case. Water to meet all demands would continue to be pumped from ground water supplies in a conjunctive management scheme. Treating surface water with advanced membrane filtration to remove salinity and organic material prior to direct delivery to customers is the "with-treatment" option. It is important to note that in this example the valuation techniques are not used to calculate the TEV for ground water. Rather, they are used to evaluate the change in ground water value that results from a particular policy decision.

Identifying Changes in the Quantity and Quality of Ground Water

Initially, hydrologists must establish the quantity and quality of Tucson's ground water resources. Policy-makers need to assess how the "with-treatment" management option would change this baseline quantity and quality. Since the recharge option is considered the baseline, an accurate assessment of the impacts of artificial recharge potential is also needed.

The quality of the water that is pumped depends on where the CAP water is recharged relative to the location of recovery, the nature of the aquifer materials, the degree of blending with local ground water, the distance the water travels in the subsurface, and the presence of any source of contamination. The Groundwater Management Code allows an entity to recharge in one location and recover at a distant location within the same AMA, provided certain criteria are met.

Identifying Changes in Service Flows

The next step is to link the management decision with the changes that result in the time path of services that the ground water will provide under the alternative. This is where the critical input from scientists and hydrologists is required. The "without" scenario describes the services provided in the base case and the incremental changes that result from substituting treated surface water supplies.

Incremental Changes in Extractive Service Flows

Although Colorado River water is viewed as a high-quality water source for millions of people in the Southwest, there are several ways in which recharge using CAP water can reduce the quality of the water available for extractive uses. CAP water as treated with conventional surface water treatment methods meets all of the EPA maximum contaminant levels (MCLs), but the aesthetics, taste, and hardness of CAP water were a major issue for Tucson Water when the supply was directly delivered to customers from 1992 to 1994. Any use of CAP water in the basin, whether through direct delivery or recharge, will increase the salinity level of the aquifers within the Tucson AMA. The only way to avoid the increase in salinity is to utilize an advanced treatment approach (probably using membrane technology) to remove the salts. This technology is expensive; it is there-



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