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Desalination: A National Perspective
to decline. Therefore, communities are increasingly looking toward more saline waters, such as brackish groundwater or seawater, or otherwise “impaired” waters to address water supply needs.
There are many ways to define the salinity (salt concentration) ranges for fresh and saline waters. Water with greater than 2,000 to 3,000 mg/L total dissolved solids (TDS) is considered too salty to drink (Freeze and Cherry, 1979) or to grow most crops. The World Health Organization considers water with TDS concentrations below 1,000 mg/L to be generally acceptable to consumers, although it notes that acceptability may vary according to circumstances (WHO, 2003). The U.S. Environmental Protection Agency (EPA) notes that drinking water with TDS greater than 500 mg/L can be distasteful (USEPA, 1979). Brackish water has a salinity between that of fresh- and seawater. In more than 97 percent of seawater in the world the salinity is between 33,000 and 37,000 mg/L (Stumm and Morgan, 1996), although the Persian Gulf has an average TDS of 48,000 mg/L (Pankratz and Tonner, 2003). Water with salinity greater than that of seawater is called brine (USGS, 2003).
As noted in Table 1-1, nearly 1 percent of the world’s water exists as brackish or saline groundwater. In most inland cases, groundwater salinity results from the dissolution of minerals present in the subsurface, possibly concentrated further by evapotranspiration. Coastal aquifers form another class of brackish water, which is created from the natural mixing of seawater with groundwater that is discharging to the ocean (see also Chapter 5). The thickness of this brackish mixing zone is sometimes increased by coastal groundwater pumping. Brackish groundwater exists at elevations less than 305 m (1,000 feet) across much of the conterminous United States (Feth, 1965) (Figure 1-1) and almost certainly at