Fresh groundwater reserves in major deltas: Evolution and current state of deltaic groundwater resources

The main objective of this thesis was to assess the status and drivers of the current fresh-salt distribution. To this end, I have formulated the following research questions: 1. What processes infuence groundwater salinity in deltas at large depths? 2. How did the fresh water volumes in deltas evolve over time? 3. What are the driving factors of the natural fresh-salt distribution? 4. What are the current fresh groundwater volumes in deltas and will they be depleted? Since data on deltaic groundwater resources is often scarce and long timescales were presumably involved in their establishment, I resorted to numerical models that simulated thousands of years of variable-density groundwater flow and coupled solute transport. At large depths, hypersaline groundwater was found in multiple deltas, of which the Nile delta was studied in detail in chapter 2. Here, I used a set of analytical and numerical models to study four hypotheses as to the origins of this hypersaline groundwater. It was concluded that the hypersaline groundwater originated either from Pleistocene free-convective systems that were formed underneath sabkhas (salt flats), or from compaction-induced seepage of hypersaline groundwater. The latter hypothesis especially is also of interest to several other deltas, as these have experienced the high sedimentation rates that cause compaction as well. Whether the old groundwater that has slowly seeped upwards is still present in the present-day, active groundwater system, depends on the connection to the sea. When there is an open connection, this water is flushed out of the system. The evolution of fresh groundwater reserves was studied with three-dimensional variable density groundwater flow models for both the Nile delta and an idealized generic delta, driven by paleohydrological forcings. It was found that long timescales were involved. In general, this evolution was as follows, starting initially salt from the latest Pleistocene highstand (120 ka), the saline aquifers are flushed during the last Pleistocene lowstand until the start of the Holocene (11 ka). From then, sea-levels rise so rapidly that sedimentation cannot keep up with it, resulting in a marine transgression around 8.5 – 6.5 ka. During the Holocene, fresh groundwater reserves reduce strongly. For example, in some of the simulations of the Nile Delta, fresh groundwater reserves are even quartered. After the Holocene transgression had passed, fresh groundwater volumes started to replenish again, but usually not to their Pleistocene levels. A global sensitivity analysis was conducted on a generic model of an idealized delta to assess the factors that determine the natural deltaic fresh-salt groundwater distribution. In total, 23 inputs of the model were included in the sensitivity analysis. It was found that the model was the most sensitive to the hydraulic conductivity of the aquifers, which are often also the most uncertain. This input both facilitates the flushing of aquifers during the Pleistocene, as well as infiltration of saline groundwater during the Holocene. The other input the model was strongly sensitive to, was the thickness of the groundwater system