The Scheldt river drains a densely populated and industrialized area in northern France, western Belgium and the south-west Netherlands. Mineralization of the high organic load carried by the river leads to oxygen depletion in the water column and high concentrations of dissolved nitrogen and phosphorus compounds. Upon estuarine mixing, dissolved oxygen concentrations are gradually restored due to reaeration and dilution with sea water. The longitudinal redox gradient present in the Scheldt estuary strongly affects the geochemistry of nutrients.Dissolved nutrients in the water column and dissolved nitrogen species in sediment porewaters were determined for a typical summer and winter situation. Water column concentration-salinity plots showed conservative behaviour of dissolved Si during winter. During summer (and spring) dissolved Si may be completely removed from solution due to uptake by diatoms. The geochemistry of phosphorus was governed by inorganic and biological processes. The behaviour of nitrogen was controlled by denitrification in the anoxic fluvial estuary, followed by nitrification in the upper estuary (prior to oxygen regeneration). In addition, nitrogen was taken up during phytoplankton blooms in the lower estuary.Dissolved inorganic nitrogen species in porewaters from the upper 20 cm of sediments were obtained from a subtidal site in the middle of the lower estuary. Dissolved nutrient concentrations were low in the upper 10-15 cm of the sandy and organic poor (<1% POC) sediments mainly as a result of strong sediment mixing.The porewater profiles of ammonium and nitrate were evaluated quantitatively, using a one-dimensional steady-state diagenetic model. This coupled ammonium-nitrate model showed ammonification of organic matter to be restricted to the upper 4 to 7 cm of the sediments. Total nitrification ranged from 3.7-18.1 mmol m -2 d -1, converting all ammonium produced by ammonification. The net balance between nitrification and denitrification depended on the season. Nitrate was released from the sediments during winter but is taken up from the water column during summer. These results are in good agreement with data obtained from the independently calibrated water column model for the Scheldt Estuary (VAN GILS et aL, 1993).
Hydrodynamic and water quality 3D modelling of the Nam Theun 2 Reservoir (Lao PDR): predictions and results of scenarios related to reservoir management, hydrometeorology and nutrient input Modélisation 3D de l'hydrodynamique et de la qualité d'eau du réservoir de Nam Theun 2 : prédictions et résultats de scénarios liés à la gestion du réservoir, à l'hydrométéorologie et à l'apport de nutrimentsV. Chanudet (1) , J. Smits (2) , J. Van Beek (2) , P. Boderie (3) , F. Guérin (4,5,6) , D. Serça (7) , C. Deshmukh (7 Abstract -A 3D water quality model has been applied to predict medium term evolution of the water quality in the Nam Theun 2 Reservoir and also to quantify the effect of various scenarios. 15-year simulations show that the oxygen concentration will continue to increase in the water column although the hypolimnion will remain anoxic in some areas of the Reservoir. In parallel, the concentration of reduced compounds will decrease with time. The significance of the hydrodynamics in water quality evolution is pointed out with two scenarios in which natural or human forcings have been modified. The comparison of simulations made for years with contrasted hydrometeorological conditions shows that and duration of major hydrometeorological related events (rainfall, flood and air temperature drop) have a major influence on the seasonal evolution of water quality in the whole Reservoir. Simulations also show that the physico-chemical quality of the water released downstream of the power house would have been different if the commissioning had been carried out immediately after the impoundment. Finally, the model has been used to quantify the impact of an increase of the NO 3 -and PO 4 3-incoming flux consecutive to potential changes in the watershed land use. The fluxes have been multiplied by a factor two separately (2 scenarios) and together. While the additional load of NO 3 -has almost no impact on physico-chemistry and phytoplankton activity, the INTRODUCTIONWater quality in reservoirs depends on many parameters and processes, including hydrodynamics. Interactions are so numerous (Stumm & Morgan, 1996) that it is difficult to predict or assess precisely the effect on water quality of a modification of forcing data (from change in the watershed to global change). A possible approach to overcome this difficulty is numerical modelling. Since the 1970s many water quality and ecological processbased models have been developed (Jorgensen et al., 1996) and the studies of water quality and ecosystem problems with numerical models are increasing rapidly (Cerco & Cole, 1993;Jorgensen, 1999;Gin et al., 2001;Arhonditsis & Brett, 2005;Chao et al., 2007). Some well-established threedimensional water quality models have been successfully applied to simulate water quality variables in rivers, lakes, estuaries and ocean environments (Cerco & Cole, 1995;Luyten et al., 1999;Antenucci et al., 2000;Wool et al., 2001;Hydroqual, 2004;Romero et al., 2004; Danish Hydraulic Institute, 2001;Deltares, 2013). This type of model has be...
The Ganga Basin in India experiences problems related to water availability, water quality and ecological degradation because of over-abstraction of surface and groundwater, the presence of various hydraulic infrastructure, discharge of untreated sewage water, and other point and non-point source pollution. The basin is experiencing rapid socio-economic development that will increase both the demand for water and pollution load. Climate change adds to the uncertainty and future variability of water availability. To support strategic planning for the Ganga Basin by the Indian Ministry of Water Resources, River Development and Ganga Rejuvenation and the governments of the concerned Indian states, a river basin model was developed that integrates hydrology, geohydrology, water resources management, water quality and ecology. The model was developed with the involvement of key basin stakeholders across central and state governments. No previous models of the Ganga Basin integrate all these aspects, and this is the first time that a participatory approach was applied for the development of a Ganga Basin model. The model was applied to assess the impact of future socio-economic and climate change scenarios and management strategies. The results suggest that the impact of socio-economic development will far exceed the impacts of climate change. To balance the use of surface and groundwater to support sustained economic growth and an ecologically healthy river, it is necessary to combine investments in wastewater treatment and reservoir capacity with interventions that reduce water demand, especially for irrigation, and that increase dry season river flow. An important option for further investigation is the greater use of alluvial aquifers for temporary water storage.
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