Correspondence/Rebuttal pubs.acs.org/est
Comment on “Regulatory FOCUS Surface Water Models Fail to Predict Insecticide Concentrations in the Field; Environ. Sci. Technol. 2012, 46, 8397−8404″ would like to question the suitability of the first two hypotheses for evaluating the protectiveness of the FOCUS surface water modeling approach1 for the EU. Hypothesis 1: “A maximum of 10% of the FOCUS step 3 PECs underestimate the measured field concentrations (MFCs)” This hypothesis is only suitable for assessing the “protectiveness” of the FOCUS sw modeling approach (which comprises a suite of 3 models and 10 generic scenarios) for the EU if all the following conditions are met: (1) real and modeled pesticide application scenarios are similar (substance, application rate/ type/season), (2) the study areas are representative for the distribution of agro-environmental scenarios (soil, climate, crops) in the EU, and (3) measured and modeled concentrations are indeed comparable with each other because they have the same underlying conditions (type of water body, integration period, environmental compartment, unit etc.). After a close examination of the 22 field studies Bach and Hollis 2 found that of the eight EU studies only three are suitable at all for a comparison with modeling results. For the studies performed outside Europe, where climatic, soil, water body and/or legal conditions may be very different from EU conditions, the relevance for the EU has not been demonstrated. The probability that the remaining set of suitable field studies is representative for the distribution of agro-environmental scenarios in the EU is therefore very low. Hypothesis 2: “The calculated FOCUS PECs exhibit a high correlation with the measured insecticide concentrations in water and sediment. The degree of correlation improves from step 1 to step 4.” Regarding the first part of this hypothesis, the authors state “Figure 1 shows that there is no positive relationship in terms of a statistically significant slope >0 in a linear regression model between predicted and measured concentrations. For all FOCUS steps, the data points form a rectangular cluster that spans several orders of magnitude on both axes. The complete absence of a relationship between the predicted and measured data highlights the importance of the inherent FOCUS model restrictions.” There are two explanations for this observation: (1) The 22 field monitoring studies are quite different from each other in terms of boundary conditions (weather, soil, catchment, water body, application practice). Hence, the degree and direction (i.e., better or worse case) of correspondence between a field study and the FOCUS scenario assigned to it will vary between the studies. Given the uncertainty in the assignment of FOCUS scenarios to field studies (cf., ref 2) and the variable correspondence between them, one cannot expect a good or any correlation for steps 1−4 when plotting all studies in the same plot. A correlation between FOCUS results and monitoring data can only be expected if only
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one monitoring data set is used, or at least only studies with similar boundary conditions. (2) In addition, there is a serious plotting artifact: One modeled data point was incorrectly plotted against several measured ones. The authors report: “If multiple insecticide concentrations were reported in a publication, only the peak concentrations that originated from different entry events were classified as separate events; therefore, multiple concentrations in one publication can be regarded as independent.” In fact, different measured concentration data points for the same substance in the same study come from the same time series. They are therefore not independent. For a comparison with PECmax only the maximum measured value should be used. With respect to the second part of hypothesis 2, the authors observe: “In addition, the relationship did not improve from FOCUS step 1 to step 4”. This is absolutely normal, since the unchanged FOCUS scenarios (steps 1−4) are independent of the monitoring studies. Only with the “realistic step 3” simulations, where the FOCUS scenarios were adapted to fit the monitoring studies, a better correlation could be expected. I would also like to comment on some other statements in the paper: • “(Luo and Zhang stated) that state PRZM is known to inadequately predict the pesticide transport associated with soil erosion.” This statement from ref 4 is made with reference to another statement in the USEPA release notes of the DOS-based version PRZM 3.1.2.3:5 “Pesticide associated with eroded solids is only removed from the top-most compartment. Changing the compartment size may result in dramatic changes to the pesticide mass removed by erosion.” The second sentence “dramatic changes” is merely a misinterpretation of the PRZM source code, as was confirmed by the code manager.6 The effect of the thickness of the top compartment is indeed only an indirect one due to pesticide leaching and volatilization losses from this layer. As one can see from Figure 2 in ref 4, differences in bifenthrin erosion losses simulated with PRZM 3.1.2 for compartment thicknesses ranging from 0.03 to 1 cm were less than a factor of 2, which disproves the USEPA statement. • “A similar situation holds true for FOCUS step 4 results, as almost a third of the PECsw values underpredicted the insecticide MFCs (Table 3). This result is remarkable when it is considered that FOCUS step 4 is the most realistic standard tier available in European regulatory exposure modeling and is almost exclusively used in risk Published: February 25, 2013 3015
dx.doi.org/10.1021/es3037884 | Environ. Sci. Technol. 2013, 47, 3015−3016
Environmental Science & Technology
Correspondence/Rebuttal
(5) U.S. EPA. Pesticide Root Zone model (PRZM) 3.1.2.3 release note, 2012. http://www.epa.gov/ceampubl/gwater/przm3/prz3reln. html. (6) Cheplick, M. Waterborne Environmental, Inc. Personal communication, 2010.
assessment for insecticides currently registered in the European Union.” The purpose of step 4 simulations is to account for the implementation of mitigation measures. If no mitigation measures (drift-reducing techniques, grassed buffer strips) were implemented in the field monitoring studies, it is no point comparing measured concentrations with the results of step 4 simulations. • “For the field studies that reported surface runoff as an insecticide exposure pathway, insecticide application was simulated as a granular application to exclude spray drift as an entry route for the PEC calculations.” This is justifiable only if surface runoff was reported to be the only input pathway. • “...However, this malpractice would not explain the 23% underestimation of MFCsw values resulting from FOCUS step 3 calculations, as this step does not include any pesticide application restrictions for farmers (e.g., nospray buffer zones).” Even without “no spray buffers” the minimum distance between the sprayed area and the water surface in FOCUS step 3 is 1 m for arable crops and 3 m for vines, orchards and hops. • “Compared to organochlorines and organophosphates, highly toxic pyrethroids had significantly lower ratios of PECsw to MFCsw for all FOCUS standard steps.” This may be explained by lack of comparability of measured and modeled pyrethroid concentrations: Conventional analytical methods measure not only dissolved, but also particle-bound and DOM-bound pyrethroids in water samples,3 whereas the PECsw calculated by FOCUSsw reflect the dissolved phase only. It is not really known whether pyrethroids bound to suspended sediment are equally or less bioavailable and toxic to aquatic organisms than dissolved pyrethroids. To be on the safe side, one should therefore use the total (dissolved + suspended) predicted concentration in future aquatic risk assessments, which can already be calculated by the TOXSWA model used in FOCUS.
Stefan Reichenberger*
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Footways S.A.S., 10 Avenue Buffon 45071 Orléans Cedex 2, France
AUTHOR INFORMATION
Corresponding Author
*Phone: +33 (0)238 63 63 11; fax: +33 (0)238 63 64 60; eMail:
[email protected]. Notes
The author declares no competing financial interest.
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REFERENCES
(1) FOCUS. FOCUS surface water scenarios in the EU evaluation process under 91/414/EEC. Report of the FOCUS Working Group on Surface Water Scenarios; EC Document Reference SANCO/4802/ 2001-rev.2, 2001. (2) Bach, M.; Hollis, J. M. Comment on “Regulatory FOCUS surface water models fail to predict insecticide concentrations in the field”. Environ. Sci. Technol., 2013, 47, 1177−1178, DOI: 10.1021/es303575j. (3) Liu, W.; Gan, J. J.; Lee, S.; Kabashima, J. N. Phase distribution of synthetic pyrethroids in runoff and stream water. Environ. Toxicol. Chem. 2004, Vol. 23, No. 1, pp. 7−11 (4) Luo, Y. Z.; Zhang, M. H. Environmental modeling and exposure assessment of sediment-associated pyrethroids in an agricultural watershed. Plos ONE 2011, 6, No. e15794. 3016
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