Sustainable Water Management - Sustainable Practices in Geoenvironmental Engineering - page 78

of water in more than 300 cities (WRI, 1994). It was estimated in the Global Environment
Outlook 3 report by the United Nations Environment Program (UNEP, 2002) that about
half of the world's population will not have suficient water by the year 2032. This would
be the result of the currently unsustainable practices that are currently using about 50%
of the earth's freshwater supply (UN-FPA, 2001). What will happen to the ecosphere and
to human life when the population reaches 9 billion by 2100 (UNDP, 2001)? Even now,
dehydration is occurring in many areas to the extent that this will affect biodiversity and
increase the requirements for agricultural irrigation.
Per hectare of land, 10 million liters of water each season is required for production
of 8000 kg of corn (Pimentel et al., 1996). Water use for irrigation has decreased due to
improved agricultural practices. Treatment of groundwater as a scarce resource may be
required. Water markets have been proposed (Dorf, 2001) as a means of valuing water as
a resource. Conservation by farmers, and urban users will increase as the price of water
increases.
3.2.2.1 Excessive Groundwater Abstraction and Land Subsidence
In regions where the underlying geology consists of interlayering of soft aquitards and
aquifers, excessive groundwater abstraction from the interlayered aquifers can cause sub-
sidence of the ground surface. A good case in point is the Quaternary sediments that
underlie many coastal cities such as Shanghai, Bangkok, and Jakarta. In Bangkok, the capi-
tal of Thailand, for example, the city is situated on a low-lying, lat deltaic plain known
as the Lower Central Plain (known also as the Lower Chao Phraya Basin)—about 30 km
north of the Gulf of Thailand. The basement bedrock gently inclines southward toward
the Gulf of Thailand, and the strata overlying the basement bedrock consists of a com-
plex mix of unconsolidated and semiconsolidated sediments of the Tertiary to Quaternary
geological age. The thickness of the strata ranges from about 400 m in the north to more
than 1800 m in the south, with a stratigraphic proile that shows ive discernible separate
aquifers layers separated by unconsolidated and semiconsolidated sediments, all of which
are overlain by a stiff surface clay layer. Excessive long-term groundwater abstraction has
resulted in subsidence in the region. This causes severe looding of the region. Yong et al.
(1991) reported that with the drainage system existent at that time, extensive looding last-
ing for periods of 6 to 24 hours occurs with rainfall exceeding 60 mm.
For coastal cities in similar situations, land subsidence due to prolonged excessive
groundwater abstraction can reach the stage where the land surface will reach levels
below sea level. When such occurs, unless containment dikes are built, seawater intru-
sion causing local looding and contamination of the aquifers can occur. The impacts to
human health in respect to contaminated water, in addition to physical problems associ-
ated with looding can be severe. Buildings and other structures have been known to
suffer considerable structural distress due to uneven settlement of footings and foun-
dations. By some accounts, delivery of potable water from a central source located in
a “safe” region is required when lood waters compromise the existent drinking water
sources.
3.2.2.2 Uses of Water
By all indications, if sustainable water usage is to be obtained, water treatment and
reuse will need to increase and demand will need to decrease. Decreased water quality
through contamination decreases the amount of available water. In the industrialized