Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “Regulatory FOCUS Surface Water Models Fail to Predict Insecticide Concentrations in the Field”
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for the purpose of our study. From study (20), the concentrations for chlorpyrifos in bed sediment and bifenthrin and Tau-fluvalinate in suspended sediment were extracted from Figures 8 and 9 (see above for our general comment on the use of MFCsed). Furthermore, Bach and Hollis criticize study (21) because it was conducted in the “Altes Land” fruit- growing region, which is not covered by the FOCUS spray drift scenarios. However, both MFCsw values taken from this study were not underpredicted by the FOCUS model results. One major criticism proposed by Bach and Hollis involved the selection of data points, particularly the noninclusion of concentrations below the peak levels and especially below the detection limit. We did not neglect events with measured concentrations below the overall peak. If two events were independent, we included the peak value of each event although one of these two peak values might have been lower than the other. The FOCUS modeling approach predicts overall maximum concentrations based on realistic worst-case conditions (Step 3), which are also used for the acute risk assessment in place of exposure frequencies. Given that the measured concentrations are most likely underestimating the peak concentrations,3 the maximum MFCs were taken from the considered studies. This underestimation holds especially true for the majority of studies, which did not employ eventtriggered sampling (see Figure 2 in ref 1). Furthermore, Bach and Hollis criticize the selection of field concentrations from study (18); however, one of the two sites was intentionally omitted from our study,1 as its size (15 m width) exceeded those represented by the FOCUS water-body scenarios. Only one of the two substances was detected at the other study site (18) and thus included in our analysis. A further criticism by Bach and Hollis states that only 8 out of the 22 field studies were from the EU. This is, however, not relevant for FOCUS step 1 and 2 because these steps are not specific to any climate, crop, topography or soil type.4 Furthermore, we specifically reported in our study1 that the comparison of step 3 PECsw to MFCsw derived only from EU studies leads to an even higher degree of underprediction than the average from the global data. Overall, we disagree with Bach and Hollis that the studies that they cited are not appropriate for providing suitable MFCs. Nevertheless, excluding studies (2), (8), (20), (21), and (22) (i.e., those potentially influenced by farmyard runoff and structural pest control, or those coming from the “Altes Land”; 37 out of 122 cases) would result in an amount of underprediction in step 3 of 29% in surface water and 68% in sediment, even higher than the values given in our original paper.1 This result highlights that insecticide concentrations and thus MFC increase if only ascertained nonpoint sources are considered.
e appreciate the comment of Bach and Hollis on our study evaluating FOCUS predictions for insecticides in surface water.1 We emphasize that they concur with our overall conclusion “that the FOCUS modelling approach is not reliable in predicting insecticide concentrations when compared to realworld surface water concentrations.” For our study, we compiled field studies containing as much information necessary for FOCUS modeling as possible.1 In their comment, Bach and Hollis claim that a substantial number of the studies used by us are not suitable for providing measured field concentrations (MFCsw and MFCsed) and are consequently inappropriate for evaluating FOCUS predictions. In the following, we respond to their criticism. We agree that in study (1; reference numbers in brackets refer to the Supporting Information for ref 1), the source of insecticide input is irrigation return water. However, this is included in the FOCUS modeling approach. The only concentrations selected from study (5) were those that originated from spray drift during vineyard applications, not those potentially affected by insecticide wash-off from roads. One of the two MFCs from study (14) was clearly related to agricultural nonpoint source pollution, as this particular insecticide was only detected in field runoff and not as an input from point sources or farmyard entries. In studies (2), (8), and (22), it is not possible to definitively exclude that the MFCs resulted from sources other than runoff, drainage and spray drift. However, for this reason, we consciously selected no MFCs from study (2) from the upper watershed, which is more influenced by residential and commercial inputs. From study (8), no MFCs were taken from the larger east-side tributaries of the San Joaquin River, where urban land use prevails. For the western tributaries, where pyrethroids were most often detected, the authors of study (8) state that agricultural sources are strongly dominant and that pyrethroids are transported to stream channels as a result of irrigation and stormwater runoff. Studies (4), (8), (11), and (17) were criticized by Bach and Hollis because the sediments were sampled from a depth of 0− 3 cm, whereas the calculations of predicted environmental concentrations (PECsed and PECsw) in our study1 were performed for the FOCUS standard setting of 5 cm. Most importantly, as already mentioned in our study,1 PECsed plays virtually no role in regulatory risk assessment, and our principal conclusions refer exclusively to MFCsw. Nevertheless, we recalculated the PECsed values for those field studies based on the sampled sediment depths. The recalculated values yielded an even greater underprediction of MFCsed (51% instead of 42%). Indeed, study (13) provides only poor information on water and sediment sampling itself. However, especially for transient aqueous-phase insecticide exposure, any inadequate sampling methodology would at most produce an underestimate of the actual peak concentrations.2 As all other relevant information for FOCUS modeling is included, study (13) was well suited © 2012 American Chemical Society
Published: October 18, 2012 1179
dx.doi.org/10.1021/es3040957 | Environ. Sci. Technol. 2013, 47, 1179−1180
Environmental Science & Technology
Correspondence/Rebuttal
Bach and Hollis further commented that the change of base flow in step 3 realistic means that the FOCUS scenario is incorrect. We intentionally adapted the flow to make the FOCUS step 3 realistic results more realistic. The FOCUS group states that base flow ”usually represents only a very minor fraction of the total flow in a FOCUS surface water body, as soon as drainage or runoff occurs.”4 Therefore, we do not agree that the FOCUS step 3 realistic scenarios that we used, which are not part of the regulatory pesticide exposure assessment, are incorrect. Finally, we conclude that the field studies and methodologies used in our study are definitely adequate to prove that the FOCUS surface water models completely fail to predict insecticide concentrations in the field and that our main conclusions remain unaltered by the comment of Bach and Hollis.
Anja Knab̈ el* Sebastian Stehle Ralf B. Schaf̈ er Ralf Schulz
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Institute for Environmental Sciences, University Koblenz-Landau, Landau, Germany
AUTHOR INFORMATION
Corresponding Author
*Phone: +49-6341-28031313; E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Knäbel, A.; Stehle, S.; Schäfer, R. B.; Schulz, R. Regulatory FOCUS surface water models fail to predict insecticide concentrations in the field. Eviron. Sci. Technol. 2012, 26, 8397−8404. (2) Leu, C.; Singer, H.; Stamm, C.; Muller, S. R.; Schwarzenbach, R. P. Variability of herbicide losses from 13 fields to surface water within a small catchment after a controlled herbicide application. Environ. Sci. Technol. 2004, 38, 3835−3841. (3) Crawford, C. G. Sampling strategies for estimating acute and chronic exposures of pesticides in streams. J. Amer. Water Resour. Assoc. 2004, 40, 485−502. (4) 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.
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