A three-dimensional palaeohydrogeological reconstruction of the groundwater salinity distribution in the Nile Delta Aquifer
<p>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 previ...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-12-01
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| Series: | Hydrology and Earth System Sciences |
| Online Access: | https://www.hydrol-earth-syst-sci.net/23/5175/2019/hess-23-5175-2019.pdf |
| Summary: | <p>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<span class="inline-formula"><sup>3</sup></span>, mainly due to uncertainties in
the lithology offshore and at larger depths. This range is smaller
(1511–1989 km<span class="inline-formula"><sup>3</sup></span>) 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.</p> |
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| ISSN: | 1027-5606 1607-7938 |