City leaders in a coastal community asked residents not to drink water from the tap or use it for cooking. Because of a drought and low groundwater levels, the community's well field was experiencing saltwater intrusion.

City officials contacted Florida Gateway College to ask what they should do to correct the problem. In the interim, the city made bottled water available to residents, while contemplating desalination equipment to make the water drinkable again.

The people of this small community are not alone: Saltwater intrusion into drinking water wells is a growing phenomenon in coastal regions. Dania Beach, Pompano Beach, Hallandale Beach, Lantana and Lake Worth are among Florida cities dealing with the problem.

The threat to coastal drinking water security is linked to the drying up of inland springs, rivers and lakes. Drought is a factor, but over-pumping of the aquifer is the greater and ever-present threat to the well fields. The indifference of politicians and water managers is mystifying.

How much salt?

Water that is saline contains significant concentrations of dissolved salts. The secondary contaminant level for total dissolved solids (TDS) in drinking water is 500 ppm. The parameters for salinity in water are:

Freshwater: Less than 1,000 ppm TDSSlightly saline water: 1,000 to 3,000 ppmModerately saline water: 3,000 to 10,000 ppmHighly saline water: 10,000 to 35,000 ppmAverage ocean salinity: 35,000 ppm

The scarcity of freshwater and the need for additional water supplies is critical on a global basis. Water issues are likely to be a determining factor in world stability in the near future. Many areas do not have freshwater in the form of surface water, such as rivers and lakes, and their limited underground water resources are becoming more brackish as water continues to be removed from aquifers.

Getting the salt out

Modern technologies in desalination now make it possible to consider desalination plants to ensure production of water in a sustainable manner.

Solar desalination occurs in nature to produce rain, which is the main source of freshwater on earth. The formation of sea ice is also a process of desalination as well: Salt is expelled as the water freezes, so that the overall salinity of the ice is much lower than that of seawater. Also, willow trees and reeds absorb salt and other contaminants, effectively desalinating water. That attribute is used in constructed wetlands that treat sewage.

Today, desalination plants convert seawater to drinking water on ships and in many arid regions of the world. The technology also treats water fouled by natural and manmade contaminants. Some facts about desalination:

An estimated 30 percent of the world's irrigated areas suffer from salinity.There are roughly 12,500 desalination plants around the world, producing more than 300 million gallons of freshwater per day.The largest users of desalinated water are the Middle East and North Africa.The United States uses 6.5 percent of the world's desalination capacity.

Rise of membrane technology

Early desalination plants used distillation — an evaporative technology that is essentially a duplicate of the natural solar-driven process. Today, desalination has evolved into a membrane treatment technology. As the technology advances, membranes are being used in water treatment plants, in homes as point-of-use or point-of-entry devices, and in reclamation facilities to remove dissolved and suspended minerals.

Membrane processes use semi-permeable membranes and pressure to separate contaminants from water. Membranes used in treatment facilities are classified according to pore size. From the largest pore size to the smallest, the types of membranes are microfiltration, ultrafiltration, nanofiltration and reverse osmosis.

The type of membrane used depends on the constituents to be removed from the water. Microfiltration and ultrafiltration are used for particle, sediment, algae, bacteria and virus removal. Desalination systems use reverse osmosis.

Reverse osmosis uses less energy than thermal distillation, and that has reduced desalination costs. However, desalination remains energy intensive, and future costs depend on the price of energy and the efficiency of desalination technology.

Alternatives to desalination

Communities faced with saltwater intrusion can take a variety of measures to avoid the costly step of desalination. Water conservation and efficiency remain the most cost-effective practices. Wastewater reclamation for irrigation and industrial reuse also helps protect potable water supplies and reduce demands on aquifers. Urban runoff and stormwater capture and treatment also provide benefits in restoring and recharging groundwater.

In the case described earlier, the coastal community received a reprieve from the saltwater intrusion when a tropical storm arrived, recharging the groundwater in the well field. The fact remains that if over-pumping continues, the community leaders have only succeeded in delaying the need for a desalination process.

ABOUT THE AUTHOR

John Rowe, Ph.D., is a professor of Water Resources at Florida Gateway College in Lake City, Fla.