Where Do Chlorides Come From?

When excessive sodium and chloride appear in source water for a drinking water system, it’s important to pinpoint the source and explore options for treatment.

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Leaders of a small community water system in Florida inquired with specialists and Florida Gateway College about the origin of chlorides in source water.

College representatives explained that the majority of chloride ions found in groundwater near the coast are of marine origin. Chloride ions can be introduced to groundwater through mixing with salt water, either as lateral intrusion from saline water bodies or as highly mineralized water up-coning from deeper aquifers.

Sodium and chloride also occur in domestic wastewater, which means finding their source is important: Their presence may indicate nearby improper disposal of sewage. Elevated sodium and chloride must be considered an indication of increased risk of more serious bacterial or chemical pollution until a more detailed analysis identifies the actual origin.

Finding the source

Chloride concentrations in groundwater worldwide average close to 6 mg/L. Chlorides in Florida’s groundwater are significantly higher because of saltwater intrusion and a marine aerosol effect. The average chloride concentration in seawater is 19,000 mg/L. The drinking water standard for chloride is 250 mg/L. At concentrations above 250 mg/L, chloride can impart a salty taste. Elevated sodium and chloride increase water’s ionic conductance, and thus increase the potential for corrosive water damage to plumbing fixtures.

The college explained to the community that its raw water source in the Floridan aquifer system moves from areas of high potential head to areas of low potential head. The potentiometric surface in 1960 ranged from more than 60 feet above sea level to about 30 feet above sea level in the depression in the south-central part of the county. In 1999, the potentiometric surface ranged from about 53 feet above sea level to 17 feet above sea level within that depression.

A comparison of 1960 potentiometric surface with that of 1999 showed that heads in most of the county had declined from 5 to 26 feet since 1960. The degree of decline in the potentiometric surface varies depending on the location. Long-term hydrographs of Floridan aquifer system monitoring wells indicate a downward trend of water levels in many wells in the county. Water levels showed declines of about 25 to 30 feet from 1940 to 2001.

The potentiometric surface of the Floridan aquifer system has gradually declined primarily as a result of increased pumping. Associated with this decline has been increased potential for movement of saline water into the freshwater zones of the aquifer system. Gradual but continual increases in chloride in water from the aquifer have been observed in a number of wells in the county. The potential for saltwater intrusion is expected to increase as population growth places greater demands on groundwater resources of northeastern Florida.

Controlling concentrations

Chloride concentrations also have increased with time. At present, chlorides in samples from only a few wells exceed the 250 mg/L. However, continued declines in aquifer water levels increase the risk of further water-quality degradation. As water levels have declined about 24 feet in 70 years, chloride concentrations have increased by 63 mg/L.

The U.S. EPA has set a maximum contaminant level of 250 ppm chloride, which is the point at which water starts to taste salty. Typical background levels of sodium and chloride are less than 20 mg/L. At present, there are no health-based standards for sodium or chloride under the Federal Safe Drinking Water Act.  

Normally, the best method to control sodium and chloride in drinking water is to better manage activities that add salt in the recharge area of the water supply source. A water resource management plan is paramount if saltwater intrusion is to be avoided. However, there are drinking water treatment technologies that will remove chloride.

Reverse osmosis (RO) places water under pressure against a special membrane. The membrane allows water molecules to move through but prevents the passage of salt and other dissolved minerals. However, RO for high-volume needs is inefficient and costly.

Distillation first boils water to produce steam. The steam is then condensed to produce purified drinking water. Salts and other mineral impurities stay in the boiling chamber, which requires periodic cleaning to remove the accumulated minerals. Distillation is not effective for organic contaminants. It is also costly to operate and is only feasible for small volumes.

Deionization is a method similar to water softening, but this process uses strong acids and bases rather than salt to regenerate the system. It is an effective method for removing chlorides from raw water. Bottled water is also an option while long-term treatment solutions are being investigated.

About the author

John Rowe, Ph.D., is a professor of water resources at Florida Gateway College in Lake City, Fla. He can be reached at john.rowe@fgc.edu.   



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