From Freshwater to Marine Environments: Spatial Variation in Chemical Biodegradation Rates Applying a Modified OECD 309-Type Experiment

The rate of biodegradation in aquatic environments is a large source of uncertainty in chemical exposure assessments. Currently, there are insufficient biodegradation data and limited understanding of spatial variation in biodegradation, particularly across diverse aquatic environments. This study investigated the spatial variation in chemical biodegradation rate constants (<i>k</i>) across seven distinct aquatic environments in Australia including marine, mangrove, estuarine, and freshwater ecosystems, using a modified OECD 309 test. Significant spatial variation in <i>k</i> was found for 36 of 38 chemicals (ANOVA, <i>p</i> < 0.05). In addition, the average log <i>k</i> for the chemicals was strongly correlated with the average log <i>k</i> of the same chemicals in 18 European rivers measured using the same method (Pearson <i>r</i> = 0.94, <i>p</i> < 0.001), illustrating intercontinental consistency in the relative biodegradation behavior of the chemicals. However, the standard deviation in log <i>k</i> (which is a relative metric equivalent to a fold difference) was larger across the Australian sites than across the European sites for many chemicals. These findings provide two key insights: first, that diverse aquatic environments demonstrate greater spatial variability in <i>k</i> than rivers; and second, that despite this spatial variability, the modified OECD 309 method can reliably rank chemicals according to their biodegradability and thus be useful in persistence assessments.