Today, vast numbers of organic compounds are in commerce, many of which end up in the environment as contaminants. Chemical biodegradation affects the fate of almost all organic compounds in the environment and is a key process in eliminating contaminants from the environment. The rate of biodegradation is often the largest source of uncertainty in assessing chemical exposure. It is usually quantified in the laboratory using biodegradation simulation tests, such as the OECD 309 test. However, the environmental relevance of existing standard procedures for assessing the biodegradation of compounds in aquatic environments has been questioned. In addition, no systematic investigation of the spatiotemporal variability in biodegradation rates provided by these tests has been done for multiple and diverse compounds. If a chemical has biodegradation rate constants that vary across space and time, the variability is likely to cause uncertainties in evaluating its environmental persistence and risk. This thesis aims to quantify the spatial and temporal variability in biodegradation rate constants through a more environmentally relevant test and to establish a scientific framework for more accurate extrapolation of rate constants across space and time.

In Paper I, a modified OECD 309 test was developed to increase the environmental relevance of the biodegradation rate constant outcomes. The results demonstrated that a modified OECD 309 test at low spiking concentration can provide good reproducibility and comparable results to no spiking. The environmental relevance of the measured rate constant can be further increased by focusing on initial biodegradation kinetics. Following the modified OECD 309 protocol developed in Paper I, the biodegradation rate constant was quantified for 96 compounds during four seasons in four river segments in Paper II to study the seasonality, and it was quantified for 97 compounds in 18 European river segments in Sweden, Germany, Switzerland, Spain, and Greece in Paper III to study the spatial variability. The rate constants of almost all compounds varied seasonally and spatially in the experimental rivers with a median standard deviation corresponding to a fold difference of two and three, respectively. Biodegradation frequently deviated from Arrhenius-type temperature dependence. The seasonal and spatial variability of the rate constants was not fully explained by the variation in pH-dependent bioavailability, bacteria cell density, tested environmental factors, and the contamination level in the river segment. In Paper IV, we explored whether we could describe the spatiotemporal variability by a proxy of total microbial biomass or a benchmark chemical. We compiled a comprehensive dataset consisting of 2265 rate constants for 97 compounds measured in 38 modified OECD 309 tests in European (Papers II and III) and Australian aquatic ecosystems. The results indicated that a single proxy, either a total biomass proxy or a universal benchmark, could not well describe the biodegradation rate constants of compounds across ecosystems. The rate constants of some compounds showed significant correlations in space and time. Grouping chemicals based on these correlations increased the accuracy of predicting biodegradation by group-specific benchmarking.