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. . w OJ CJl ~ salinity control program also results in a saving of usable water that can be returned to the river system, additional economic benefits re- sult. The marginal value of water in the Lower Colorado Basin is esti- mated at $40.00 per acre-foot (a 1978 report estimate). The agricultural benefits to Mexico equal ab,out half of the U, S. agricultural and residential benefits from salinity control, It is in- appropriate to add them to U.S. benefits, for purposes of justifying U.S. costs of a salinity control program. Nevertheless, these benefits would be of significant value to a neighboring country and would contri- bute to improved U.S.-Mexican relations as well as being socially de- sirable. For purposes of this analysis, Mexican benefits are consider- ed as a reinforcing but indirect benefit. Salinity control also has amenity benefits, some of which (i.e., softness of water used for residential purposes) are included in the economic benefit estimates. A reduction in TDS of drinking water can be measured either in reduced cost, for persons who would otherwise pay for water softening or for bottled drinking water, or in improved taste for persons who would drink the Colorado River water. Salinity in drinking water is objectionable largely because of its effect on taste (at concentrations above 300 or 400 mg/l of sulfates) and its associated hardness. However, saline water is known to have negative health effects on certain sensitive persons, particularly at higher sodium levels. Gas- trointestinal effects are' associated with high levels of sulfates (over 600 mg/l), among persons unconditioned to such levels.17 Other non-quantified amenity benefits from salinity control in- clude improved habitat for certain fish and wildlife species and support for some non-crop vegetation, although water of up to 3,000 mg/l TDS can provide recreational use and wildlife habitat. On a benefit/cost equation, it appears that a salinity control program will have a total benefit to U.S. water users (as a group) of $450,000 per mg/l of salinity reduction at Imperial Dam plus $40/af of water saved or added to the Lower Colorado River supply. Consumptive use of water, whether for salinity control or other purposes, will have an opportunity cost of $40/af consumed. Salinity Control as a Beneficial Use of Water Salinity in the Colorado River must be controlled and maintained at or below specific levels. That is mandated by Minute 242, l7U.S. Environmental Protection Agency, "Proposed National Secon- dary Drinking Water Regulations," Federal Register, Vol. 42, No. 62, March 31, 1977. III-16 , -'