Saltwater Intrusion The fresh ground-water resource of the some wells become saline, thus contaminating United States is surrounded laterally and below the water supply. The location and magnitude by saline water. This is most evident along coastal of the ground-water withdrawals with respect areas where the fresh ground-water system comes to the location of the saltwater determines how into contact with the oceans, but it is also true in quickly and by how much the saltwater moves. much of the interior of the country where deep Even if the lateral regional movement of saltwater saline water underlies the freshwater. The fresh is negligible, individual wells located near the ground-water resource being surrounded by salt- saltwater/freshwater boundary can become saline water is significant because, under some circum- as a result of significant local drawdowns that stances, the saltwater can move (or intrude) into cause underlying saltwater to “upcone” into the the fresh ground-water system, making the water well (Figure 25B). unpotable. Freshwater is less dense than saline water and tends to flow on top of the surrounding or underlying saline ground water. Under natural conditions, the boundary between freshwater and saltwater maintains a stable equilibrium, as shown in Figure 25A. The boundary typically is not sharp A and distinct as shown in Figure 25A, but rather is a Land surface gradation from fresh to saline water known as the Water table zone of diffusion, zone of dispersion, or the transi- tion zone. When water is pumped from an aquifer that contains or is near saline ground water, the Ocean saltwater/freshwater boundary will move in Fresh ground water response to this pumping. That is, any pumpage will cause some movement in the boundary between the freshwater and the surrounding Saline ground water saltwater. If the boundary moves far enough, B Well discharge Land surface Pumped well Figure 25. Relation of fresh and saline ground water. Water table (Modified from U.S. Geological Survey, 1984.) (A) In coastal areas, fresh ground water discharges to the surrounding saline surface-water bodies by flowing Fresh ground water over the denser saline ground water. (B) In both coastal and inland areas, large drawdowns in an individual well can cause underlying saline water to migrate upward into the well and cause contamination of the water being Saline ground water discharged. 64 In 1969, the Task Committee on Saltwater Counties are being affected by saltwater intru- Intrusion of the American Society of Civil sion. The threat of saltwater intrusion is always Engineers (1969) indicated that saltwater intru- present on Long Island, New York, and Cape Cod, sion of some type is an existing problem in Massachusetts, because saltwater bodies surround nearly every State. Examples of saltwater intru- both localities. A specific example of saltwater sion are especially numerous along the coasts intrusion into the Old Bridge aquifer of New (U.S. Geological Survey, 1984). Some prominent Jersey (Schaefer and Walker, 1981) is shown in examples follow. Figure 26. Los Angeles and Orange Counties in California operate artificial-recharge programs to control saltwater intrusion caused by ground- water withdrawals. In Hawaii, several aquifers susceptible to saltwater intrusion underlie the A island of Oahu. In Florida, saltwater intrusion 1,000 occurs in the Jacksonville, Tampa, and Miami areas. Farther north on the Atlantic Coast, saltwater intrusion is occurring near Brunswick 100 and Savannah, Georgia, and on Hilton Head Island, South Carolina. In New Jersey, aquifers underlying parts of Atlantic, Gloucester, 10 Monmouth, Cape May, Ocean, and Salem IN MILLIGRAMS PER LITER CHLORIDE CONCENTRATION, 1 1965 1960 1950 1955 1970 1975 1977 B Figure 26. Saltwater intrusion into the Old Bridge aquifer, New Jersey. (Modified from Schaefer and Raritan Bay Union Beach Walker, 1981.) well field (A) A composite graph of chloride concentration Keyport – 20 in water samples from wells screened at about the same Harbor 250 10 depth in the Union Beach Borough well field. Chloride – 30 36 concentration in water samples from the Union Beach – 40 well field increased significantly above background 35 levels beginning in about 1970 and increased steadily 35 after that time. (B) As pumping in the area caused N water levels to decline below sea level, saline ground 0 1 MILE water moved landward and caused the increase in 0 1 KILOMETER chloride (and dissolved solids) in wells near the shore. Because of the increasing chloride and dissolved solids, EXPLANATION – 40 Line of equal head, 1977—Contour interval pumpage was curtailed in the 1980’s, and the well field 10 feet. Datum is sea level 10 Line of equal chloride concentration, in was abandoned in the early 1990’s and replaced by milligrams per liter, 1977—Contour interval is variable wells farther inland. Generalized direction of ground-water flow 65 An inland area where saltwater intrusion is Withdrawals from the overlying aquifers in these an important issue is the Mississippi River alluvial areas increase the potential for saltwater intrusion plain in Arkansas. For example, ground-water from below. withdrawals from the alluvial aquifer for irriga- In summary, the intrusion of saltwater or tion near Brinkley, Arkansas, have caused upward movement of saline water from the underlying mixing of fresh ground water with the surrounding Sparta aquifer into the alluvial aquifer (Morris and saltwater, caused by withdrawals of freshwater Bush, 1986). A confining unit separating the aqui- from the ground-water system, can make the fers is discontinuous, and the intrusion appears to resource unsuitable for use. Thus, ground-water occur mainly where the confining unit is absent. development plans should take into account poten- Many of the deeper aquifers in the central tial changes in water quality that might occur part of the United States contain saline water. because of saltwater intrusion. 66 MEETING THE CHALLENGES OF GROUND-WATER SUSTAINABILITY As we have seen, the sustainability of yield water to wells. Subsequently, methods ground-water resources is a function of many were applied to evaluate various effects of ground- factors, including decreases in ground-water water development on surface-water bodies, land storage, reductions in streamflow and lake levels, subsidence, and saltwater intrusion. Starting in the loss of wetland and riparian ecosystems, land late 1970’s, increasing concerns about contamina- subsidence, saltwater intrusion, and changes in tion of ground water by human activities led to an ground-water quality. Each ground-water system awareness of the great difficulty and expense of and development situation is unique and requires cleaning up contaminated aquifers and drew an analysis adjusted to the nature of the water attention to the importance of prevention of issues faced, including the social, economic, and ground-water contamination. With time, it has legal constraints that must be taken into account. become clear that the chemical, biological, and A key challenge for achieving ground-water sustainability is to frame the hydrologic implica- physical aspects of ground-water systems are tions of various alternative management strategies interrelated and require an integrated analysis, in such a way that they can be properly evaluated. and that many issues involving the quantity, Ground-water scientists have developed an quality, and ecological aspects of surface water expanding capability to address issues associated are interrelated with ground water. Thus, ground- with the development and sustainability of water hydrologists are challenged continually by ground-water resources. Early efforts focused on the need to provide greater refinement to their methods of evaluating the effects of ground-water analyses and to address new problems and issues pumping on an aquifer’s long-term capacity to as they arise. A key challenge for achieving ground-water sustainability is to frame the hydrologic implications of various alternative management strategies in such a way that they can be properly evaluated. 67 The Importance of Ground-Water Data The foundation of any good ground-water thicknesses of hydrogeologic units from lithologic analysis, including those analyses whose objective and geophysical well logs, water-level measure- is to propose and evaluate alternative manage- ments to allow construction of predevelopment ment strategies, is the availability of high-quality water-level maps for major aquifers as well as data. Principal types of data commonly required water-level maps at various times during develop- are listed in Table 2. Some, such as precipitation ment, ground-water sampling to document pre- data, are generally available and relatively easy to and post-development water quality, and simulta- obtain at the time of a hydrologic analysis. Other neous measurements of streamflow and stream data and information, such as geologic and hydro- quality during low flows to indicate possible geologic maps, can require years to develop. Still contributions of discharging ground water to other data, such as a history of water levels in different parts of ground-water systems, require surface-water quality. Many of the types of data foresight in order to obtain measurements over and data compilations listed in Table 2 need to time, if they are to be available at all. Thus, a key be viewed on maps. Thus, Geographic Informa- starting point for assuring a sustainable future tion Systems (GIS) typically