Geoscience Reference
In-Depth Information
Chapter 6
Ground Water Desalination
William M. Alley
INTRODUCTION
Desalination is a water-treatment process that removes salts from water. Concerns
about the sustainability of freshwater supplies, as well as rapid advances in membrane
and other water-treatment technologies, are fostering a renewed interest in desalina-
tion as a partial solution to increased water demand. It is projected that more than 70
billion dollars will be spent worldwide over the next 20 years to design and build new
desalination plants and facilities (Sandia National Laboratories, 2002).
Desalination presents the possibility of providing freshwater not only from the
ocean, but also from saline ground water. The potential importance of desalination
is exemplifi ed, for example, by New Mexico where approximately 75% of ground
water is too saline for most uses without treatment (Reynolds, 1962, p. 91). Earth-
science issues related to desalination of ground water include the distribution of saline
ground water resources, the chemical characteristics and suitability of ground water
for desalination, effects of extraction of saline ground water on connected freshwater
resources, and disposal of residual products. These issues are discussed by posing a
series of basic questions.
WHAT IS SALINE GROUND WATER?
Salinity is a term used to describe the amount of salt in a given water sample. It usually
is referred to in terms of total dissolved solids (TDSs) and is measured in milligrams
of solids per liter (mg/l). Water with a TDS concentration greater than 1,000 mg/l
commonly is considered saline. This somewhat arbitrary upper limit of freshwater is
based on the suitability of water for human consumption. Although water with TDS
greater than 1,000 mg/l is used for domestic supply in areas where water of lower TDS
content is not available, water containing more than 3,000 mg/l is generally too salty to
drink. The US Environmental Protection Agency has established a guideline (second-
ary maximum contaminant level) of 500 mg/l for dissolved solids. Ground water with
salinity greater than seawater (about 35,000 mg/l) is referred to as brine.
WHERE IS SALINE GROUND WATER LOCATED?
Little is known about the hydro-geology of the parts of most aquifers that contain
saline water compared to the parts that contain freshwater, because the need to utilize
saline ground water has been limited. Most ground water resource evaluations have
been devoted to establishing the extent and properties of freshwater aquifers, whereas
evaluations of saline water-bearing units have been mostly devoted to determining the
effects on freshwater movement.
