Impact of experimentally derived surface water half-lives on the accuracy of exposure modeling for readily biodegradable substances

The half-lives of chemical substances in river water are important for assessing environmental persistence and estimating chemical concentrations through exposure modeling. Reports on the half-lives of readily biodegradable substances are limited, and regulatory guidelines provide default values for these substances in the absence of actual data. However, few studies have assessed the effect of experimentally derived half-lives on exposure estimates compared to default values. In this study, we investigated the half-lives of 12 readily biodegradable substances, including surfactants and tertiary amines, and evaluated the impact of the experimentally derived values on the accuracy of model estimation. Most experimentally derived half-lives were less than 24 h, which is considerably shorter than the default values of 120 and 360 h commonly used in regulatory assessments. When half-lives exceeded 60 h, biodegradability in surface water had a limited effect on concentration estimates in The National Institute of Advanced Industrial Science and Technology - Standardized Hydrology-based AssessmeNt tool for chemical Exposure Load model, whereas half-lives below 24 h led to decreased concentration estimates. When concentration estimates using experimentally derived and default values were compared with environmental monitoring data, the experimentally derived half-lives improved the accuracy of the model estimations. These findings suggest that the experimentally derived half-lives of readily biodegradable substances considerably affect exposure estimates. Therefore, readily biodegradable substances that show high exposure concentrations based on default values should be investigated further in higher-tier assessments with experimentally derived values, especially for surface water half-lives.