GROUND-WATER RESOURCES FOR THE FUTURE Desalination of Ground Water: Earth Science Perspectives 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 desalination 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). “By 2020, desalination and water ����������� purification technologies will contrib- ������������������������������������� ute significantly to ensuring a safe, Less than 500 sustainable, affordable, and adequate 500 to 1,000 More than 1,000 water supply for our nation.” � ��� ��������� Inadequate information � ��� �������������� Guiding vision for Desalination and Water Purification Technology Roadmap prepared Figure 1. Depth to saline ground water in the United States (generalized from Feth and others, by the U.S. Bureau of Reclamation and 1965) Sandia National Laboratories (2003). in milligrams of solids per liter (mg/L). parts that contain freshwater, because Desalination presents the possibility Water with a TDS concentration greater the need to utilize saline ground water of providing freshwater not only from than 1,000 mg/L commonly is considered has been limited. Most ground-water the ocean, but also from saline ground saline. This somewhat arbitrary upper resource evaluations have been devoted water. The potential importance of limit of freshwater is based on the suit- to establishing the extent and properties desalination is exemplified, for example, ability of water for human consumption. of freshwater aquifers, whereas evalua- by New Mexico where approximately Although water with TDS greater than tions of saline water-bearing units have 75 percent of ground water is too saline 1,000 mg/L is used for domestic sup- been mostly devoted to determining the for most uses without treatment (Reyn- ply in areas where water of lower TDS effects on freshwater movement. olds, 1962, p. 91). Earth-science issues content is not available, water containing Much of the work to characterize related to desalination of ground water more than 3,000 mg/L is generally too saline ground-water resources in the include the distribution of saline ground- salty to drink. The U.S. Environmental United States was done in the 1950s, water resources, the chemical charac- Protection Agency has established a 1960s, and 1970s. Surveys of saline teristics and suitability of ground water guideline (secondary maximum contami- water resources of several States (for for desalination, effects of extraction of nant level) of 500 mg/L for dissolved example, Winslow and Kister, 1956), saline ground water on connected fresh- solids. Ground water with salinity greater and of selected areas within States (for water resources, and disposal of residual than seawater (about 35,000 mg/L) is example, Hood, 1963), were published in products. These issues are discussed by referred to as brine. the late 1950s and early 1960s. Krieger posing a series of basic questions. and others (1957) undertook a prelimi- Where is saline ground water nary survey of the saline water resources What is saline ground water? located? of the United States during this period. Later, Feth and others (1965) prepared Salinity is a term used to describe the Little is known about the hydro- a generalized map of the depth to saline amount of salt in a given water sample. geology of the parts of most aquifers ground water for the conterminous It usually is referred to in terms of total that contain saline water compared to the United States (fig. 1). This map provides dissolved solids (TDS) and is measured U.S. Department of the Interior Fact Sheet 075–03 U.S. Geological Survey Printed on recycled paper October 2003 a preliminary perspective on the loca- the USGS developed quantitative assess- that might be of interest for saline-water tion of saline ground-water resources, ments of 25 of the Nation’s most impor- development. Thus, surface and subsur- but provides limited information about tant regional aquifer systems, as part of face geophysical techniques to supple- critical factors required to understand the the Regional Aquifer-System Analysis ment information from wells may prove development potential of the resources (RASA) Program (USGS Professional useful tools for saline water-resource such as aquifer hydraulic conductivity Paper Series 1400−1425). Reports on the evaluation. and well yields. geochemistry of some of these aquifer Reports published in U.S. Geological systems and the Ground-Water Atlas of How do we characterize the Survey (USGS) Professional Paper Series the United States (Miller, 2000) produced chemistry of saline ground water 813 in the 1970s present very general- as a part of the RASA Program contain with respect to suitability for ized analyses of ground-water resources maps and data about dissolved-solids and within each of the 21 Water Resources specific ion concentrations for selected desalination? Regions of the United States. Many areas (fig. 2). The dissolved-solids concentration of of these reports contain a short sum- Subsurface data on water chemistry saline ground waters typically is less than mary statement about the saline water and hydrology typically are restricted that of seawater. Saline ground water, resources of a region, including use of to near-surface, freshwater aquifers and however, has a greater tendency than sea- saline aquifers as a source of treatable to sediments containing deeper, com- water to precipitate sulfate (for example, water, place to store freshwater, or place monly highly saline, oil-field water. Few gypsum), carbonate (for example, to dispose of liquid waste. Subsequently, wells are located in intermediate areas calcite), and silicate scales (fig. 3). The tendency of saline ground water to form scale is important to its suitability for use in desalination. Saline ground water also may contain some constituents, such ������� as arsenic, elevated radioactivity, and ���������� dissolved organic material, at greater ��� ���� concentrations than in seawater. Although a relatively large body of information ����������� ���������������������������������� exists on the chemical behavior of solutes �������������������������� within desalination systems, relatively ���° 0 to 1,000 little research has been undertaken on the ��° 1,000 to 3,000 chemical suitability of water from various 3,000 to 10,000 aquifers as sources for desalination. Bedrock What are the effects of using saline ground waters on fresh- ���° water resources and the environ- ment? ���° Although one might at first consider saline ground water as a “new” source ��° of water, the parts of aquifers contain- ing saline water commonly are con- ������� nected hydraulically to parts of the same ���������� aquifer or aquifer system that contain freshwater. Thus, development of one ������������������ resource affects the other, as well as ���������������������� ������������������������������� potentially affecting the flow and quality of surface-water bodies connected to the ground-water system. These effects may extend over many years or decades. Techniques are needed to predict the � �� ��� ����� effects of saline ground-water develop- � �� ��� ���������� ment in three dimensions and with time. Variable density numerical models have Figure 2. Dissolved-solids concentrations in basin-fill aquifers in Arizona and New Mexico are been used successfully to simulate the typically less than 1,000 mg/L, but can be much higher. Areas of higher concentrations include low subsurface occurrence and movement parts of some basins where dissolved solids are concentrated by evapotranspiration from shallow of saltwater in coastal environments in ground water, basins with smaller fluxes of water naturally circulating through the ground-water many locations. The use of these models system, near deposits of saline minerals, and in some areas of salt buildup from intensive irriga- to quantify the movement of inland saline tion. (modified from Kister, 1973; Robson and Banta, 1995) water resources would aid in understand- ing the impacts of utilizing saline water mations have associated water that is only resources for desalination. slightly saline (less than 10,000 mg/L), �� Disposal of the volumes of brine notably in the intermountain basins of the �� ������� produced as a residual product of desali- western United States. Coal-bed methane ������ nation is a primary environmental issue water in the Powder River Basin of Wyo- �� ������ associated with desalination. Residual ming is generally less than 3,000 mg/L �� products from desalination of seawater TDS. The likely presence of organic �� are pumped to the sea. The expense of compounds in this water complicates �� geologic disposal of residual products treatment. from inland saline ground water is �� a major factor that affects the cost- Summary Remarks �� effectiveness of ground-water desalina- tion. Disposal of residual products of Desalinated ground water potentially ���������������������������������� � �������� �������� ���������� desalination requires careful consider- represents an increasing component of ������������ ����� the Nation’s water supply; however, ����������������������������� ation of their chemical composition. For example, disposal in