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1050 GROUND WATER QUALITY Because the results of specific conductance measurements shown in Table 3 are larger than expected, four water samples were obtained from selected wells for detailed analyses by the Colorado Department of Public Health. Each well that was sampled was chosen because either it had a large specific con- ductance measurement or was used frequently. The samples were obtained at: 1. The measuring flume of the company-owned wells while the north field (wells 5-7) were pumping on September 4, 1970; 2. An individually-owned well located in T22S, R59W, Sec. 2lbdd (watt-hour meter no. 11946) on September 2, 1970; 3. An individually-owned well located in T22S, R59W, Sec. l5abb (watt-hour meter no. 13259) on September 2, 1970; and 4. An individually-owned well located in T22S, R59W, Sec. l8bcc (watt-hour meter no. 12303) on September 2, 1970. The results of the analyses are shown in Tables 4A-4D. In Irrigation of Agricultural Lands, an American Society of Agronomy publication, the four important criteria that determine quality of irrigation water: total concentration of dissolved solids, sodium concentations, bicar- bonate concentration and boron concentration are explained, and the U. S. Salinity Laboratory procedure for classification of irrigation water is des- cribed. The following general discussion is paraphrased from the reference. Total concentration is probably the single most important criterion of irrigation water quality because the salinity of the soil is usually related to, and often determined by the salinity of the irrigation water. Thus, plant growth may be impaired or prevented depending on the salt content of the water. The total concentration may be expressed in terms of parts per million (ppm) of dissolved solids or as specific electrical conductance in micromhos per centi- meter ~mho/cm). The latter is preferred because conductance is more easily determined. Specific electrical conductance is the conductance of a cube of water one centimeter on a side at 250 Centigrade. Conductance is the ability of a substance to conduct electricity, i.e., the opposite of resistance. Chemically pure water is a poor conductor, but as salts are added the conductance improves. The conductance measurements shown in Table 3 were made with a temperature com- pensated Beckman RB3 Solu Bridge. Sodium (Na+) in irrigation water can form exchangeable sodium in the soil which can result in harmful effects. It can make a moist soil impermeable to air and water, and on drying, this soil is hard and difficult to till. Dense crusts form that interfere with germination and seedling emergence. The most reliable index of this sodium hazard in irrigation water is the sodium-absorption- ratio, SAR. It is defined as: - 10 -