De ecologische risico's van bestrijdingsmiddelen in beeld: een toxische druk indicator in de bestrijdingsmiddelenatlas oppervlaktewater

Abstract

Pesticides are a main source of chemical pollution in Dutch surface waters, where they occur as unintended

mixtures. This often causes mixture effects, which may be more harmful than expected from exposure to

individual substances. The degree of harm is not simply evaluated on the basis of exceedances of protective

water quality standards. To address this issue more effectively, the Institute of Environmental Sciences Leiden (CML) and the National Institute for Public Health and the Environment (RIVM) have developed a new mixture toxic pressure indicator, now incorporated into the Atlas pesticides in surface water (Pesticides Atlas, BMA, www.pesticidesatlas.nl). This report explains the background and development of the new Atlas products developed with this indicator, which focus on mixture toxicity as a measure for the net impact of unintended chemical mixtures on aquatic life. The new indicator is based on decades of RIVM- and global research into mixture toxic pressure methods and combines toxicity data using Species Sensitivity Distributions (SSDs). According to this method, exposure to mixtures is characterized by quantifying the mixture toxic pressure, expressed in the dimensionless multi-substance Potentially Affected Fraction (msPAF) metric. The mixture toxic pressure predicts which proportion of aquatic species is potentially affected by the mixture. Calibration studies confirm that higher mixture toxicity correlates with greater ecological impacts on water organisms. Six new products have been created, including interactive maps, monthly and annual trend graphs, and risk attribution tables. They build on the concepts of Quantitative Risk Analysis and risk attribution, quantifying both the severity (chance × impact) of ecological harm and the relative contributions of individual substances. Products 1 through 4 show the spatial and temporal trends in toxic pressure, while the remaining products identify which specific substances cause disproportionate harm within a mixture. Together, the products allow evaluation of where impacts are likely highest, which compound(s) likely cause this, and whether novel uses or programs of measures induce a change in mixture toxic pressure. For the indicator, if important compounds are not measured or represented with accurate concentration measurements in the monitoring data (i.e. below LoQ), the mixture toxic pressure metric underestimates the true local toxic pressure. Key findings indicate that overall mixture toxicity shows a substantial spatial variability, which has—on average—decreased over time, but also that exposure peaks and potential effects from newer, insufficiently tested substances remain an issue. A lack of SSD data for these new substances likely underestimates the actual toxic pressure. Synthetic pyrethroids (such as cypermethrin and deltamethrin) currently appear to be the largest contributors to mixture toxicity, and the data reveal that mixture effects can be high even if individual substance concentrations do not exceed regulatory norms. Looking ahead, collection of representative environmental monitoring data combined with supplementing toxicity data for data-poor compounds will be crucial. The new indicators in the BMA help users such as water managers, governments, authorisation boards, manufacturers, and farmers(-organisations) to better determine where in the Netherlands the highest ecological risks in surface waters occur and which substances are primarily responsible. Additionally, a regional or local application of the mixture toxic pressure indicator will enable water authorities and farmers to take targeted measures, for instance by focusing on the main contributors in their area. In conclusion, the toxic pressure indicator in the Atlas provides valuable summary information to both prioritize protective measures and evaluate the effectiveness of previous policies. Moreover, the use of this kind of method—offering one overarching and nuanced view on chemical pollution instead of numerous separate maps or trends evaluated via the One Out, All Out principle of the EU-Water Framework—has been recommended in a recent amendment to the Water Framework Directive. As such, the new Atlas products offer additional insights, which stakeholders (regulators, industry, academia) have deemed a valuable foundationfor both practical and policy-related decisions on pesticide use in the Netherlands.