Significance Of Chloride
Chloride in reasonable concentrations is not harmful to humans. At concentrations above 250 mg/L it gives a salty taste to water, which is objectionable to many people. For this reason a secondary standard of 250 mg/L for chloride in drinking water has been set by the U.S. EPA, the same value contained in the WHO guidelines. In many areas of the world where water supplies are scarce, sources containing as much as 2000 mg/L are used for domestic purposes without the development of adverse effects, once the human system becomes adapted to the water.
The chloride content of waters used for irrigation of agricultural crops is generally controlled along with the total salinity of the water. Evapotransportation tends to increase the chloride and salinity at the root zone of irrigated plants, making it difficult for crops to take up water due to osmotic pressure differences between the water outside the plants and within the plant cells. For this reason, chloride and total salinity concentrations at or below the drinking water standards are normally specified for waters used to irrigate salt-sensitive crops.
Before the development of bacteriological testing procedures, chemical tests for chloride and for nitrogen, in its various forms, served as the basis of detecting contamination of groundwaters by wastewater. Chloride is used to some extent as a tracer in environmental engineering practice. It is inconvenient to use in many instances because of the quantities required to produce significant increases, in chloride level and because of their tendency to produce density currents. Their use as tracers has been superseded to a great extent by organic dyes which can be measured accurately in trace amounts. In groundwater studies, however, where the ratio of soil to water is very high, organic dyes tend to sorb to a sufficient degree such that they move slower than the water itself. Here, chloride, or perhaps a less prevalent halogen such as bromide, may still be useful as a tracer.
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