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12 Estimating Economic Impacts of Salinity... Thus, the question of effect versus damage relates not only to water source, but to the cause of the salinity, the use to which the water is put, F\) and whether the use is appropriate or well- 0) managed. In direct answer to the question, CO . "Should every negative salinity effect be con- i-" sidered a damage?," it is tbe judgment of the authors that each negative impact should not be, and could not be, considered a damage. Instead, the impacts of current salinity should be com- pared with those of a baseline salinity level that can be justified as a "normal" standard for comparison. By using one or more selected baseline salinity levels, a potential problem is avoided: that of measuring damages against an idealized water supply that rarely exists in nature and could never be achieved technologically or economically for most water supplies. Measuring against such an ideal water would exaggerate the true damage figure. Instead, it is proposed to measure the physical and economic effects of two water supplies - one with current salinity levels and another with a selected baseline salinity level- and subtract the latter from the former. The difference in the economic effects, or costs, between two or more salinity levels has been selected in this study as the appropriate measure of salinity "damages." Selection of a Baseline What should the baseline be? An explora- tion has identified numerous candidates. Should it be the "natural" level of salinity in the Colorado River? If so, at which point in time or at which physical spot along its course of rising salinity levels should it be measured? Should the baseline be the salinity level most commonly found throughout the U.S., despite wide varia- tions in rainfall, soil, and geology? Should it be the level which EP A recommends as a secondary drinking water standard? Table 1 presents an array of 13 possible baseline salinity levels, ranging from under 50 mg/L (a "pristine" water that might still be chemically aggressive) to 825 mg/L, the maxi- mum flow-weighted salinity level projected at Parker Dam in 2010 without adoption of salinity controls. The selection of appropriate baselines has been a matter of intense discussion among the authors, with the Bureau of Reclamation, and with the Work Group of the Colorado River Basin Salinity Control Forum. Agreement was reached regarding the selection of two baselines which, in conjunction with current salinity levels would bound the range of salinity damages. They are: 334 mg/L TDS and 500 mg/L TDS. There are other salinity levels which could have been considered as baseline values, as illustrated by table 1. Some of them will be mentioned briefly following a discussion of the two selected baseline TDS numbers. 334 mg/L TDS. EPA's 1971 The Mineral Quality Problem in the Colorado River Basin determined a natural TDS level at Hoover Dam based on natural point and diffuse sources of 334 mg/L TDS for both 1960 and projected 2010 conditions at Hoover Dam. This is based on the 1942-1961 hydrologic record. It was pointed out that it is not entirely consistent with virgin flow and salt load assumptions used in the longer period of record of the CRSS data base? 500 mg/L TDS. The EPA Secondary Drink- ing Water Standard of 500 mg/L TDS is widely used in reports and discussions about salinity. It is the standard widely used by various health agencies and by the Environmental Protection Agency, although this number is not based on any formal scientific investigation. It is selected as a baseline because it is a widely recognized and accepted TDS parameter in the area of water quality. The 500 mglL standard also is 1 he goal of the Metropolitan Water District of Southern California for its blended water supply. Other Possible Baselines Initial studies considered baselines of 200 mg/L TDS, 250 mg/L ITDS, 295 mg/L TDS, and 349 mg/L TDS. The 200 mgIL baseline corres- ponded to the hardness goal (80-100 mgIL) in the AWWA's policy statement for Potable Water adopted in the 1960's (currently being revised). 3EPA, Summa~ Rf!port, pp. 15 and 22; David Merritt, Colorado River Water Conservation District, September 15, 1987.