Chloride and water quality

Chlorides are salts resulting from the combination of the gas chlorine with a metal. Some common chlorides include sodium chloride (NaCl) and magnesium chloride (MgCl2). Chlorine alone as Cl2 is highly toxic and it is often used as a disinfectant. In combination with a metal such as sodium it becomes essential for life. Small amounts of chlorides are required for normal cell functions in plant and animal life.

Methodology: Although analysis can be performed by potentiometric titration using the computer aided titrimeter (CAT) with an ion-specific chloride electrode, Ion Chromatography (IC) is the preferred method of identification. For an in-depth explanation of Ion Chromatography see Appendix II-D. Essentially, an analytical column is used to separate out various anions. The time required for the anion to pass through the column indicates its concentration. To determine the identification of the anion, the IC uses a conductivity meter. Since each anion has a different conductivity, its identity can easily be determined.

Environmental Impact: Chlorides are not usually harmful to people; however, the sodium part of table salt has been linked to heart and kidney disease. Sodium chloride may impart a salty taste at 250 mg/L; however, calcium or magnesium chloride are not usually detected by taste until levels of 1000 mg/L are reached.

Chlorides may get into surface water from several sources including:

1) rocks containing chlorides;

2) agricultural runoff;

3) wastewater from industries;

4) oil well wastes;

5) effluent wastewater from wastewater treatment plants, and;

6) road salting.

Chlorides can corrode metals and affect the taste of food products. Therefore, water that is used in industry or processed for any use has a recommended maximum chloride level. Chlorides can contaminate fresh water streams and lakes. Fish and aquatic communities cannot survive in high levels of chlorides. Table VI below shows the effects of chlorides on fish:

Table VI

Chloride Above These Levels Can Be Toxic
mg/L (PPM)
Short Exposure
Long Term
2,540 400 Snail
6,570 430 Fathead minnow
6,740 900 Rainbow trout
8,000 800 Channel catfish
8,390 850 Carp

Criteria: Public Drinking Water Standards require chloride levels not to exceed 250 mg/L. Criteria for protection of aquatic life require levels of less than 600 mg/L for chronic (long-term) exposure and 1200 mg/L for short-term exposure.

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