Many chemicals are released into surface water and biodegradation is one important mechanismto remove the contaminants. Their actual concentrations in water depend on their persistence.The OECD 309 test is the most recognized method to assess the persistence of chemicals in theaquatic environment. However, the quantity of sediment used in the OECD 309 suspensiontests is not clearly defined. To investigate the influence of sediment concentrations and thebacterial cell count on the biodegradation, we compared the biodegradation rate constants of amixture of organic chemicals in suspension tests with 5 different water-sediment ratios. Thesediment concentrations were 125, 50, 20, 8 and 3 g L-1. The 5 test systems were spiked with129 chemicals including pharmaceuticals, agrochemicals, food additives, and cosmetics at aconcentration of 1 μg mL-1 each. The chemical degradation was monitored by analyzing thewater subsamples collected at different time points during the 14-day incubation period. It wasthe first time that the effects of sediment concentrations on biodegradation were systematicallystudied for a broad range of chemicals. The bacterial cell count was measured to test whether itcorrelated with sediment concentration and biodegradation rate constant. In the present study,k could be quantified for at least 2 sediment concentrations for 10 chemicals. The resultsshowed that the biodegradation rate constants of the chemicals, k, were positively correlatedwith sediment concentration for 10 chemicals; 6 of these correlations were significant. The totalbacterial cell count showed a positive correlation with sediment concentration, but it was notsignificant. Also, k was positively correlated with total cell count, but these correlations werenot significant for any of 10 chemicals. It is concluded that sediment plays an important role inchemical biodegradation. Sediment-associated total cell count can be one factor contributing tothis effect. The variable results of chemical biodegradation rate constants caused by differentsediment concentrations in the present suspension tests suggest the need for a standardizedsediment concentration in OECD 309 tests.

This thesis presents the development of a novel numerical model capable of evaluating suspended sediment dynamics under breaking waves, and is based in the open source Computational Fluid Dynamics software, OpenFOAM®. The hydrodynamics were determined by solving the incompressible, Reynolds-Averaged Navier-Stokes equations for a two-phase fluid using the Finite Volume method, along with a Volume of Fluid scheme that modelled the interface between the air and water phases. A new library of five turbulence models was developed to include weakly compressible effects through the introduction of density variations in the conservation equations. This library was thoroughly evaluated against existing physical data for surf zone dynamics. A skill score was applied, based on the MSE, to rank the models, with the nonlinear k−ε performing best overall, and the k−ω predicting turbulent kinetic energy most accurately. Furthermore, the numerical model was shown to predict the near-bed hydrodynamics well, through comparison with new in-house physical data obtained in the COAST laboratory. Suspended sediment concentrations were determined using an advection-diffusion methodology, with near-bed processes modelled using a flux based approach that balances entrainment and deposition. The model was validated against existing experimental data for steady state flow conditions, as well as for regular and breaking waves. The agreement was generally good, with the results indicating that the model is capable of capturing complicated processes such as sediment plumes under plunging breakers. The validated model was applied to investigate the properties of the sediment diffusivity, which is a vital parameter in suspended sediment dynamics. In physical experiments, sediment diffusivity is commonly estimated implicitly, based on the vertical concentration profile. In this work, this approach was applied to the numerical concentration predictions, and compared with the value directly determined within the model. The estimated value was generally acceptable providing that large horizontal concentration gradients were not present, and diffusion dominated flow advection. However, near the breaking point of waves, large errors were observed at mid-depth of the water column, which strongly correlates with a region of large flow advection relative to diffusion. Therefore, when using this estimation, caution is recommended since this approach can potentially lead to substantial discrepancies.