Criteria presented in the following sections can be used to evaluate the quality of water relative to existing or anticipated beneficial uses. It should be noted that these criteria are merely guides to the appraisal of water quality. Except for those constituents considered toxic to human beings, these criteria should be considered as suggested limiting values. A water which exceeds one or more of these limiting values need not be eliminated from consideration as a source of supply, but other sources of better quality water should be investigated.
The physical and chemical quality of drinking water is judged in relation to a set of criteria contained in Title 17, Part 1, Chapter 5, Subchapter 1, of the California Administrative Code and also set forth by the Environmental Protection Agency (1975,
The physical characteristics that are recommended for limiting in drinking water are: turbidity -- not to exceed 0.5 turbidity units for water exposed to significant sewage hazards; color -not to exceed 15 units on the cobalt scale; and odor -- not to exceed 3 on the threshold odor number.
Limiting concentrations for most mineral constituents in drinking water are shown on Table 25. Upper limits for total solids, electrical conductivity, chloride, and sulfate are shown on Table 26. Limiting concentrations for fluoride are shown on Table 27.
Although hardness is not included in any of the above standards, it is important to domestic and industrial uses. Excessive hardness in domestic water supplies causes increased consumption of soap and formation of scales in pipes and fixtures. Table 28 shows the relative degrees of hardness in water.
The quality of water required by industry varies widely because of the many purposes to which water is put. These requirements are too variable to allow any broad generalization; however, for most processes, industry is generally willing to accept water that meets drinking water standards. One factor that is of
Table 28
RELATIVE DEGREES OF HARDNESS OF DRINKING WATER
Range of Hardness Expressed as CaCO3 Relative Classification
0 - 100 Soft
101 - 200 Moderately hard
Greater than 200 Hard
primary importance to most industries is that the concentrations of the various mineral constitutents in water be relatively constant because variations in concentrations of constituents usually require continued attention and added expense.
The concentration of salts in irrigation water is rarely high enough to cause immediate injury to crops. Good soil drainage is probably more important to crop growth because even when excellent- quality water is applied, poorly drained land could go out of production due to the salt build-up in water retained in the soil. The continual concentration of salts in the soil solution is the result of evapotranspiration by plants which use only the water fraction, leaving the salts behind. The soil solution can be rendered less saline only by downward leaching and by periodic application of less saline water.
A discussion of the quality of irrigation water must include the effects that the mineral constitutents contained in the water will have on both the plant and the soil. Potential detrimental effects on plant growth by salts occur in three areas: physical, chemical, and indirect. The physical effect that applied water has on plants is by the suppression of water uptake, i.e., the osmotic effect; the chemical effect is on the metabolic reactions of the plants to the mineral constituents in the water, i.e., the toxic effects; and the indirect effect occurs through changes in soil structure, permeability, and aeration.
Because of all of the variables involved, a classification of irrigation water cannot be rigidly set. A set of water quality guidelines has been proposed by Ayers and Branson (1974). These guidelines, which are shown on Table 29, show recommended limits for irrigation water as well as for water used for livestock and poultry.
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