Abstract
Abstract. Holocene marine transgressions are often put forward to explain observed groundwater salinities that extend far inland in deltas. This hypothesis was also proposed in the literature to explain the large land-inward extent of saline groundwater in the Nile Delta. The groundwater models previously built for the area used very large dispersivities to reconstruct this saline and brackish groundwater zone. However, this approach cannot explain the observed freshening of this zone. Here, we investigated the physical plausibility of the Holocene-transgression hypothesis to explain observed salinities by conducting a palaeohydrogeological reconstruction of groundwater salinity for the last 32 ka with a complex 3-D variable-density groundwater flow model, using a state-of-the-art version of the SEAWAT computer code that allows for parallel computation. Several scenarios with different lithologies and hypersaline groundwater provenances were simulated, of which five were selected that showed the best match with the observations. Amongst these selections, total freshwater volumes varied strongly, ranging from 1526 to 2659 km3, mainly due to uncertainties in the lithology offshore and at larger depths. This range is smaller (1511–1989 km3) when we only consider the volumes of onshore fresh groundwater within 300 m depth. In all five selected scenarios the total volume of hypersaline groundwater exceeded that of seawater. We also show that during the last 32 ka, total freshwater volumes significantly declined, with a factor ranging from 2 to 5, due to the rising sea level. Furthermore, the time period required to reach a steady state under current boundary conditions exceeded 5.5 ka for all scenarios. Finally, under highly permeable conditions the marine transgression simulated with the palaeohydrogeological reconstruction led to a steeper fresh–salt interface compared to its steady-state equivalent, while low-permeable clay layers allowed for the preservation of fresh groundwater volumes. This shows that long-term transient simulations are needed when estimating present-day fresh–salt groundwater distributions in large deltas. The insights of this study are also applicable to other major deltaic areas, since many also experienced a Holocene marine transgression.
Highlights
Palaeohydrogeological conditions have influenced groundwater quality in the majority of large-scale groundwater systems (Edmunds, 2001; Jasechko et al, 2017)
A 3-D variable-density groundwater flow and coupled salt transport model was used for a palaeohydrogeological reconstruction of the salinity distribution of the Nile Delta Aquifer
None of the evaluated palaeohydrogeological scenarios reached a steady state over the last 9 ka, meaning that the transient boundary conditions during the Holocene must have had an influence on the current groundwater salinity
Summary
Palaeohydrogeological conditions have influenced groundwater quality in the majority of large-scale groundwater systems (Edmunds, 2001; Jasechko et al, 2017) These conditions can especially be found in deltaic areas, where the effects of marine transgressions are often still observed in groundwater salinities (Larsen et al, 2017). Their low elevation allowed for far-reaching marine transgressions, leading to a large vertical influx of seawater, and hampered subsequent flushing with fresh water after the marine regression. The physical feasibility of this hypothesis, has not often been tested in the form of palaeohydrogeologic modelling, with a few notable exceptions (Delsman et al, 2014; Larsen et al, 2017; Van Pham et al, 2019). Delsman et al (2014) conducted a de-
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