Under natural conditions, the seaward movement of freshwater prevents saltwater from encroaching coastal aquifers, and the interface between freshwater and saltwater is maintained near the coast or far below land surface. This interface is actually a diffuse zone in which freshwater and saltwater mix, and is referred to as the zone of dispersion (or transition zone) ( and ). Ground-water pumping can reduce freshwater flow toward coastal discharge areas and cause saltwater to be drawn toward the freshwater zones of the aquifer. Saltwater intrusion decreases freshwater storage in the aquifers, and, in extreme cases, can result in the abandonment of supply wells. Saltwater intrusion occurs by many mechanisms, including lateral encroachment from coastal waters and vertical upconing near discharging wells ( and ). 

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What is saltwater intrusion?

Salt water intrusion occurs in coastal freshwater aquifers when the different densities of both the saltwater and freshwater allow the ocean water to intrude into the freshwater aquifer. These areas are usually supporting large populations where the demanding groundwater withdrawals from these aquifers is exceeding the recharge rate. Figure 2 gives a rough illustration of what an overdrawn aquifer may look like. This can cause lateral and vertical intrusion of the surrounding saltwater, and evidence of saltwater intrusion has been found throughout the eastern seaboard of the U.S. (USGS, 2007). The encroaching seawater will encounter an area known as the zone of dispersion, where the freshwater and saltwater mix and form an interface, as illustrated in Figure 3. This interface moves back and forth naturally because of fluctuations in the recharge rate of freshwater back into these coastal aquifers (Ranjan, 2007). Aquifers are naturally replenished by precipitation and surface waters that saturate into the ground and work their way through the soil and geologic material to the water table.

What are the problems associated with groundwater depletion?

Declining Water Tables

Probably the most detrimental effect that groundwater depletion causes is the lowering of the water table. The water table is the area underneath the ground that is completely saturated with freshwater and can be drilled into and extracted as a freshwater resource. As the water level declines, extraction of water may prove to be more difficult. If the water level drops below the well, then it must be re-drilled and set at a lower depth. This can be quite an expensive procedure, especially for a residential consumer with an independent well system. As the water table declines, extraction of the freshwater becomes more difficult and expensive, and the rate of water that can usually be pumped out of the well will decline (USGS, 2005). Keeping this in mind, if water tables continue to decrease, extraction of groundwater for all different activities will become increasingly more expensive as time progresses. Currently in Vermont, the price of well drilling is going up. On average, a 250 foot well will cost somewhere around $4,000 (Helmich, 2000). However, the more complicated the project gets due to location, underground geological rock formation, and lower water tables, the higher the cost of drilling a well becomes. If this is true for a non coastal, lightly populated area such as Vermont, you can extrapolate the costs of well drilling in a densely populated, coastal area such as coastal South Carolina and Georgia, where the water table is extremely low.  

Changing the Interface

Another problem that takes place when saltwater intrusion occurs in coastal freshwater aquifers is the changing of the saltwater- freshwater interface. Also known as the zone of dispersion or transition zone, this is the area where the body of saltwater and freshwater meet and form a hydrologic barrier. The natural hydrologic movement of the underground freshwater towards the ocean usually prevents the seawater from intruding into the coastal aquifer system (Ranjan, 2007). Over-pumping of these coastal aquifers will cause a fluctuation in the amount of freshwater moving towards the coastal discharge areas and will allow for the oceanic water to move inland, into the aquifer system. This will result in higher chlorinated concentrations of water and less available storage space for the freshwater in the aquifer. Figure 6 accurately shows how the saltwater freshwater interface can intrude into the confined coastal aquifer. When the interface moves inland, the deeper wells will begin to withdraw saline contaminated freshwater. This means that the water must be treated before human consumption. The other option is to stop using this well until the natural recharge rate of the aquifer can force the saltwater freshwater interface back to its normal position. This process can take a long time and it reduces the freshwater availability of this region.  

What causes saltwater intrusion?

When groundwater levels in aquifers are depleted faster than they can recharge. This is directly related to the position of the interface and determines the amount of saltwater that can intrude into the freshwater aquifer system. Since saltwater intrusion is directly related to the recharge rate of the groundwater, this allows for other factors that may contribute to the encroachment of seawater into the freshwater aquifers. Climatic variables, such as precipitation, surface runoff, and temperature can play a big role in affecting saltwater intrusion. With lower precipitation amounts and warmer temperatures, the recharge rate will be much less due to lack of groundwater present and increased evaporation (Ranjan, 2007). Along with this, other factors may influence the groundwater recharge rate indirectly. An example of this would be the rising carbon dioxide emissions in the atmosphere. Increasing carbon dioxide levels can lead directly to an increase in average surface temperatures, indirectly increasing the evaporation rate and affecting the recharge of freshwater into the coastal aquifers. Figure 4 illustrates a situation where major pumping of well water has lead to a cone of depression in the water table. Figure 4 illustrates a situation where major pumping of the well water has lead to a cone of depression in the water table. When this occurs, it will move the saltwater freshwater interface inland, resulting in a higher saline concentration in the aquifers' water, rendering it useless for human consumption, unless it is treated.

Figure 4. Saltwater Intrusion Situation (University of Florida, 2000))

Another factor that directly affects coastal aquifer depletion is land use planning and management. Different activities such as irrigating crops and industrial processing can require a substantial amount of freshwater resources to be withdrawn. Figure 5 shows the groundwater withdrawals for the southeastern U.S. region in 1985 (Miller, 2002). If certain wells are relying on these coastal aquifers to provide enough freshwater to support agricultural, industrial, municipal, and residential demands, then the recharge rate of the aquifer must be able to keep up. The over-pumping of these coastal aquifers has decreased the underground water table level and decreased the abundance, pressure, and storage capacity of the freshwater aquifer. This will cause the zone of dispersion to move inland and drastically reduce the freshwater that is available from the well and it may result in contamination of the freshwater aquifer and eliminate it as a potential freshwater source.

Changing the Interface

Another problem that takes place when saltwater intrusion occurs in coastal freshwater aquifers is the changing of the saltwater- freshwater interface. Also known as the zone of dispersion or transition zone, this is the area where the body of saltwater and freshwater meet and form a hydrologic barrier. The natural hydrologic movement of the underground freshwater towards the ocean usually prevents the seawater from intruding into the coastal aquifer system (). Over-pumping of these coastal aquifers will cause a fluctuation in the amount of freshwater moving towards the coastal discharge areas and will allow for the oceanic water to move inland, into the aquifer system. This will result in higher chlorinated concentrations of water and less available storage space for the freshwater in the aquifer. Figure 6 accurately shows how the saltwater freshwater interface can intrude into the confined coastal aquifer. When the interface moves inland, the deeper wells will begin to withdraw saline contaminated freshwater. This means that the water must be treated before human consumption. The other option is to stop using this well until the natural recharge rate of the aquifer can force the saltwater freshwater interface back to its normal position. This process can take a long time and it reduces the freshwater availability of this region.  

What is saltwater intrusion?

Salt water intrusion occurs in coastal freshwater aquifers when the different densities of both the saltwater and freshwater allow the ocean water to intrude into the freshwater aquifer. These areas are usually supporting large populations where the demanding groundwater withdrawals from these aquifers is exceeding the recharge rate. Figure 2 gives a rough illustration of what an overdrawn aquifer may look like. This can cause lateral and vertical intrusion of the surrounding saltwater, and evidence of saltwater intrusion has been found throughout the eastern seaboard of the U.S. (). The encroaching seawater will encounter an area known as the zone of dispersion, where the freshwater and saltwater mix and form an interface, as illustrated in Figure 3. This interface moves back and forth naturally because of fluctuations in the recharge rate of freshwater back into these coastal aquifers (). Aquifers are naturally replenished by precipitation and surface waters that saturate into the ground and work their way through the soil and geologic material to the water table. 

What is saltwater intrusion?

Salt water intrusion occurs in coastal freshwater aquifers when the different densities of both the saltwater and freshwater allow the ocean water to intrude into the freshwater aquifer. These areas are usually supporting large populations where the demanding groundwater withdrawals from these aquifers is exceeding the recharge rate. Figure 2 gives a rough illustration of what an overdrawn aquifer may look like. This can cause lateral and vertical intrusion of the surrounding saltwater, and evidence of saltwater intrusion has been found throughout the eastern seaboard of the U.S. (). The encroaching seawater will encounter an area known as the zone of dispersion, where the freshwater and saltwater mix and form an interface, as illustrated in Figure 3. This interface moves back and forth naturally because of fluctuations in the recharge rate of freshwater back into these coastal aquifers (). Aquifers are naturally replenished by precipitation and surface waters that saturate into the ground and work their way through the soil and geologic material to the water table. 

What is saltwater intrusion?

Salt water intrusion occurs in coastal freshwater aquifers when the different densities of both the saltwater and freshwater allow the ocean water to intrude into the freshwater aquifer. These areas are usually supporting large populations where the demanding groundwater withdrawals from these aquifers is exceeding the recharge rate. Figure 2 gives a rough illustration of what an overdrawn aquifer may look like. This can cause lateral and vertical intrusion of the surrounding saltwater, and evidence of saltwater intrusion has been found throughout the eastern seaboard of the U.S. (). The encroaching seawater will encounter an area known as the zone of dispersion, where the freshwater and saltwater mix and form an interface, as illustrated in Figure 3. This interface moves back and forth naturally because of fluctuations in the recharge rate of freshwater back into these coastal aquifers (). Aquifers are naturally replenished by precipitation and surface waters that saturate into the ground and work their way through the soil and geologic material to the water table.