Saltwater Intrusion

2.10.1 Saltwater Intrusion

During the last several decades, groundwater use in coastal areas worldwide has dra- matically increased due to rapid population growth. With this increase came the public recognition that groundwater supplies are vulnerable to overuse and contamination. Groundwater development depletes the amount of groundwater in storage and causes reductions in groundwater discharge to streams, wetlands, and coastal estuaries, and lowered water levels in ponds and lakes. Contamination of groundwater resources has

GroundwaterSystem

resulted in degradation of some drinking-water supplies and coastal waters. Although overuse and contamination of groundwater are common for all types of aquifers, the prox- imity of coastal aquifers to saltwater creates unique challenges with respect to ground- water sustainability. Two main concerns are saltwater intrusion into freshwater aquifers and changes in the amount and quality of fresh groundwater discharge to coastal saltwa- ter ecosystems. Saltwater intrusion is the movement of saline sea water into freshwater aquifers caused primarily by groundwater pumping from coastal wells. Because saltwa- ter has high concentrations of total dissolved solids and certain inorganic constituents, it is unfit for human consumption and many other uses. Saltwater intrusion reduces fresh groundwater storage and, in extreme cases, leads to the abandonment of supply wells when concentrations of dissolved ions exceed drinking-water standards (Barlow, 2003). The problem of saltwater intrusion was recognized as early as 1854 on Long Island, New York (Back and Freeze, 1983), thus predating many other types of drinking-water contamination issues in the news.

When natural conditions in a coastal aquifer are altered by groundwater withdrawal, the position and shape of the freshwater-saltwater interface, as well as the mixing zone thickness, may change in all three dimensions and result in saltwater intrusion (encroach- ment). The presence of leaky and discontinuous aquitards, and pumping from different aquifers or different depths in the same aquifer, may create a rather complex spatial rela- tionship between freshwater and saltwater. Figure 2.121 shows a simple case of saltwater intrusion caused by well pumpage from an unconfined homogeneous aquifer resting on an impermeable horizontal case. As the pumping rate and drawdown increase, the in- terface continues to move landward until it reaches the critical hydraulic condition. At this point the hydraulic head at the groundwater divide caused by pumping and the interface toe are positioned on the same vertical. Any further increase in the pumping rate or lowering of the hydraulic head will result in a rapid advance of the interface until

Pumping well

face Land sur

Initial water table

Critical head

Sea level

Freshwater Saltwater

Initial interface

Critical interface toe

F IGURE 2.121 Changing freshwater-saltwater interface position resulting from a single well pumping in a coastal unconfined aquifer. (Modified from Strack, 1976; Bear, 1979.)

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new equilibrium is reached, with the interface toe landward of the well (Bear, 1979). In many cases this lateral intrusion of saltwater would result in complete abandonment of the well.

Well pumping above the saltwater-freshwater interface will cause upconing of the denser saltwater, which is not necessarily always accompanied by a significant lateral landward movement of the interface. This upconing may reach the well and also result in cessation of pumping due to unacceptable concentrations of total dissolved solids and other constituents in the extracted water. However, unlike in the case of complete lateral saltwater encroachment, once the pumping stops and the hydraulic head of freshwater increases, the cone of dense saltwater dissipates relatively quickly driven by gravity.

Strack (1976), Bear (1979), Kashef (1987), and Bear et al. (1999) provide analytical solu- tions for calculating the positions and movement of the saltwater-freshwater interface for various cases of groundwater extraction including stratified aquifer-aquitard systems. There are several excellent commercial and public domain (free) computer programs for three-dimensional numeric modeling of density-dependent groundwater flow. SUTRA developed by the USGS (Voss and Provost, 2002) is an example of a program widely used for modeling saltwater-freshwater interactions in coastal groundwater systems.