Geoscience Reference
In-Depth Information
17.1.2 Salinization Under Lateral Flow Conditions
Groundwater and seawater are primary components of the hydrological system of
a coastal aquifer. Seawater intrusion, induced by over-exploitation (over-pumping)
of coastal groundwater resources as a result of increasing demand for freshwater,
occurs with changes in the pressure balance between groundwater and seawater
(Werner et al.
2012
and references therein). Seawater intrusion is expressed by
lateral flow into a freshwater aquifer and involves the simultaneous transport of
water and dissolved salts; spatiotemporal changes in salinity cause density dif-
ferences that influence the flow pattern.
Seawater intrusion is a key process that can lead to irreversible changes in
coastal aquifer chemistry. The total salt concentration and the ionic composition in
the aquifer change as a result of seawater intrusion and may degrade freshwater
quality to a point that renders it unportable or environmentally harmful. Moreover,
once salts enter an aquifer, they can advance further inland by dispersive and
diffusive mixing. We discuss below several examples of lateral saltwater intrusion
into a coastal aquifer, selected from different environmental settings.
An example of groundwater salinization by seawater intrusion is given by El
Moujabber et al. (
2006
), who examined a site situated south of Mount Lebanon
(Choueifat area) on the Mediterranean coast of Lebanon. The analysis was based
on a 3-year, monthly monitoring of six wells, in which groundwater samples were
collected and analyzed for chemical content (salt content and composition), and
compared to the seawater chemistry. Mediterranean seawater has a salinity of
about 40 g/L and exhibits a predominance of Cl
-
and Na
+
. Chlorides (Cl
-
) are in
excess of alkali ions (Na
+
and K
+
), and Mg
2+
is greatly in excess of Ca
2+
, as well
as of SO
4
2-
and HCO
3
-
. In contrast, continental groundwater is rich in HCO
3
-
,
SO
4
2-
, and Cl
-
, with Ca
2+
the predominant cation accompanied by Mg
2+
.In
general, the alkali ions Na
+
and K
+
are present in smaller amounts.
The measured variation in cation and anion ratios is illustrated in Fig.
17.3
.
Overall, the curves indicate seasonal variation with highest ratios during the
summer months, as a result of intensive groundwater exploitation, and a relative
steady state observed during the rainy period, characterized by natural aquifer
recharge and lack of need for pumping of irrigation water. However, this steady
state period during the winter did not minimize groundwater salinization. More
specifically, disregarding the summer months, the Na
+
/Ca
2+
curve reveals a slowly
increasing trend of seawater salt penetration into the aquifer over the years. The
Mg
2+
/Ca
2+
curve shows an overall increase in Mg
2+
, the SO
4
2-
/Cl
-
curve indicates
aSO
4
2-
decrease coupled with an increase in Cl
-
, and the HCO
3
-
/Cl
-
curve
indicates a drop in the concentration of HCO
3
-
(contrary to the Cl
-
concentration).
Seawater intrusion due to freshwater over-pumping in a different climatic
environment is reported by Barlow and Reichard (
2010
). In a review of saltwater
intrusion in coastal regions of North America, these authors examine the case of
Cape May County (New Jersey, USA) peninsula. The freshwater aquifer in this
peninsula is bounded laterally by saline groundwater, affected by the Atlantic
