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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/hess-2019-151
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2019-151
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 02 May 2019

Research article | 02 May 2019

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Hydrology and Earth System Sciences (HESS).

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

Joeri van Engelen1,2, Jarno Verkaik1,2, Jude King1,2, Eman R. Nofal3, Marc F. P. Bierkens1,2, and Gualbert H. P. Oude Essink1,2 Joeri van Engelen et al.
  • 1Department of Physical Geography, Utrecht University, Utrecht, 3584 CB, The Netherlands
  • 2Unit Subsurface and Groundwater systems, Deltares, Utrecht, 3584 BK, The Netherlands
  • 3Research Institute for Groundwater, National Water Research Center (El-Kanater El-Khairiya), Egypt

Abstract. The Nile Delta is an important agricultural area with a fast-growing population. Though traditionally irrigated with surface water, the delta increasingly relies on groundwater. However, saline groundwater extends far land inward, rendering groundwater close to the coastal zone useless for consumption or agriculture. To aid groundwater management decisions, hydrogeologists reconstructed this saline and brackish groundwater zone using variable-density groundwater models with very large dispersivities. However, this approach cannot explain the observed freshening of this zone as observed by hydrogeochemists, who hypothesize that the coastal saline zone is the effect of the Holocene transgression. Here, we investigated physical plausibility of this hypothesis by conducting a palaeo-reconstruction of groundwater salinity for the last 32 ka with a complex 3D variable-density groundwater flow model, using state-of-the-art model 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 fresh water 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 consider the volumes of onshore fresh groundwater within 300 m depth. Regardless of the variance, in all cases the total volume of hypersaline groundwater exceeded that of sea water. We also show that during the last 32 ka, the total fresh groundwater volumes significantly declined, with a factor ranging from 1.9 to 5.4, due to the rising sea-level. Compared to a steady-state solution with present-day boundary conditions, the palaeo-reconstruction improved our validation for the saline zone (5 g/L–35 g/L TDS). Also, under highly permeable conditions the marine transgression simulated with the palaeo-reconstruction led to a steeper fresh-salt interface compared to its steady-state equivalent, while low permeable clay layers allowed for the preservation of volumes of fresh groundwater. This shows that long-term transient simulations are needed when estimating present-day fresh-salt groundwater distribution in large deltas. The insights of this study are also applicable to other major deltaic areas, given the wide-range of lithological model scenarios used in this study and since many deltas also experienced a Holocene marine transgression.

Joeri van Engelen et al.
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Video supplement: A three-dimensional palaeo-reconstruction of the groundwater salinity distribution in the Nile Delta Aquifer J. van Engelen https://doi.org/10.5281/zenodo.2628427

Joeri van Engelen et al.
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Short summary
The Nile Delta is an important agricultural area with a fast-growing population, relying increasingly on groundwater. However, saline groundwater extends far land inward, rendering groundwater close to the coastal zone useless for consumption or agriculture. It normally is assumed that this is caused by mixing due to velocity differences, but here we show that it might also be caused by the coastline being located more landinwards 8000 years ago.
The Nile Delta is an important agricultural area with a fast-growing population, relying...
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