Are Sorption and Degradation Processes Adequately Accounted for in Surface Water Risk Assessments for Plant Protection Products?
This article outlines the processes which drive the fate and behaviour of plant protection products (PPPs) in surface waters, and to consider how well these are accounted for in regulatory systems. Even with best practice, it is possible that under certain circumstances, PPPs will reach surface water via drift, run-off, point source contamination, and drainage. In risk-based regulatory systems, the concentration of PPP reaching the water body is estimated then compared to the concentration that has been observed to cause adverse effects in non-target organisms. At its most basic, this procedure compares the worst case exposure estimate to the lowest concentration at which an adverse effect was observed. If this simple calculation results in an adequate margin of safety, this part of the risk assessment is passed. If not, then refinements to both the exposure or effect sides of the calculation can be made to reflect a more realistic in-use situation. This overview focuses on the exposure side of this calculation. There are areas of overlap with effects testing and attempts better to integrate exposure and effects testing are currently being made by groups such ELINK. Mathematical models that take account of factors such as the type of water body, the use of mitigation measures to reduce entry and the route of entry of the PPP are often used to refine exposure estimates. However, there are few options to use more realistic sorption and degradation data to refine exposure estimates. This is due to both an absence of regulatory guidelines for such studies and the difficulties of using the data in existing models. As a result, often only the data from laboratory adsorption/desorption and water/sediment studies (conducted in the dark) are used. A new development in safety assessment in Europe is the proposal to refuse or remove registration of a compound if certain undesirable properties are demonstrated above an arbitrary acceptable limit. Such cut-off criteria will apply irrespective of whether the compound passes risk assessments. Persistence in surface waters and sediments may be used as part of one such “cut-off” criterion. At present there is no need or requirement to demonstrate that a PPP degrades to pass risk assessments. As a result for some PPPs there will be no evidence available that the compound is not persistent. Additional studies may, therefore, become necessary to demonstrate that under more realistic conditions sufficient degradation will take place. A number of large scale water-sediment studies with aquatic plants (microcosm studies) are described and the degradation rates obtained in these studies have been compared to those from laboratory water/sediment studies. These data suggest that the current suite of laboratory studies fail to account for a number of potentially important dissipation processes. In Europe, this may be addressed to a limited extent by the revision of Council Directive 91/414 which regulates PPPs. The current draft of which, provides the option of conducting water/sediment studies under UV light and of determining photolysis rates in natural waters. Even so it is not clear how the results from such studies could be subsequently used in the models to estimate exposure. Furthermore, the contributions of aquatic biota to both sorption and degradation would still be excluded.
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