Water Quality: Salinity and Chloride

May 26, 2006

Chloride, in the form of the Cl– ion, is one of the major inorganic anions, or negative ions, in saltwater and freshwater. It originates from the dissociation of salts, such as sodium chloride or calcium chloride, in water.               

                                                NaCl(s) ---->  Na+(aq) + Cl–(aq)

                                                CaCl2(s) ---->  Ca2+(aq) + 2 Cl–(aq)

These salts, and their resulting chloride ions, originate from natural minerals, saltwater intrusion into estuaries, and industrial pollution.

There are many possible sources of manmade salts that may contribute to elevated chloride readings. Sodium chloride and calcium chloride, used to salt roads, contribute to elevated chloride levels in streams. Chlorinated drinking water and sodium-chloride water softeners often increase chloride levels in wastewater of a community.

In drinking water, the salty taste produced by chloride depends upon the concentration of the chloride ion. Water containing 250 mg/L of chloride may have a detectable salty taste if the chloride came from sodium chloride. The recommended maximum level of chloride in U.S. drinking water is 250 mg/L.

Some sources of Chloride Ions
River streambeds with salt-containing minerals
Runoff from salted roads
Mixing of seawater with freshwater
The Use of Water softeners
Irrigation water returned to streams
Chlorinated drinking water

Salinity is the total of all non-carbonate salts dissolved in water, usually expressed in parts per thousand (1 ppt = 1000 mg/L). Unlike chloride (Cl–) concentration, salinity is a measure of the total salt concentration, comprised mostly of Na+ and Cl– ions. Even though there are smaller quantities of other ions in seawater (e.g., K+, Mg2+, or SO4), sodium and chloride ions represent about 91% of all seawater ions. Salinity is an important measurement in seawater or in estuaries where freshwater from rivers and streams mixes with salty ocean water. The salinity level in seawater is fairly constant, at about 35 ppt (35,000 mg/L), while brackish estuaries may have salinity levels between 1 and 10 ppt. Since most anions in seawater or brackish water are chloride ions, salinity can be determined from chloride concentration. The following formula is used:

salinity (ppt) = 0.0018066 5 Cl– (mg/L)


A Chloride Ion-Selective Electrode can be used to determine the chloride concentration, which is converted to a salinity value using the above formula.

Salinity can also be measured in freshwater. Compared to seawater or brackish water, freshwater has much lower levels of “salt ions” such as Na+ and Cl–; in fact, these ions are often lower in concentration than hard-water ions such calcium (Ca2+) and bicarbonate (HCO3–). Because salinity readings in freshwater will be significantly lower than in seawater or brackish water, readings are often expressed in mg/L instead of ppt (1 ppt = 1000 mg/L).

Salinity is also of interest in bodies of water where seawater mixes with freshwater, since aquatic organisms have varying abilities to survive and thrive at different salinity levels. Saltwater organisms survive in salinity levels up to 40 ppt, yet many freshwater organisms cannot live in salinity levels above 1 ppt.