Saltwater Intrusion

Saltwater intrusion is the contamination of freshwater aquifers and other freshwater resources by saltwater, rendering them useless for freshwater consumption. Intrusions usually occur in coastal areas and marine islands. Most saltwater intrusion is caused by human activities such as water diversion projects and irrigation and once started, it can be very difficult, if not impossible, to reverse.

Coastal and Marine Island Aquifers

Saltwater intrusion commonly occurs in coastal and marine island aquifers. In these locations, the freshwater from the groundwater in an aquifer will rest atop the underlying saltwater in the aquifer. This layering of freshwater on top of saltwater results from the slightly higher density of the saltwater because of the amount of dissolved salts it contains. In the coastal regions of continents and marine islands, the freshwater thins and tapers as the coastline is reached, producing a classic lens-shaped cross-section to the freshwater body as it floats on the underlying saltwater. For this reason, the freshwater in aquifers in coastal regions is commonly called the freshwater lens.

Ideally, pure freshwater has a density of 1 gram per cubic centimeter. Saltwater, in contrast, has a density of 1.025 grams per cubic centimeter, a density difference of one part in forty. Freshwater floats on saltwater, much like a piece of wood floats on water. How high something floats on an underlying, denser liquid depends on the density difference, or contrast, between the floating material and the underlying liquid. In the case of the freshwater lens, the density contrast of one part in forty means that for every centimeter that the freshwater lens floats above the saltwater, it must displace downward into the saltwater at least 40 centimeters. Freshwater is not a solid, like a piece of wood; it is a liquid and will flow unless contained. When a freshwater lens floats on saltwater, the level of the freshwater in the water table is above the level of the saltwater, so it tends to flow in the direction of the source of the saltwater. Typically, that direction is seaward. As the freshwater flows seaward, the lens becomes thinner. This thinning will continue until the lens almost disappears unless the freshwater lost seaward is replaced by new freshwater entering the lens from sources above or inland of the lens. A freshwater lens is an example of dynamic equilibrium, or stability through motion. A freshwater lens remains stable only if it is recharged by new freshwater at a rate equal to the flow loss of the lens to the sea. This principle of dynamic equilibrium in the freshwater lens is called the Ghyben-Herzberg principle, developed by two scientists who independently discovered the phenomenon.

The boundary between the freshwater lens and the underlying saltwater is called the mixing zone. Other terms used to describe this boundary include the halocline, the diffusion zone, and the dispersion zone, depending on the nature of the process of interest. This boundary can have a variety of characteristics. It can be sharply defined, in which case the change from freshwater to saltwater can occur over a distance of a few centimeters or less, a situation in which the term “halocline” (salt boundary) is typically used. The water can also change from fresh to salt over a broad zone of brackish water meters or tens of meters thick with an ever-increasing salt content. Freshwater and saltwater are miscible liquids and mix very easily. (Oil and water are examples of immiscible liquids, or liquids that do not mix or dissolve each other.) The reasons that the mixing zone can be sharp or broad are not well understood and may be controlled in part by the size, shape, and chemistry of the pores in the aquifer.

Saltwater intrusion occurs when the dynamic equilibrium of the freshwater lens is upset. If the loss of freshwater from the lens is increased or if the flow of new freshwater to the lens is decreased, the lens will thin and, in the process, migrate inland. Inland migration of the freshwater lens means that some portion of the coastal aquifer that once contained freshwater now contains saltwater, as the saltwater replaces or intrudes into the freshwater aquifer. Changes in climate, sea level, or river flow paths are ways in which this process can occur naturally. Most saltwater intrusion, however, occurs because of human modification of the freshwater flow system.

Active and Passive Intrusion

Active saltwater intrusion occurs when excessive pumping of freshwater wells distorts the freshwater lens by water removal. This distortion can cause two separate problems. The first problem is a decrease in the water available in the freshwater aquifer, causing saltwater to intrude from the ocean direction, pushing the freshwater lens inland. The second situation is called upconing, where excessive pumping from a well produces an inverted cone of depression in the mixing zone. Upconing effectively draws saltwater upward into the freshwater aquifer. Normally, when a well is pumped in an aquifer, the water is removed from the vicinity of the well and is replaced by flow from the surrounding aquifer. Depending on how fast the aquifer transmits water and how fast water is pumped, the water table around the well is lowered in a characteristic way called the cone of depression. If a well is pumped too fast, the cone of depression can reach the bottom of the aquifer, and the well will go dry. After a period of no pumping, the aquifer will be able to fill the cone of depression enough so that pumping can again produce water.

In a freshwater lens, this situation is doubly complex. The bottom of the freshwater aquifer is essentially the saltwater table, which is at a higher pressure than the freshwater table. As pumping reduces the amount of freshwater pressing down on the saltwater, the pressure of the underlying saltwater can rise through an area that reflects the cone of depression formed at the freshwater table. The Ghyben-Herzberg principle requires the mixing zone to rise as the thickness of the freshwater decreases because of pumping. Eventually, the bottom of the freshwater well is reached by this upconing of the mixing zone, and the well begins to draw increasingly brackish water and then finally saltwater. When this happens, the well is ruined for further human use. In some cases, cessation of pumping will allow the freshwater lens to reestablish itself, but in most cases, the upconing process produces long-term saltwater contamination of the freshwater lens.

Passive saltwater intrusion can occur from a more subtle distortion of the freshwater lens. Because of the dynamic equilibrium of the lens, it can be distorted not only by freshwater removal at wells but also by interruption of the mechanism by which it is recharged by surface water. Human activities that alter the surface drainage and infiltration of freshwater in the recharge zone of an aquifer will cause the loss of recharge for that aquifer. If recharge is interrupted, the freshwater lens retreats inland as saltwater intrudes into the aquifer. Activities such as digging flood-control canals, widespread paving of the land surface, and river diversions can rob the underlying freshwater lens of needed recharge. A long time may pass before the effects of human activities governing recharge are detected in an aquifer, making it difficult to reverse the process.

Other Effects

Coastal and marine island regions are not the only areas where saltwater can intrude into a freshwater aquifer. In areas where arid climates predominate, irrigation is often necessary to grow crops. It is not uncommon for irrigation water to be transported into the area by pipeline or canal because the local aquifer cannot withstand the demand for water. In this situation, the excess water on the surface can mobilize the salts in the ground and transport them downward into the water table. This movement is not true saltwater intrusion, but it can result in the loss of the freshwater aquifer by salt contamination.

Many types of rock and sediment are deposited in the ocean. At a later time, they may be uplifted from the sea and form dry land that receives rainfall and develops a freshwater aquifer. These aquifers' deeper regions may contain waters left behind from the original marine environment, called connate water. Connate water originated as saltwater, and it can intrude into the aquifer's freshwater portion during overpumping. The aquifer can thus become contaminated by salt and ruined. In some cases, the deeper waters of an aquifer are brines, where the salt content has become concentrated above that found in the oceans. Brines are potent contaminants of freshwater aquifers because they have a high salt concentration.

The intrusion of saltwater into an aquifer can significantly impact the aquifer material itself. The chemistry of saltwater is very different from that of freshwater. Some aquifer materials can become altered by saltwater intrusion, and they will stay changed even if the saltwater is later forced out by replenishing the original freshwater. Aquifers in islands and coastal regions developed in limestone strata show examples of the changes that can occur. In limestone aquifers, both the freshwater lens and the underlying saltwater are usually saturated with the dissolved mineral calcite, which is the principal component of limestone. However, when freshwater and saltwater mix, they can dissolve quantities of calcite that neither one could dissolve alone. The result is the development of areas of dissolved rock in the limestone. The development of caves and the increase in the number of pores in the limestone caused by this dissolving process changed the characteristics of the aquifer. If the mixing zone moves around because of distortions of the freshwater lens, the aquifer can fundamentally change as the dissolving area reaches more of the aquifer. The changes in sea level during the ice ages caused that to happen in many places. The Bahamas' famous blue holes and underwater caves owe their origin to this process. Human activity and subsequent saltwater intrusion in limestone coastal areas may promote this process in the twenty-first century.

Study of Saltwater Intrusion

The study of saltwater is essential to ensure drinking water is healthy for humans and animals. Climate change has made the study of saltwater intrusion even more critical. The study of saltwater intrusion is usually a very straightforward task. The primary difficulty lies in the change in water use that the public must accept to rectify the problem. The first step in analyzing a freshwater lens is to calculate a water budget for the aquifer. A water budget is much like a bank budget; there are inputs and outputs, and when the two are out of balance, a net change occurs in the budget. In an aquifer, the input is the aquifer's recharge by freshwater from the surface. This recharge includes rainfall over the aquifer area, of which a certain percentage infiltrates the ground to reach the water table. It also includes water brought into the area from other regions by rivers and streams, a certain percentage of which will also sink to the water table. These input values can be measured by considering rainfall, evaporation, the use of water by plants (transpiration), and, finally, the water that runs over the ground surface. The difference between how much water comes in and how much is lost to runoff, evaporation, and transpiration is the amount that infiltrates the ground to recharge the aquifer. If the aquifer is in equilibrium, the recharge amount must equal the discharge amount.

Observation wells are used to measure what is happening down in the aquifer. These wells identify the location of the water table. If they penetrate deeply enough, they can also tell where the mixing zone and the saltwater are. These wells are not used as a water source but as a means of monitoring the aquifer. If the water table is at a constant level (or seasonally fluctuates about a constant level), the aquifer is in equilibrium. If the input has been calculated, the output can be estimated. As the aquifer is used as a water supply, the observation wells follow any changes produced in the water table.

Saltwater intrusion is easily detected in observation or supply wells because of the change in salinity that occurs. Sometimes, it is detected as a change in the taste of the water, but chemical testing may also be used. Because freshwater is a poor conductor of electricity and saltwater is a good conductor, a relatively simple device called a conductivity meter can be used to easily and quickly check the presence of salt in a well. A drop in the water table and a rise in the mixing zone will be directly measurable in an observation well, and remediation can begin. However, once a supply well has been contaminated by saltwater, it is too late to save that well in the short term. It must be shut down, and an alternate water supply must be located. The observation well can predict when saltwater intrusion will damage supply wells by monitoring the position of the mixing zone and the water table. It is then possible to change water-use patterns at the supply wells, increase aquifer recharge, or both, thereby preventing a problem. In some cases, the well that is damaged is not the well causing the problem. Imagine a coastal well that is functioning normally. If a well inland of the coastal well pumps too much freshwater, the saltwater will intrude into the aquifer and damage the well closer to the sea long before the inland overpumping well is damaged.

Once a saltwater intrusion problem has been detected or predicted through monitoring of the aquifer, a course of action is needed to rectify the situation. The necessary action is often relatively simple: Water use can be reduced through conservation, allowing the aquifer to keep the saltwater out of the freshwater lens. This approach results in a reduction in the aquifer's output. Sometimes, however, conservation will not work because the demands of necessity are too high. In that case, the situation can be stabilized by increasing the recharge by developing reservoirs and holding ponds. In extreme cases, water has been brought in by pipeline to an area, and recharge is accomplished by freshwater injection into the aquifer. When upconing has occurred, correcting the situation by increasing recharge may not be possible because the freshwater may simply flow around the problem area. It may take many years before the well is again usable. In the United States, the Army Corps of Engineers constantly monitors cases of saltwater intrusion, especially in major bodies of water such as the Mississippi River around the city of New Orleans. In 2023, concerns over a wedge of saltwater moving toward the city due to low levels of rain necessitated planning and mitigation efforts to protect the city’s drinking water supply. Increased rainfall and the slowing of the movement of the saltwater by engineers, along with government aid, slowed the progression of the saltwater. 

Principal Terms

aquifer: a rock or sediment structure that is saturated with groundwater and is capable of delivering that water to wells and springs

brackish water: water with a salt content between that of saltwater and freshwater; it is common in arid areas on the surface, in coastal marshes, and in salt-contaminated groundwater

cone of depression: a cone-shaped depression produced in the water table by pumping from a well

freshwater: water with less than 0.2 percent dissolved salts, such as is found in most streams, rivers, and lakes

freshwater lens: the shape of the freshwater table in aquifers of coastal areas or marine islands that floats on top of a denser, underlying saltwater

groundwater: water located beneath the ground in interconnected pores beyond the soil-root zone

mixing zone: the area of contact between a freshwater lens and the underlying saltwater

saltwater: water with a salt content of 3.5 percent, such as is found in normal ocean water

upconing: the upward flexure of the mixing zone toward the ground surface produced by excessive groundwater withdrawal by wells, analogous to an inverted cone of depression

water table: the upper surface of groundwater in an aquifer, with a direct connection overhead to the atmosphere such that the water pressure is equal to atmospheric pressure

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