Geomorphic Controls on the Transformation of Wastewater Pollutants in Rivers

<p dir="ltr">Microbial transformations, such as nitrification and biodegradation, are important removal mechanisms for many wastewater pollutants. Although in-stream removal rate coefficients are often assumed to be spatially and temporally constant, they are likely to be affected by channel shape and size, which controls the extent of interactions between the water column and fixed biofilms. This thesis hypothesizes that transformation rate coefficients are inversely proportional to hydraulic radius (the ratio of channel cross-sectional area to wetted perimeter) and further modulated by sediment size, which influences biofilm colonisation and hyporheic exchange. These hypotheses were tested through mesocosm experiments, fieldwork and exposure modelling. In the mesocosm experiments, nitrification and biodegradation were monitored in stirred tanks with three different water depths and three different sediment sizes (sand, gravel and cobbles). Rate constants were inversely related to depth, highlighting channel morphology as a key control. Nitrification rate constants were highest in the gravel and lowest in sand, suggesting intermediate grain sizes optimize biofilm growth and solute exchange. To determine the extent of channel geometry controls on pollutant behaviour in the field, dye tracing experiments were conducted in two UK rivers with contrasting channel morphologies: (1) the River Maun – a small/shallow river, and (2) the River Calder – a large/deep river. Rate constants were derived for ammonium, caffeine and linear alkylbenzene sulfonate (LAS). Higher rate coefficients were observed in the Maun, which supports the proposed hypothesis. Building on these findings, a gridded model was constructed to predict wastewater pollutant concentrations in rivers that incorporates channel geometry characteristics. The model was applied to predict LAS concentrations in four UK river catchments. The model performed well and highlighted the importance of river channel geometry as the Damköhler number (the ratio of reaction rate to advection rate) increases. This thesis highlights the need to consider geomorphology in higher-tier chemical exposure models.</p>