Saltwater Intrusion (NT)
SALTWATER INTRUSION, NON-TECHNICAL
By Darrel Dunn, Ph.D., PG, Consulting hydrogeologist. (Professional Synopsis)
This is a non-technical webpage on coastal saltwater intrusion (aka seawater intrusion). It is intended to introduce the layman to characteristics of coastal saltwater intrusion that are illustrated in the paper on this website titled "Three-Dimensional Analysis of Saltwater Intrusion, City of Pompano Beach, Broward County, Florida." Coastal saltwater intrusion is the movement of seawater into freshwater aquifers. It can lead to contamination of drinking water sources.
Some background information may be helpful in understanding saltwater intrusion. Buoyancy is important. An example of buoyancy in seawater is icebergs. It is well known that most of an iceberg is below the surface of the ocean. This effect is due to Archimedes' Principle which holds that the buoyant or lifting force on an object is equal to the weight of the fluid displaced by that object. This principle was discovered by Archimedes sometime prior to his death in 212 B.C. If the weight per unit volume (weight density, for example pounds per cubic foot, lb/ft3) of the object is less than the weight density of the fluid, the object will float. The weight of the fluid displaced by the floating object will equal the weight of the floating object. In the case of an iceberg with a weight density of 56 lb/ft3 floating in seawater with a weight density of 64 lb/ft3, the proportion of the iceberg submerged would be 56/64 or 0.875 (87.5 percent).
A similar concept applies to coastal groundwater systems. Freshwater is less dense than saltwater. So if a static lens of fresh groundwater were floating on saltwater in porous material beneath the surface of an island in the ocean, the weight of the freshwater lens would equal the weight of the saltwater it displaces. Such a freshwater lens is shown in Figure 1, which is a cross-section through a hypothetical oceanic island where the water table rises to 20 feet above sea level at the center of the island. In this case the weight of the freshwater displaces a volume of saltwater of equivalent weight. Furthermore, the scientists W. Badon-Ghyben of Holland (1888) and A. Herzberg of Germany (1901) independently recognized that any vertical column of water in a static freshwater lens extending from the water table to the interface is balanced by the weight of an equivalent column of saltwater extending from sea level to the same location on the interface. This concept is similar to Archimedes' Principal, but the weight of a column of saltwater displaced by a column of freshwater will equal the weight of the column of freshwater. If the weight density of the freshwater is 62.4 lb/ft3 and the weight density of the saltwater is 64 lb/ft3, the proportion of the freshwater column below sea level would be 62.4/64 or 0.975 (97.5 percent). So if the freshwater column were 43 feet high the proportion below sea level would be 0.975 X 43, which is 41.925 feet. This result is sometimes approximated as 40 feet, which is easy to remember and use. More generally, the depth to a sharp freshwater-saltwater interface below sea level is about 40 times the height of the water table above sea level. This relation is referred to as the Ghyben-Herzberg relation. It reveals why a significant fresh groundwater resource may exist near an ocean coast, as illustrated by Figure 1, where the freshwater-saltwater interface is at 800 feet below sea level at the center of the island. The concept of a freshwater column within the freshwater lens mentioned above is also illustrated in Figure 1.
Rather than use a freshwater column to explain the Ghyben-Herzberg relation, one could note that the pressure in the saltwater and the freshwater at any point on the sharp interface must be the same (since they are not separated), and the pressure in the freshwater is produced by depth below the water table while the pressure in the saltwater is produced by depth below sea level.
Figure 1. Ghyben-Herzberg relation for a hypothetical island.
Dynamic Coastal Freshwater-Saltwater System
A number of factors cause actual depths to saltwater to differ from the Ghyben-Herzberg relation:
- A static freshwater lens is not possible. A static lens such as that shown in Figure 1 would not be stable. It would tend to decay through movement of the freshwater toward the ocean unless recharge by rainfall (or other source) balanced the movement of water from the water table to the ocean. However, such movement would involve a change in pressure at the freshwater-saltwater interface. The change in pressure would cause a change in the elevation of the interface; hence a change in depth to the interface below the water table.
- A truly sharp interface is not possible. If saltwater were in contact with freshwater the sodium and chloride and other dissolved constituents that are at high concentration in the saltwater would diffuse into the freshwater causing a transitional interface rather than a sharp one.
- Rise and fall of the water table due to variation in recharge by rainfall, seepage from streams, or other recharge mechanisms would cause rise and fall of the freshwater-saltwater interface which would result in mixing of freshwater and saltwater.
- Tidal fluctuations of sea level would cause fluctuating pressure at the freshwater-saltwater interface near the coast, affecting the stability of the freshwater-saltwater interface.
- Any anthropogenic rise and fall of the water table, such as variation in pumping of water wells and artificial groundwater recharge, would cause mixing.
There are other factors complicating the freshwater-saltwater relationship:
- Long term (thousands of years) changes in sea level can affect the occurrence of freshwater and saltwater. For example if a coastal area was below sea level and the land emerged due to decline in sea level, all of the saltwater in the subsurface my not have yet been replaced by freshwater.
- Variations in permeability of the subsurface material affect the rate of movement of the freshwater-saltwater interface. The adjustment of the interface to change in water table elevation will be slower in low permeability layers than in high permeability layers.
- Improperly designed saltwater intrusion monitoring wells may contribute to saltwater movement between permeable layers.
Nevertheless, the Ghyben-Herzberg relation helps explain the presence of freshwater in coastal areas. Although Figure 1 illustrates the occurrence of freshwater beneath an island, the same principle holds for a continental coast. Near a coast the Ghyben-Herzberg relation would predict that the depth to saltwater below sea level would be 40 times the height of the water table above sea level. However, due to complicating factors such as those listed above, the actual depth to saltwater can be significantly different from that predicted by the Ghyben-Herzberg relation. Figure 2 illustrates the effect of such complicating factors. It is an interpretive cross-section of the freshwater-saltwater relation along a line through a municipal wellfield in Pompano, Florida. (Too see the technical report that contains this cross-section, click here.) At the time represented by the cross-section (1972), the average water table elevation at monitoring well G-2064 was probably about 1.3 feet, which would indicate saltwater at a depth of about 52 feet by the Ghyben-Herzberg relation. However, the well was sampling freshwater at a depth of 200 feet. In this case, it may be that the water table was previously higher, and the interface had not yet adjusted to the lower water table elevation. This slow rate of adjustment would be due to low permeability of the subsurface materials containing the interface. In this Pompano study the lower water table in 1972 was caused primarily by (1) pumpage in the Pompano Wellfield, (2) below average rainfall recharge, and (3)construction of drainage canals. (Well 12 is in the Pompano Wellfield.)
Figure 2 also illustrates a transitional interface. Monitoring well G-2063 was sampling water with a chloride concentration of 9000 mg/L (milligrams per liter), whereas seawater has a concentration of about 19,000 mg/L. The cross-section with no vertical exaggeration shows that the interface was not steep.
Figure 2. Vertical cross-section from the Atlantic Ocean through the Pompano Wellfield, Broward County, Florida, representing 1972 conditions.
Coastal Saltwater Intrusion
Anthropogenic reduction of water table elevation and reduction of pressure in aquifer layers near a sea coast may induce the movement of saltwater into freshwater aquifers. Such migration of saltwater is known as saltwater intrusion (aka seawater intrusion). Figure 3 is an example of such intrusion from the aforementioned study. It shows the result of movement of seawater into a permeable aquifer layer that is connected to the ocean. In this case the movement may be dominantly the result of pumping in the Pompano Wellfield and a period of reduced rainfall recharge of the aquifer. In the case illustrated in Figure 3, the intrusion in the permeable layer (which is the producing layer in the Pompano Wellfield) resulted in the presence of salty water overlying freshwater. This saltwater intrusion occurred about the time of peak production rates in the Pompano Wellfield.
Figure 3. Saltwater intrusion in the permeable producing layer of the Biscayne Aquifer near the Pompano Wellfield, 1979.
Such saltwater intrusion is reversible, as may be seen in Figure 4. The cross-section in Figure 4 is along the same line as Figure 3. It shows a seaward movement of the freshwater interface between 1979 and 1994. This was a period of declining pumpage in the Pompano Wellfield. A period of above average rainfall began in 1991. Rise of the water table due to reduced pumpage and increased rainfall would tend to mitigate saltwater intrusion.
Figure 4. Saltwater intrusion in the permeable producing layer of the Biscayne Aquifer near the Pompano Wellfield, 1994.
When water wells are located sufficiently far from the coast, the upward movement of the saltwater interface may be localized beneath the well. Such movement is called upconing. It is illustrated diagramitically in Figure 5. The rise of the saltwater is much greater than the decline in the water table. There may be a large time lag between the fall of the water table and the rise of the interface. Furthermore, the interface will not be sharp, as shown in the diagram. Consequently, the salinization of the well (or wellfield) will tend to be delayed and slow. The vertical exaggeration of the diagram is great.
Figure 5. Diagram of upconing beneath a water well.
Computer Analysis of Saltwater Intrusion
Actual coastal groundwater systems involving freshwater and saltwater are complex, as may be seen by examining the technical study of the Pompano area referenced above. A useful analysis of such complex systems involves obtaining adequate monitoring well data and developing a conceptual description of the system based on the monitoring well data. Due to the complexity of the system and incompleteness of monitoring well data, there will be uncertainty in the results of the conceptual analysis. Such uncertainty may be reduced and questions raised by the conceptual analysis may be addressed by using computer techniques to simulate the freshwater-saltwater system. Once the computer simulation is deemed to adequately agree with the real data from the monitoring wells, the simulation (model) may be probed to isolate the simulated effects of the various factors influencing the saltwater intrusion, such as water well pumpage, rainfall, artificial drainage, artificial recharge, and sea level rise.
Such computer simulation was accomplished for the Pompano area. The results are reported in a paper titled "Effect of Sea-Level Rise on Salt Water Intrusion near a Coastal Well Field in Southeastern, Florida" (128). The model results were that water well pumpage in the Pompano Wellfield had the greatest simulated effect on accelerating simulated saltwater intrusion, and that a period of below average rainfall had less effect in the model. Simulated artificial recharge accomplished by over-irrigating a golf course near the wellfield reduced the simulated saltwater intrusion. Simulating historical sea-level rise produced a discernible increase in saltwater intrusion, but it was less than the effect of wellfield pumping. Simulating increased rates of sea-level rise greater than the historical rate caused corresponding increases in rates of saltwater intrusion.
This modeling study applied sophisticated computer techniques to a large three dimensional saltwater intrusion model (the software used is called SEAWAT) applied to a long historical period (105 years) and a large area (more than 100 square miles). Such a large saltwater intrusion model is computationally intensive so that great computer speed was needed. This speed was accomplished with cluster computing, which links computers together to perform simultaneous computations. Even so, the accuracy of the simulation was limited. Calculated maximum salt concentrations at important monitoring well sites were only about half of the concentration in the monitoring wells, and the advance of the saltwater tongue over freshwater shown in Figure 3 was not captured. Nevertheless, the timing of increase and decrease in salt concentrations and the areal distribution of the saltwater intrusion into the permeable production layer was captured reasonably well, and the model was deemed sufficiently accurate to support inferences.
Managing Saltwater Intrusion
A large proportion of the world's population is located near an ocean coast. Many highly populated coastal areas rely entirely or partly on groundwater in aquifers that are subject to saltwater intrusion. The three-step approach to managing saltwater intrusion (monitoring, conceptual description, computer model) would be beneficial anywhere. A thorough understanding of the complexities of the freshwater-saltwater system is necessary for all three steps. Furthermore, a hydrogeologist must be able to deal effectively with uncertainty caused by suboptimal data, limitations of the analytical procedures used to produce a conceptual description, and limitations computer modeling software and hardware.
Cost effective solutions depend on local conditions. In probably all cases of saltwater intrusion some uncertainty regarding the best management strategy exists. If the strategy is too cautious the use of the freshwater resource will not be optimal. If the strategy is incautious the freshwater resource may be damaged by saltwater intrusion. Although saltwater intrusion is reversible by diminishing the activities that are causing it and/or via mitigating measures involving artificial recharge, the recovery will be slow. Consequently, it is generally best to protect the freshwater from increase in salinity to the degree that the water is unsuitable for beneficial use.
Managing saltwater intrusion is a complex issue and a qualified hydrogeologist should be consulted when it must be addressed.
Posted November 2013
Revised August 2018