Chemicals' half-lives derived from biotransformation simulation studies are central metrics for persistence assessment in international regulatory frameworks. To determine the persistence of chemicals released to the aquatic environment, paradigm shifts in recent and ongoing revisions of chemical legislation assign increasing importance to OECD 309 simulation studies. OECD 309 studies were designed to target biotransformation in natural water (pelagic test) or in water amended with sediment (suspension test). Suspension tests bear several advantages over the pelagic test, most importantly, employing higher bacterial cell densities, which promote biotransformation of various chemicals at observable rates. However, experience with suspension tests is limited. In this study, we followed the fate of 43 pharmaceuticals, pesticides, and industrial chemicals in various suspension test setups and elucidated parameters influencing biotransformation kinetics and half-lives derived thereof. Besides striking intrastudy variability between replicates, we found that differences in sediment origin and bacterial cell density resulted in chemical half-lives that were different by up to 2 orders of magnitude, making persistence classification rather uncertain. However, data suggested that test systems employing bacterial cell densities close to the upper limit of what is commonly observed in natural surface waters (i.e., 107 cells mL−1) yielded increased and more uniform biotransformation of chemicals.

Urban-use pesticides are of increasing concern as they are widely used and have been linked to toxicity of aquatic organisms. To assess the occurrence and treatment of these pesticides in stormwater runoff, an approach combining field sampling and watershed-scale modeling was employed. Stormwater samples were collected at four locations in the lower San Diego River watershed during a storm event and analyzed for fipronil, three of its degradation products, and eight pyrethroids. All 12 compounds were detected with frequency ranging from 50 to 100%. Field results indicate pesticide pollution is ubiquitous at levels above toxicity benchmarks and that runoff may be a major pollutant source to urban surface waters. A watershed-scale stormwater model was developed, calibrated using collected data, and evaluated for pesticide storm load and concentrations under several management scenarios. Modeling results show that enhanced stormwater control measures, such as biochar-amended biofilters, reduce both pesticide storm load and toxicity benchmark exceedances, while conventional biofilters reduce the storm load but provide minimal toxicity benchmark exceedance reduction. Consequently, biochar amendment has the potential to broadly improve water quality at the watershed scale, particularly when meeting concentration-based metrics such as toxicity benchmarks. This research motivates future work to demonstrate the reliability of full-scale enhanced stormwater control measures to treat pollutants of emerging concern.

Studying aquatic biotransformation of chemicals in laboratory experiments, i.e., OECD 308 and OECD 309 studies, is required by international regulatory frameworks to prevent the release of persistent chemicals into natural water bodies. Here, we aimed to address several previously described shortcomings of OECD 308/309 studies regarding their variable outcomes and questionable environmental relevance by broadly testing and characterizing a modified biotransformation test system in which an aerated water column covers a thin sediment layer. Compared to standard OECD 308/309 studies, the modified system showed little inter-replicate variability, improved observability of biotransformation, and consistency with first-order biotransformation kinetics for the majority of 43 test compounds, including pharmaceuticals, pesticides, and artificial sweeteners. To elucidate the factors underlying the decreased inter-replicate variability compared to OECD 309 outcomes, we used multidimensional flow cytometry data and a machine learning-based cell type assignment pipeline to study cell densities and cell type diversities in the sediment and water compartments. Our here presented data on cell type composition in both water and sediment allows, for the first time, to study the behavior of microbial test communities throughout different biotransformation simulation studies. We found that sediment-associated microbial communities were generally more stable throughout the experiments and exhibited higher cell type diversity than the water column-associated communities. Consistently, our data indicate that aquatic biotransformation of chemicals can be most robustly studied in test systems providing a sufficient amount of sediment-borne biomass. While these findings favor OECD 308-type systems over OECD 309-type systems to study biotransformation at the water–sediment interface, our results suggest that the former should be modified toward lower sediment–water ratios to improve observability and interpretability of biotransformation.