A three-dimensional palaeohydrogeological reconstruction of the groundwater salinity distribution in the Nile Delta Aquifer

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 km 3 , mainly due to uncertainties in the lithology offshore and at larger depths. This range is smaller (1511-1989 km 3 ) 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.

A global dataset of surface water and groundwater salinity measurements from 1980–2019

“… Groundwater TDS data for the Nile Delta aquifer ( van Engelen et al . ) 29 was provided by Joeri van Engelen. These data include three datasets consisting of TDS measurements, synthesized from literature, collected with the selection criteria of including measurement data from less than 250 m depth.…”

“…Two of these datasets had unspecific dates, and samples were thus assumed to be from the 1 st of each reported month (see further specification of the data in van Engelen et al . 29 ). The TDS data was then converted to EC, using a regional specific conversion factor, from literature sources (see section Conversions of TDS to EC for specifics on how this was done).…”

“…Due to reported improved predictability of EC-TDS relationships when using region-specific conversion factors ( f ), we included additional groundwater TDS measurements only for regions with available reported region-specific f values. This resulted in the inclusion of three additional groundwater datasets to the final database; one from Idaho 31 , one from California 32 and one from Egypt 29 . Together these datasets added 3,477 sampling locations and a total of 9,654 measurements to the groundwater database.…”

“…This conversion factor has been developed for the continental United States, by the US Geological Survey and is widely used cross-regionally within the US 20 , 33 . For the TDS groundwater data from Egypt (from the Nile delta) 29 , we converted TDS to EC using the region-specific conversion factor f of 0.64. This factor value has been derived from local measurement data in the Nile delta itself 34 .…”

The Issue of Groundwater Salinization in Coastal Areas of the Mediterranean Region: A Review

“…Besides the physically based density dependent numerical models that need a large number of field data to be calibrated, there has recently been a growing number of papers that have employed the surrogate models, such as non-linear regressive techniques, to describe the SWI phenomenon [160][161][162][163][164], or using simple spreadsheet macros to plot the hydrochemical facies evolution [165]. In general, numerical models have been widely used to quantify the different origins of salinization via density dependent flow and transport models [166][167][168][169][170][171][172][173][174][175]. Finally, only a few papers have been conducted involving the coupling major ions, isotope data, or geophysical investigations to achieve a multidisciplinary model construction [176,177].…”