can be useful, since several of the problems encountered with field measurements of persistence can be addressed with benchmarking.

biodegradation tests involving sediment-water mixtures (OECD 308)15 or soil (OECD 307).16 In some cases, read-across and/or the use of QSARs (Quantitative Structure−Activity Relationships) have also been developed from laboratory test data to provide alternative methods for estimating the degradability of chemicals that have not yet been subjected to the screening tests.17,18 An underlying assumption of this approach is that laboratory measurements can be extrapolated to the field. Confidence in this assumption varies widely depending on the degradation mechanism. For hydrolysis the confidence is quite high. First, the influence of the two environmental variables of relevance for the hydrolysis rate, temperature and pH, can be readily studied in the laboratory. Second, the values of pH and temperature in the environment are generally well characterized, which means that the laboratory results can be extrapolated to environmental conditions with relative confidence.4 For photodegradation and biodegradation, on the other hand, our confidence is lower. One reason is the high variability observed in simulation tests that use natural materials. Boethling et al. (2009),19 in their review of persistence assessment, illustrate this with a study showing that 26 measured half-lives for the primary degradation of di(2ethylhexyl)phthalate in soil and sediment ranged from 8 to 630 days. In a laboratory study of aniline and 4-chloroaniline degradation in natural waters, the half-lives were 20 times longer in water taken from a pristine lake than in water taken from the Baltic Sea close to an urban area.10 Our intuition says that if this kind of variability is observed under controlled laboratory conditions, the variability in the environment is likely greater. In addition, we lack understanding of how environmental variables influence the degradation rates. For instance, Boethling et al.19 concluded that our understanding of the influence of temperature and pH on biodegradation was insufficient to justify scaling of biodegradation rates according to these environmental variables. For other variables such as the ability of microbial communities to degrade chemicals, our understanding is even more rudimentary. Reflecting on the role of laboratory tests in persistence assessment, Boethling et al.19 note “The relationship between chemical behavior in laboratory tests and in the environment is often unclear”. Bluntly expressed, the tools that we are using may not be good predictors of environmental persistence. This problem has been recognized since at least the early 1990s.20 One strategy to address this problem is to promote the standardization of laboratory experiments to improve test precision. However, this must not be confused with reducing the variability of the degradation in the environment or making the test more relevant for field conditions. By itself, improved standardization does not lead to greater confidence in the lab-tofield extrapolation of persistence measurements. Another strategy is to conduct studies of persistence in the field. Field measurements could lead to more relevant degradation half-lives and, more importantly, to the development of validated extrapolation procedures. However, field studies of persistence are difficult. One generally must resort to measuring chemical dissipation (i.e., loss due to the combined effect of degradation and physical removal processes like volatilization and advection) rather than just degradation. This necessitates the quantification of both chemical inflows and chemical inventories in the environmental system, which can be costly or even impossible. Even when dissipation can be measured, it is frequently difficult to infer degradation from dissipation. As a result, the scientific literature contains comparatively few

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THE BENCHMARKING CONCEPT The idea behind the benchmarking concept is to measure the relative behavior of chemicals, rather than the absolute value of a given behavioral characteristic. It is familiar to analytical chemists who conduct trace analysis. They often employ an internal standard chemical that is selected to have properties similar to those of the analyte, with the exception that it is not present in the sample. A defined quantity of the internal standard is added to the sample at the beginning of the analytical procedure. At the end of the procedure the ratio of the amounts of the analyte and the internal standard is quantified (not their absolute amounts). This ratio is multiplied by the amount of internal standard added to give the amount of analyte in the sample. Applied to environmental chemistry, benchmarking can be used to compare the behavior of a chemical of interest with the behavior of a chemical that we already have some understanding of. The latter is referred to as the benchmark chemical. Analogous to the internal standard in analytical chemistry, the effectiveness of the benchmarking concept relies on the similarity between the behavior of the benchmark chemical and the chemical of interest in the environmental system being studied, with the (possible) exception of the behavioral characteristic that is being investigated. In the case of persistence, this can mean measuring the degradation rate of the chemical of interest relative to the degradation rate of another (benchmark) chemical with an already well characterized degradation behavior.

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APPLYING BENCHMARKING CONCEPTS IN THE LABORATORY Currently, positive controls are employed in several OECD biodegradation test protocols. For instance, the ready biodegradation test (OECD 301)21 prescribes the use of parallel test systems that are dosed with a reference chemical (sodium benzoate or sodium acetate) instead of the test chemical. These systems are used as positive controls to test the proper functioning of the procedure. A broader set of reference chemicals suitable for use in a range of screening tests for biodegradation has been proposed.22 This application stops short of benchmarking, however, as the reference chemical is used only for quality assurance purposes and not in the evaluation of the test chemical’s degradation. In experiments that measure atmospheric photodegradation, on the other hand, benchmarking is routinely used to quantify half-lives. For instance, to study direct photolysis a benchmark chemical is introduced simultaneously with the test chemical to a photoreaction chamber, whereby the benchmark chemical is chosen to have sorption properties similar to that of the test chemical but not to be subject to photolysis. The dissipation of the benchmark chemical reflects the nonphotolytic losses (dilution, sorption) that the test chemical may also be subject to. The change in the concentration of the test chemical relative to the benchmark chemical over time represents the loss of the test chemical due to photolysis.23 A similar approach has been developed to study photochemical oxidative degradation (e.g., by hydroxyl radicals) in the atmosphere. A chemical with a known reaction rate constant is used as the benchmark chemical, and the B


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physical dissipation process (water advection) as the test chemicals (a range of pharmaceuticals) but no observable degradation (as reported in the literature and supported by a mass balance of one of the lakes) was selected. Using the change in the ratio of the concentrations of the test chemical and the benchmark chemical between the WWTP effluent and the lake outlet, together with the half-life for water residence in the lake, the degradation of the test chemical could be quantified. To illustrate, when the ratio of test:benchmark fell by a factor of 2 between the inlet and outlet of the lake, then it was clear that half of the test chemical had been degraded while it resided in the lake, and that the degradation half-life of the test chemical was equal to the half-life of water in the lake (see Box 1). In this way it

reaction rate of the test chemical relative to this reference chemical is measured.24,25

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APPLYING BENCHMARKING CONCEPTS IN THE FIELD An established application of the benchmarking concept in the field is the use of persistent tracers to account for the dispersion of a test chemical. For example, reaeration and volatilization in streams has been measured by simultaneously introducing a volatile gas (the test chemical) and a persistent, nonvolatile dye (the benchmark chemical) into the water body. The change in concentration of the dye accounted for the dispersion of the two tracers as they moved down the river. Measuring the change in concentration of the gas relative to the dye over time enabled quantification of volatilization and the determination of the reaeration coefficient.26,27 Tracer techniques have also long been used to study chemical degradation in the field. The photodegradation of pesticides has been studied by cospraying the pesticide with a nonphotodegradable benchmark chemical. The latter is intended to correct for dispersion and physical loss processes from the atmosphere, and the photodegradation rate of the pesticide is determined from the change in relative concentration to the benchmark chemical over time.28 Pesticide attenuation in aquifers and soils is measured by injecting a persistent tracer such as bromide into the aquifer or soil together with the pesticide, whereby the tracer corrects for diffusion and dispersion effects. If knowledge of physical attenuation of the pesticide is available, then pesticide degradation can be quantified.29−33 Chemical degradation in surface waters has been studied by introducing the chemical of interest together with a persistent tracer, and then using the change in the ratio of the chemical of interest and the tracer over time to quantify dissipation and infer degradation.34−36 The use of compound specific isotope analysis (CSIA) to quantify chemical degradation in the environment can also be viewed as an application of the benchmarking concept. CSIA makes use of the fact that the biodegradation rate of some chemicals is dependent on the isotopic composition of the molecule. When this is the case, the concentration of the more rapidly degraded isomer decreases more rapidly than the concentration of the other isomer. This can be observed as a change in the isotope ratio of the substance over time. If the relationship between the rate of change of the isotope ratio and the rate of biodegradation of the chemical is known (e.g., from laboratory experiments), then field observations of the rate of change of the isotope ratio can be used to determine the biodegradation rate.37 Whereas in the other examples of benchmarking given above the degradation rate of the benchmark chemical is the known quantity, in the case of CSIA it is the relative degradation rate of the two chemicals (isotopes) that is known. Degradation of chlorinated solvents in groundwater is routinely studied using CSIA, and the methodology has been used to study the degradation of many other compounds.38,39 Enantiomer ratios can be used in the same manner as isotope ratios when the degradation of the chemical is enantiomer specific.40 Recently, the tracer technique was taken a step further. Instead of intentionally releasing the test and benchmark chemicals, existing environmental contaminants were used so that no manipulation of the environment was required.41,42 Studies were conducted in two lakes where wastewater treatment plants were the dominant source of the chemicals. A benchmark chemical (the artificial sweetener acesulfame K) with the same dominant

Box 1 Using benchmarking to measure persistence in a lake When a lake (with water (blue), sediment (black) and suspended sediment (brown)) is at steady state, the rate of chemical entering the lake is equal to the rate of chemical being removed from the lake. Removal can occur via physical processes (advection, volatilization, or burial in sediment) or via degradation. We have a test chemical (“Test”) for which we would like to know the persistence. We choose a benchmark chemical (“BM”) which is subject to the same physical removal processes as the test chemical, but for which degradation is a negligible removal process. If the lake is well-mixed, then the change in the concentration ratio CTest/CBM between the inflow and the outflow gives quantitative information on the relative importance of degradation compared to physical removal of the chemical. The degradation half-life t0.5R (persistence) of the chemical can be calculated from the change in this ratio and the half-life for physical removal of the chemical τ (see equation). The figure shows a hypothetical example where the concentrations in the inflow and outflow are shown in red. The concentration ratio CTest/CBM decreases by a factor of 2 between the inflow and the outflow. Applying the equation, the persistence of this test chemical is equal to the half-life for its physical removal. If the concentration ratio can be measured with high accuracy, then the challenge in the persistence measurement is determining τ. When advection is the dominant physical removal process, then τ can be estimated from the hydraulic residence time of the lake. When volatilization or burial are important processes, then benchmarking techniques can be used to estimate τ.43

was possible to compare persistence across two lake systems, and do some initial exploration of the seasonality of chemical persistence.41,42 Furthermore, a theoretical framework was developed to allow this method to be applied to chemicals subject to other physical dissipation mechanisms in lakes such as C


no other significant sources of the chemical in the study region

further inputs of chemical during the chemical’s journey are negligible

strong chemical sources with low dilution in the environment sensitive analytical methods

lakes with water residence times of several months (for existing hazard criteria for persistence in water)

length in time of the chemical’s journey is similar to or greater than the persistence half-life of interest

target and benchmark chemical concentrations can be measured in the environment

a point source or riverine input with many chemicals (e.g., a municipal WWTP outfall)

there is a benchmark chemical with the same dominant physical dissipation mechanism as the target chemical and with known persistence

concentration ratio of target chemical to benchmark chemical is constant target and benchmark chemicals come from the same point source at the beginning of the journey or riverine input both chemicals are emitted (point source) or flow in (riverine input) at a constant rate

favorable situations single point source of chemical emission to a lake single source of riverine input of chemical to a lake multiple upstream sources of the chemical that have become wellmixed by the time they reach the starting point for the study

requirements

chemical concentration is well characterized at the beginning of the chemical’s journey (in the context of the persistence measurement)

weak chemical sources with high dilution in the environment insensitive analytical methods new chemical not yet in use

fast flowing streams or rivers (for existing hazard criteria for persistence in water)

a point source releasing just one chemical

concentrations of the benchmark and target chemicals at the source have strongly different temporal variability on a time scale relevant for the persistence measurement

target and benchmark chemicals come from sources that are widely separated spatially

atmospheric deposition is not negligible diffuse inputs from adjacent land (e.g., for agrochemicals) multiple municipal WWTP outfalls (e.g., for down-the-drain chemicals)

multiple point sources of chemical emission to a lake multiple sources of riverine input of chemical to a lake

unfavorable situations

Table 1. Requirements for the Application of Benchmarking to Study Persistence in the Field, Together with Some Favorable and Unfavorable Situations for Meeting These Requirements When Studying Persistence in Water Bodies

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opens the door to quantifying persistence of a broader range of chemicals in a broader range of environments than tracer techniques and CSIA currently allow. For instance, one could compare persistence in tropical and boreal lakes, oligotrophic and eutrophic lakes, brown water and blue water lakes. This could be augmented by measurements under somewhat more controlled conditions in outdoor mesocosms.46,47 Such measurements can lead to a stronger empirical basis for characterizing the influence of different environmental variables on environmental persistence. Since different environmental variables influence each of the different chemical degradation mechanisms (hydrolysis, photodegradation, biotransformation), it would be expedient to identify chemicals subject to just one form of degradation for studying the influence of environmental variables on that mechanism. This knowledge could be integrated for chemicals subject to multiple forms of degradation, which in turn could lead to a description of the spatial and temporal variability of chemical persistence in our diverse and changing environment, and eventually to the production of persistence maps. Another opportunity arising from a rich data set of field persistence measurements would be the comparison of laboratory tests with environmental data. For instance, such a comparison could be used to test the following three hypotheses (in order of increasing relevance of the laboratory data and decreasing likelihood). Do the laboratory and field data show: A. Same Rank Order of Persistence. Good agreement between the rank-order of the persistence of different chemicals in the laboratory and in the field is a prerequisite for the use of laboratory data to assess environmental persistence. It is also an important condition for implementing threshold benchmarking. Should the rank order not be the same, then a measurement showing a greater persistence of X over Y in the laboratory would not necessarily mean a greater persistence of X in the field. B. Equal Relative Persistence. Laboratory measurements can be more valuable if the relative magnitude of persistence of different chemicals are similar in the laboratory and the field. If chemical X is twice as persistent as chemical Y in the laboratory, and one knows that Y has a persistence in the field of Q, then one knows that chemical X will have a persistence of 2Q in the field. Indeed, a framework for quantifying relative environmental persistence based on laboratory experiments has been proposed.48 C. Equal Persistence. Laboratory measurements would be most useful for chemical assessment if they give a good approximation of the spatially and temporally aggregated persistence in the environment. The comparison of laboratory and field measurements must consider the spatial and temporal variability of persistence in the field. To address this, one could envisage establishing one or several reference lakes for assessing chemical persistence in the aquatic environment. A lake with a hydraulic residence time of 2−4 months, one significant municipal wastewater effluent source and negligible upstream input of contaminants can be used to measure the persistence of chemicals in the half-life range of the regulatory thresholds for persistence hazard.41 This can be done without system manipulation for existing chemicals that are being released to water from urban environments, and it could also be applied to new chemicals by adding these to the wastewater effluent stream in a controlled manner. The experimental effort is low, being limited to collection of wastewater effluent and outflowing lake water in a well characterized system. The major challenge lies in the quantification of the test and benchmark chemicals at the levels

volatilization and sedimentation, whereby dissipation via these mechanisms is also quantified using benchmarking techniques.43 Some regulatory frameworks stipulate a threshold-based assessment of persistence (REACH, for instance, classifies a chemical as persistent (P) if its half-life in water exceeds a threshold of 40 days11). Benchmarking can be tailored to directly support such threshold-based assessment. Instead of comparing the degradation half-life of the chemical of interest with a half-life threshold, one could compare the persistence of the chemical of interest with the persistence of another (benchmark) chemical that has been judged to have a persistence at the threshold between what is acceptable and what is unacceptable. We call this threshold benchmarking.41 Persistence evaluation using threshold benchmarking is very simple experimentally. One must simply follow the test:benchmark concentration ratio over time. If the ratio increases, then the test chemical is more persistent than the benchmark chemical, and it follows that the chemical fulfils the persistence hazard criterion. This is a trivial measurement task compared with a properly calibrated determination of a degradation half-life. Threshold benchmarking can be applied in both laboratory and field measurements of persistence.

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LIMITATIONS AND POTENTIAL OF THE BENCHMARKING APPROACH IN PERSISTENCE ASSESSMENT In addition to reflecting on the long tradition of useful application of benchmarking concepts in analytical and environmental chemistry, we note that there are limitations to its application in persistence assessment. Two conditions for its application are that it must be possible to follow the test chemical and the benchmark chemical on a journey, and that no significant new inputs of either of the chemicals occurs during this journey. Although it is relatively easy to design a laboratory experiment that fulfils these criteria, it is more challenging to find suitable situations in the field. One approach has been to release chemical mixtures into the environment (the tracer technique described above), but permission to do such experiments can be difficult to obtain, they can be ethically problematic, and they can require complex and costly sampling strategies to capture transient signals. In addition, biodegradation can vary widely depending on the pre-exposure of the microbiological community to the chemical and chemical concentration,44,45 so the results of chemical release experiments may only be relevant for limited scenarios (e.g., an accidental chemical release) and not to situations involving chronic, lower-level exposure. Nonmanipulative benchmark studies circumvent these problems, but they are constrained by the need to identify situations in the field where the chemical of interest and appropriate benchmarking chemicals are present in measurable quantities. The required absence of further inputs during the chemicals’ journey can also be a difficult constraint. One must look for appropriate environmental scenarios, for instance in a waterbody downstream of a major emission source, in the atmosphere downwind of a major emission source, or in soil following application of sewage sludge, whereby the source must be the dominant input of both the chemical of interest and the benchmark chemical. Table 1 elaborates on the conditions constraining the application of benchmarking to measuring persistence in water bodies. Despite these limitations, benchmarking has unexploited potential to contribute to the measurement and understanding of chemical persistence. Benchmark-based measurement of persistence in the field without experimental manipulation E


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Environmental Science & Technology encountered in the environment. Benchmarking would significantly enhance the value of the reference lake approach. In laboratory persistence tests, benchmarking could also lead to added value. For example, the reference chemical in the ready biodegradation test (OECD 301)21 could be used as a true benchmark chemical for persistence. By evaluating the percent removal of the test chemical relative to that of the reference chemical, it could be possible to improve test precision. Another example is the OECD 308 guideline15 on aerobic and anaerobic transformation in aquatic sediment systems, where the interpretation of the test results is constrained by mass transfer limitations on chemical removal, as well as potential differences in the biodegradation potential of the sediment chosen for the test.49 It may be possible to address these limitations by using benchmark chemicals to quantify the mass transfer and biodegradation properties of the test system. Benchmarking could also support existing regulatory frameworks by making higher level tests for assessing persistence more transparent in their performance and thereby more useful. For instance, the current EU regulations foresee a test hierarchy with the ready biodegradation test as the initial step, followed by enhanced biodegradation tests, inherent biodegradation tests, and simulation tests. Chemicals failing a lower tier test are passed on to the next higher tier.11 The higher the tier, the better the test is believed to simulate environmental conditions for contaminant biodegradation. However, the higher tier tests are also more complicated and more difficult to standardize, making it difficult to judge the value of a specific test result and to interpret conflicting results from different tests or test tiers. Comber and Holt (2010)22 proposed using a number of reference chemicals with a range of biodegradation behaviors to evaluate the performance of biodegradation tests. If such a group of chemicals were to be used as benchmark chemicals within different tiers of a test system, they would both increase transparency regarding the relevance of higher tiered tests and allow test variability to be quantified. Figure 1 summarizes our vision of how benchmarking could contribute to an improved understanding of persistence. A central challenge is bridging the gap between the laboratory and the environment. We propose that a suitable starting point would be to compare well-studied laboratory systems with a wellstudied environmental system (e.g., a lake or a field), which we have labeled “reference environment”. This comparison would lead to methods for translating the results of laboratory persistence studies into persistence in the reference environment (“calibration”). Persistence measurements in other, diverse environmental systems would generate understanding of the temporal and spatial variability of persistence. As outlined above, this would be used to refine and calibrate theoretical frameworks (that may rely on input from laboratory experiments) to extrapolate persistence in the reference environment in space and time, eventually enabling a comprehensive assessment of persistence in the real environment. Absolute benchmarking (i.e., benchmarking that leads to quantification of the degradation half-life) techniques would be a core element of both laboratory and field measurements of persistence. Threshold benchmarking would come to play in regulatory assessment of persistence hazard.

Figure 1. Schematic of how benchmarking could contribute to persistence assessment. Both absolute benchmarking and threshold benchmarking are used in lab tests and field measurements, absolute benchmarking primarily in the context of risk assessment, threshold benchmarking for hazard assessment. Lab tests and field measurements are applied in a focus study of a reference environment(s), in order to define how lab test results can be extrapolated to the field. Lab studies and field measurements are also used to establish methods to extrapolate persistence from the reference environment to other environments, that is, in space and time.

end, there is a clear need for field measurements of degradation half-lives. A focused research effort on a reference environment would facilitate the generation of the consistent and comparable data sets that are required for an in-depth assessment of lab-tofield extrapolation. Field measurements of the spatial and temporal variability of degradation are also needed to build the fundamental understanding needed to extrapolate persistence from one environment to another. Creative efforts from diverse research groups are needed to illuminate the causal relationships governing this variability. Benchmarking can play an important role in both of these efforts. It would be particularly important to compare the rank order of persistence in lab studies and in the field. As noted above, if laboratory tests are to predict persistence in the field, then the rank order of laboratory persistence and field persistence must be similar. It will also be important to study how the rank order of field persistence varies in space and time, which in turn could lead to the development of more sophisticated benchmarking strategies targeted at specific degradation mechanisms. The discussion of field measurements has largely focused on chemical persistence in water bodies. However, benchmarking can also be applied to assess chemical persistence in other media, and more work is needed to explore these possibilities. For instance, in sewage sludge fertilization experiments benchmarking could be applied to study persistence in soil. The application of sewage sludge to soil is analogous to the discharge of WWTP effluent to a lake; a multitude of chemicals are released at the same location, providing a wide range of test chemicals and potential benchmark chemicals that can be followed in time. Benchmarking has also been employed to quantify chemical persistence in the atmosphere by comparing the spatial variability of concentrations at remote sites for a test chemical and chemicals with known atmospheric half-lives.50 Research into environmental persistence would benefit from the integration of modeling of contaminant fate processes and modern techniques in trace analysis. Process-based modeling can

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RESEARCH NEEDS To feel confident in the ability of laboratory tests to predict environmental persistence, we need empirical evidence that this extrapolation is valid for a broad spectrum of chemicals. To this F



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be used to design field experiments that provide the process understanding that is needed to improve lab-to-field extrapolation of persistence. Sensitive modern analytical techniques will help to exploit the opportunities for studying chemical persistence in the environment by expanding the number of existing contaminants that can be analyzed at low concentrations. Work is also needed to identify suitable threshold benchmark chemicals. These should be chemicals that have a persistence in the environment that is considered to be at the thresholds defined in regulations. In addition, they should be suitable for persistence measurement both in the laboratory and in the field, for example, not highly toxic, and present in environmental media at quantifiable concentrations. Finally, some deeply entrenched regulatory paradigms should be re-examined. In our opinion, the change with perhaps the greatest potential to improve the scientific quality of the regulatory assessment of persistence hazard would be embracing threshold benchmarking as a regulatory principle. This could reduce the tremendous effort currently spent trying to produce calibrated quantifications of degradation half-lives (whereby, for photodegradation and biodegradation, the reference point for the calibration, namely persistence in the real environment, is not even known). Instead, these resources could be invested in more relevant and strategically designed persistence studies, such as threshold benchmark measurements in the field. By studying the relative behavior of chemicals rather than their absolute behavior, we can more efficiently advance our mechanistic understanding of chemical fate. The relativity in environmental chemistry!

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Michael S. McLachlan: 0000-0001-9159-6652 Present Address §

(H.Z.) Tianjin Key Laboratory of Water Resources and Environment, Tianjin Normal University, Tianjin 300387, China.

Notes

The authors declare no competing financial interest. Biographies Dr. Michael McLachlan is head of the Department of Environmental Science and Analytical Chemistry at Stockholm University. His research interests encompass the fate and bioaccumulation of organic contaminants. Dr. Hongyan Zou is at present an assistant professor at Tianjin Key Laboratory of Water Resources and Environment of Tianjin Normal University. Her research interest is the fate and transport of organic pollutants in aqueous systems and the mechanisms of different degradation processes, which she studies using both experimental and computational measures. Dr. Todd Gouin is an environmental fate modeller with the Safety & Environmental Assurance Centre at Unilever. He is part of a team committed to ensuring the human and environmental safety of chemical ingredients used in consumer products.

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ACKNOWLEDGMENTS We thank Unilever, Bedfordshire, UK for funding this work, Matthew MacLeod for commenting on an early version of the manuscript, and Amelie Kierkegaard for assisting in the preparation of the TOC art. G


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Using Benchmarking To Strengthen the Assessment of Persistence

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