SALT INTRUSION: A THREAT TO SOURCE WATER QUALITY

By Swati Hegde, Ph.D.

February 14, 2019

Salt (salinity) intrusion is the movement of saline water into freshwater aquifers resulting in contamination of drinking water resources. Salinity intrusion can occur during the events of reduced streamflow caused by severe drought or, potentially, due to climate change-related sea level rise1. However, other significant factors such as increased ground-water pumping can increase the rate of intrusion of saline water into ground-water sources resulting in a high water treatment cost in places that rely on ground-water for a source of drinking water2. Salt intrusion also renders ground-water wells unusable due to elevated chlorine concentration. In case of surface waters, as the sea levels rise, a hydrodynamic phenomenon occurs, where the ‘salt-fronts’ progress further upstream (for example, Delaware Bay). This phenomenon is happening at an alarming rate in various regions and may diminish the quality and availability of water sources for drinking water utilities.

 

Cases of Salt intrusion:

Many case studies have reported the extent of contamination caused in crucial aquifers by salt water. In Gaza, the Mediterranean Sea is percolating through the sand, impinging on a fresh ground-water reserve—a salty invader contaminating the primary drinking water source for more than a million people3. Miami Beach, Florida, stands as a sign of the times in which coastal regions are being impacted by sea level rise. Nearly seven million people in four south Florida counties rely on the Biscayne Aquifer for their drinking water4. As a coastal aquifer connected to the floor of Biscayne Bay and the Atlantic Ocean, it is vulnerable to potential salt contamination. In the Mid-Atlantic region- the Delaware Estuary- a primary source of drinking water to Pennsylvania, New Jersey, and Delaware, is at a significant threat by saltwater intrusion. Because saltwater contains high concentrations of dissolved solids and inorganic matter, it is unfit for human consumption and other recreational uses. Saltwater intrusion affects ground-water stock negatively and, in extreme cases, results in the abandonment of supply wells when dissolved ion levels exceed drinking-water standards. Several other case studies can be found here.

Several city planning departments have been taking proactive measures to track salinity levels in the drinking water supply. The Environmental Fluid Dynamics Code (EFDC) modeling has been utilized by the departments to model salinity intrusion in York River, Indian River Lagoon, Lake Worth and Philadelphia Water Department. Salt-laced water known in the water world as “salt front” or “salt-line” is identified where the chlorine concentration is 250 mg/L5. The total dissolved solids concentration in seawater is about 35,000 mg/L, of which chloride ion is the most significant component (about 19,000 mg/L). Levels of chloride in fresh ground water along the Atlantic coast are typically less than about 20 mg/L, so there is a significant contrast in chloride concentrations between freshwater and saltwater6. The salt line’s locations fluctuate throughout a water body as the inflows can increase or decrease, resulting in dilution or concentration of chlorides in water.

 Approaches to reducing salt intrusion:

A common approach to reducing saltwater intrusion has been to reduce the rate of ground-water pumping from coastal supply wells or to move the locations of pumping further inland. Reduced coastal extractions allow ground-water levels to recover from their stressed levels, and allow space for fresh ground-water to displace the intruded saltwater. In some states like New Jersey, reductions in ground-water withdrawals in some coastal counties due to a State mandate have resulted in ground-water-level increases in aquifers that have been affected by saltwater intrusion. There have also been efforts to artificially recharge freshwater into an aquifer to increase ground-water levels and control the hydraulic movement of the invading saltwater. Specially designed Injection wells or by infiltration of freshwater at the land surface are used to accomplish artificial recharge7. The most noticeable example of the use of artificial recharge in the United States is in southeastern Florida. In that area, a widespread network of surface-water canals is used to transport fresh water from inland water-storage locations during the dry season to coastal regions, where the water is recharged through the canals to the underlying aquifer to slow saltwater intrusion in the aquifer.

In addition to conventional methods, scientific and innovative strategies are now being used to control or manage saltwater intrusion along the Atlantic coast. These include aquifer storage and recovery systems and desalination systems. Aquifer storage and recovery (ASR) is a process by which water is recharged through wells into a suitable aquifer, stored for a duration, and then extracted from the same wells when needed8. Typically, water is stored during rainy and wet seasons and pumped during dry seasons. ASR systems have been developed in New Jersey, the town of Chesapeake, Virginia, Wildwood (Cape May County), the and at several locations in Florida.

Desalination is a water-treatment process that produces freshwater by removing dissolved salts from saline or brackish waters by using a membrane-based process called reverse osmosis. Desalination systems are increasingly being adopted in the United States. One of the exciting aspects of the increased use of desalination systems is that it changes the perspective on saline or brackish water from that of a potential water problem (a contaminant) to that of a potential water source. The desalination plant in Cape May, New Jersey is capable of producing 2 million gallons treated water output per day and was installed at a total cost of USD 5 million in 19989.

Challenges and Opportunities:

Despite the regulatory and non-regulatory efforts to manage salt intrusion, there are several challenges and opportunities associated with this problem. Some of the issues that need immediate attention are,

• Periodic evaluation of the ground-water monitoring systems and estimates of ground-water use especially in areas where ground-water development has recently begun or increasing at a substantial rate
• Improved understanding of the phenomena that lead to saltwater intrusion via monitoring, modeling and simulation studies
• There is a growing need to quantify the relative importance of ground-water as a source of drinking water and contaminants to different types of coastal ecosystems
• There are a number of areas in which scientific evaluations are needed to support conventional and emerging approaches for ground-water management in coastal regions.

Because of increasing awareness of the critical role of ground-water in sustaining coastal populations, ecosystems, and economies, the time is right to review some of the essential water-management issues and scientific principles related to ground-water and to identify some of the management challenges that lie ahead.  As coastal populations and ground-water use increase, new monitoring and research efforts will be needed to characterize the occurrence and hydrodynamics of saline ground-water in different types of coastal terrains. Novel methods are required to better understand the linkages between ground-water discharge and quality and the sustenance of coastal ecosystems.