Chlorine is by a wide margin the most important and most widely distributed of the Halogens in natural water. Generally, it is present as the chloride ion Cl-. Chloride may be of natural mineral origin or derived from:
(a) sea water contamination of underground water supplies,
(b) salts spread on fields for agricultural purposes such as potassium chloride,
(c) human or animal sewage, or
(d) industrial effluents.
Chlorine is widely used in water treatment as a disinfectant or biocide. According to the data in Table 1, chloride is present in various rock types in lower concentrations than any of the other major constituents of natural water. Main chloride bearing minerals occurring in igneous rock are feldspathoid sodalite and the phosphate mineral apetite.
On the whole, igneous rocks, at least those available to circulating natural water cannot yield very high concentrations of chlorides. Considerably more important sources are associated with sedimentary rocks, particularly the evaporites. Chloride may be present in resistates as the result of inclusion of connate brine and is to be expected in any incompletely leached deposit laid down in the sea or in a closed drainage basin.
When porous rocks are submerged by the sea at any time after their formation, they become impregnated with soluble salts in which chloride plays a major role. Fine grained marine shale might retain some of this chloride for very long periods. In all these rock types, the chloride is mostly present either as sodium chloride crystals or as a solution of sodium and chloride ions.
Chloride ions do not significantly enter into oxidation reduction reactions, form no important solute complexes with other ions, do not form salts of low solubility, are not significantly absorbed on mineral surfaces and play relatively few vital biochemical roles.
Chloride is present in all natural waters, but mostly the concentrations are low. In most surface streams, chloride concentrations are lower than those of sulfate or bicarbonate. Exceptions occur where the streams receive inflows of high chloride ground water or industrial wastes or are affected by oceanic tides.
The influence of chloride from hot springs and volcanic gases is very evident in the chloride loads of some streams. Chloride ions may be retained in solution through most of the processes which tend to separate out other ions.
The differential permeability of clay and shale may be a major factor in the behavior and composition of saline ground water associated with fine grained sediments.
For example, chlorine held back while water molecules pass through a clay layer might accumulate until high concentrations were reached. The selective behavior of such a layer also influences the residual concentration of cations. The more strongly retained ions in such a solution would be ones most strongly attracted to cation exchange sites. Calcium is usually the ion preferentially held, thus a mechanism is suggested for the origin of calcium chloride brines.
The most common type of water in which chloride is the dominant anion is one in which sodium is the predominant cation. Waters of this type tend to range from dilute solutions influenced by rainfall near the ocean to brines near saturation with respect to sodium chloride.
Chlorides in drinking water are generally not harmful to human beings until high concentrations are reached. However, chlorides may be injurious to some people suffering from diseases of heart or kidneys.
Restrictions on chloride concentrations in drinking Water are generally based on palatability requirements, rather than on health. Chlorides may impart asalty taste at concentrations as low as 100 mg/L.
Chloride concentration should be below 170 mg/L to support good fresh water biota. In water supporting a good fish fauna, ordinarily the concentrations of chlorides is below 3 mg/L in 52, below 9 mg/L in 50% and below 170 mg/L in 95% of such waters.
The following concentrations of chloride have been reported to be harmful to fish.
Concentration in mg/L Type of Fish _
4,000 Bass, Pike and Perch
4,500-6,000 Carp Eggs V
8,100-10,500 small Blue Gills
On the other hand, 2,000 mg/L of chloride has been reported as not harmful for some fish. The Aquatic Advisory Commission of ORSANCO concluded that it is nearly impossible to generalize on the effects of chloride concentrations on aquatic life because each mixture of chlorides with other salts must be evaluated separately.
In summary, the basis of the foregoing information it appears that the following concentrations of chloride will not be normally deleterious to specified beneficial uses:
domestic water supply – 250 mg/L,
industrial water supply – 50 mg/L,
irrigation – 100 mg/L,
stock and wildlife – 1,500 mg/L.