Geoscience Reference
In-Depth Information
17.1 Groundwater Salinization
In their overview of global saline groundwater, van Weert et al. (
2009
) consider
terrestrial anthropogenic origins to be one of the main factors of occurrence and
genesis, identifying irrigation, coastal seawater intrusion, and waste effluent dis-
posal as the main anthropogenic drivers affecting groundwater salinization.
Groundwater salinization occurs by diffuse contamination when saline or
sewage water is used for irrigation and by anthropogenically induced intrusion of
seawater into coastal aquifers. Additionally, local (point) source groundwater
salinization may result from waste disposal on land surface, municipal disinfection
of drinking and swimming pools water, and in cold areas by large amounts of salt
spread on roads to melt ice covering. Groundwater salinization may also occur
from nonirrigated surfaces, when an increase in recharge due to agricultural
practices enhances leaching of salts from the unsaturated zone into groundwater
(Suarez
1989
). In general, due to the magnitude of the process, saline contami-
nation of an aquifer is irreversible at least on a human lifetime scale.
17.1.1 Salinization Under Downward Flow Conditions
Changes in aquifer salinity occur when the electrolyte concentration and compo-
sition of water transported from the land surface into an aquifer are different from
those of the groundwater. The salinity is controlled both by the quality of the water
reaching the land surface and by the amount of salt dissolved from the vadose zone
solid phase. Under downward flow conditions, gravitational and viscous forces
determine mixing processes and control the pattern of groundwater salinization.
For concentrated salt solutions, high density and viscosity are also key parameters.
As a consequence, downward displacement of a concentrated salt solution during
irrigation by freshwater and freshwater displacement by brines are controlled by a
dispersive process defined by the gravity number and mobility ratio (Flowers and
Hunt
2007
).
A typical example of irreversible change in groundwater salinity, following
irrigation with water having a salt concentration higher than that of groundwater,
occurred in the Coastal Plain aquifer of Israel. During the years 1940-1950, before
the development of intensive irrigated agriculture, the chloride concentration of
the Coastal Plain aquifer in Israel was less than 100 mg/L; after 30 years of
irrigation, a chloride concentration of 180 mg/L was reached. Mercado et al.
(
1975
) considered the period from 1940-1950 as a hydrologically undisturbed
period, where the major causes of groundwater salinity were chloride in rainwater,
dry fallout, and leakage from more saline aquifers in the vicinity of the Coastal
Plain. In this period, the Coastal Plain aquifer drained to the sea and a natural
equilibrium of chloride concentration was achieved. A first cause of groundwater
salinization
was
the
increase
in
aquifer
exploitation
by
intensive
irrigated
