Constructing 3D variable-density grondwater flow models for six deltas using global data sets

Abstract

One third of the world’s population lives within 100 km from the shoreline. High population densities in deltaic areas create a very high demand for high-quality fresh water. To meet his demand, groundwater is extracted. Sea level rise, land subsidence, coastal erosion and fresh groundwater extraction enhance salt water intrusion and threaten the fresh groundwater reserves. For this study six 3D variable-density groundwater flow models were constructed from global data sets to quantify the fresh groundwater gaps in 6 deltas. A global hydrogeological dataset does not exist so 6 possible geological scenarios were tested. Second, a method was created to determine the dynamic equilibrium time Tau (τ) for all six models. The dynamic equilibrium time, is the time it takes for a disturbed to reach a point where fresh and salt groundwater volumes do not change over time. The Tau helps to understand how far back in time a paleoreconstructive model should go to make a better estimation of the initial salt and fresh groundwater distribution. The time it took for the models to reach Tau was in-between 30.000 and 90.000 years. The 6 deltas have a different Tau because of the difference in geometry, precipitation and river network. Within a delta the different geological scenarios cause a difference in Tau. The higher the velocities in groundwater flow the shorter the Tau. Scenarios were both tested for a system that starts completely fresh and gradually becomes salt (the so-called fresh-to-salt scenario) as for a co-called salt-to-fresh scenario. The salt-to-fresh scenario showed that inland in deep parts of the delta, salt groundwater can be stagnant and therefore remain stable under the right conditions. The bodies of salt groundwater can originate from the times when the sea level was higher than the present sea level (last time this happened was 120.000 years ago). When deep inland salt paleo groundwater is not taken into account, the computed volume of the fresh groundwater reserve can be overestimated. This will affect the projections of water policy makers.