Biodegradation Rates of Organic Chemicals in a Subtropical River: From Laboratory to Field

Understanding the attenuation rate of organic chemicals in aquatic environments is essential for assessing their long-term ecological risks and informing chemical management. Biodegradation is a key attenuation pathway for many chemicals, however, measuring biodegradation in situ is inherently complex, resource-intensive, and subject to high environmental variability. Consequently, regulatory frameworks and research efforts often rely on laboratory tests, intended to provide controlled and reproducible conditions. Despite their widespread application, few studies have evaluated how well these laboratory results represent biodegradation in real aquatic environments. In this study, we compared biodegradation rate constants (k) from a modified OECD 309 laboratory test with those derived from a field experiment in a subtropical river (Queensland, Australia). Biodegradation was measurable in both laboratory and field experiments for 23 chemicals spanning diverse structures. Rank-order agreement between laboratory k and field k was strong (Spearman ρ = 0.7, p < 0.001), and linear association was high (Pearson r = 0.8, p < 0.001) for the 16 chemicals that did not sorb strongly. The relative magnitudes of laboratory and field log k also aligned well (slope of the linear regression = 0.8), however, uncertainty remained (average root-mean-square-error equivalent to a factor of 2). These results demonstrate that the modified OECD 309 can capture much of the rank-order and relative magnitude of biodegradation, supporting its role as a higher-tier laboratory simulation test for persistence assessment.