An integrated modelling framework to simulate unsteady flow and reactive-transport for urban river-aquifer system

In this study, a new modelling framework that addresses issues was developed and used to investigate real world problems involving the transfer of contaminants between the two systems. The tool was built from the full (internal) coupling of modified versions of stand alone river and groundwater state-of-the-art transport and water quality models to an integrated river-groundwater flow model. The exchange of flow and mass across the interface is computed and updated throughout the simulation using the conductance-based model.;Firstly, the mathematical and numerical performance of the coupled model, which was recompiled to a new computer program and named as FLUXOS, was validated on controlled cases against analytical solutions. Additionally, its sensitivity and capabilities were theoretically tested on a series of real world test cases. Secondly, the model was applied to the Ciliwung River in Jakarta, Indonesia, to investigate two distinct pollution problems in two independent studies. In both cases, the water quality issues are related to the exchange of contamination between the river and the groundwater. Simulation results with coupled and stand alone models were compared to each other, and to observations when possible, to evaluate the performance of FLUXOS and discuss its advantages and limitations.;The Ciliwung River, which is the largest river, amongst thirteen other, that flow through the city of Jakarta and empties in the Jakarta Bay, is an interesting case to investigate with FLUXOS because the poor water quality of the river, which loses water to the groundwater inside Jakarta, suggests that it may significantly contribute to groundwater contamination and deterioration, a phenomenon that has already been observed in the shallow aquifer.;In the first real world application, FLUXOS was used to bring new insights into the characteristics of the nitrogen (N) cycle in the river-aquifer systems of Jakarta, and to identify the main sources and N-contamination pathways. Besides being interesting as a complex case where the model can be challenged and show its potential, the study is motivated by recent observations in the shallow aquifer of Jakarta that point at a rise in the nitrate (NO3 -) levels. As groundwater is extensively used in the city to compensate for the limited public water supply network, a rise in NO3 - concentration can put at risk this resource, due to its consequence on human health, particularly for children, for which it can contribute to excessive levels of methemoglobin in the blood and eventually lead to death in extreme cases.;The particular focus of this investigation is on the impact of urbanisation on the nitrogen cycle of the river-aquifer system. In this context, the model was thus used to quantify river to groundwater nitrogen recharge fluxes during the wet and dry seasons for the current and some future development scenarios. The study uses field data along Ciliwung River provided by the Indonesian authorities. The data was used to define the numerical application and to validate the model. The computed fluxes were discussed in relation to estimates of leaks from septic tanks, which were calculated using different literature data. The results of the model simulations and field data were combined to discuss effective rehabilitation strategies to control the levels of NO 3- in the groundwater.;The coupled model was further used in a second real world application to investigate the impact of river regulation on the quality of river and shallow aquifer in Jakarta. This is expected to occur in the future, as the construction of a retention dam is being proposed by the Indonesian authorities to lower the flood risk in Jakarta. The study uses hydraulic and water quality data collected during a flood event at two river sections that are representative of rural/semi-urban (upstream) and urban (downstream) conditions. As in first model application, the data was also used here for set-up and validation of the coupled model. The latter was subsequently applied to the downstream section to estimate the amount of river water and dissolved oxygen that recharged the aquifer during the transient flood regime monitored. The results of the model simulations and field data were combined to quantify the positive effect of flooding on the quality of the integrated river-aquifer system and to discuss the importance of integrated solutions, which account for both flood risk and water quality problems effecting the river corridor.;Although the results from the two model applications are specific to the case study region, they exemplify through practical demonstrations how more integrated simulations of pollution dynamics can effectively help identifying contamination sources and pathways in urban environments. (Abstract shortened by ProQuest.).