Long-term development, modeling and management of nutrient loading to inland and coastal waters

Abstract: Environmental and water system protection and restoration require accurate knowledge and quantification of waterborne pollutant transport, retention-release and transformation processes taking place in the subsurface, river, lake and sea water systems, and at the interfaces between these systems. This thesis investigates the importance of spatial model resolution for such quantification and the possible long-term nutrient load development under different human perturbations and nutrient source management scenarios in the Swedish Norrström drainage basin, based on simulations with the GIS-based dynamic nutrient transport-attenuation model POLFLOW. The results indicate substantial nutrient load contributions from the subsurface water system (soil water and groundwater) to surface waters, which may largely control present and future nutrient mass loads from drainage basins to downstream inland and coastal waters. This role of the subsurface water system needs to be acknowledged, in conjunction with the effects of expected climate change and population and lifestyle developments, for achieving successful and sustained decreases in nutrient loading to inland and coastal waters. The results point also towards a need for downstream nutrient load control and abatement measures, to complement nutrient source reduction programs. The results further indicate the importance of the chosen spatial model resolution and account for subgrid variability for the accuracy of the necessary coupled transport and attenuation modeling of nutrient spreading in, and export from, river networks to coastal systems. Failure to correctly conceptualize and accurately account for the physical transport processes and their subgrid variability in this modeling may also yield systematic misinterpretation of the biogeochemical processes and process rates. Further, this may have important practical implications for the necessary abatement effectiveness and efficiency of nutrient and other waterborne pollutant loading to inland and coastal waters worldwide.