ecotox consult - Dr. Michael Meller - ecotoxicological consulting service

In higher-tier risk assessment for PPP the new EU aquatic guidance document (AGD) [1] requires the analysis of predicted exposure profiles with respect to data gained from mesocosm studies. Specifically the requirements concern two aspects:

1. Verification of a realistic to worst case dissipation in a mesocosm study to derive a RAC (regulatory acceptable concentration).
2. Assessment of predicted exposure periods exceeding the ETO-RAC (ecological threshold option) but not the ERO-RAC (ecological recovery option) to upgrade a provisional to an official ERO-RAC.

How these requirements can be applied adequately in practice is discussed in this study.

Methods

A dataset of mesocosm study results and parameters required for exposure modeling was compiled for 7 pesticides from publicly available EU assessment reports. According to the published GAPs, exposure profiles were modeled with FOCUS [2] for a total of 233 scenarios. 392 peaks were compared with measured exposure in the mesocosm studies by following methods:

• Visual comparison by expert judgement
• Area under the curve (AUC)
• Vertical differences of data points

ETO- and ERO-RAC were derived based on effect class 1 or 2 / NOEC and effect class 3A / NOEAEC, assuming assessment factors (AF) of 2 and 3, respectively. RAC values were plotted with the exposure profiles and a method for assessment of interjacent peaks was investigated. The ERO-RAC curve is derived by applying an AF of 3 on the measured concentrations of class 3A / NOEAEC. For vertical difference, the deviations of normalized mesocosm dissipation from exposure profiles is calculated evaluating the highest negative deviation in %. All calculations and graphical analysis were performed with R.

Results & Discussion

Verification of a realistic to worst case dissipation: Mesocosm dissipation data were compared to predicted exposure peaks based on the mean of all treatment levels. 233 exposure profiles were assessed. Visual comparison and AUC show similar results and are thus considered adequate for the verification of a realistic to worst case dissipation in a mesocosm study. The third evaluated method shows results with twice as much unrealistic and not worst case ratings. The automated calculation used for the vertical difference would need further improvements to be a useful tool for regulatory purposes.

Figure 1: Mesocosm concentrations as mean of all levels (black line) normalized on exposure profile peak and plotted with the simulated exposure peak (red line) for a PPP in D1 ditch scenario with one application per season on summer cereals. AUC=Area under the curve, in-plot values represent the vertical difference of the mesocosm concentration (black dots) from the predicted exposure (red line) in %.

ERO-RAC upgrading: According to the AGD [1] and ELINK [4], upgrading of a provisional ERO-RAC to an official one should be done by linking effects to exposure profiles based on the exposure period of peaks exceeding the ETO-RAC and the total effect period. Alternatively Boesten et al. [3] proposed a ‘RAC curve’ based on measured concentrations for comparison with exposure profiles. Based on this approach an ERO-RAC curve for assessing the exposure peaks exceeding the ETO-RAC but not the ERO-RAC was used in this study. Considering evaluated examples, upgrading ERO-RAC based on class 3A effects is considered acceptable:

• if all peaks were below the ERO-RAC curve, since no population relevant long-term effects were observed at this treatment level;
• if toxicologically independent exposure peaks exceed the ERO-RAC curve, the total effect period (with respect to the exposure period of peaks exceeding the ETO-RAC) is < 8 weeks after the first relevant peak.

Conclusion

• The AUC is considered adequate to characterize mesocosm dissipation rate and the verification of a realistic to worst case dissipation.
• Based on the evaluated examples linking effects to exposure, using an ERO-RAC curve is considered an acceptable alternative to the AGD proposal and sufficiently protective to meet the specific protection goal of recovery within 8 weeks after first exposure.
• More research is needed to verify the proposed methods.

References

[1] EFSA (2013) EFSA Journal, 2013;11(7):3290, 268 pp. doi: 10.2903/j.efsa.2013. 3290.
[2] FOCUS (2012) Generic guidance for FOCUS surface water scenarios. Version 1.2, December 2012.
[3] Boesten et al. (2007) Ecotoxicology and Environmental Safety 66, 291–308.
[4] Brock et al. (2010) SETAC Press, ISBN: 978-1-4398-1347-8.
[5] Ernst, J. (2015) Master Thesis: Assessment of pesticide effects based on time-variable exposure in aquatic ecosystems.