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The access of almost all 270 million U.S. residents to reliable, safe drinking water distinguishes the United States in the twentieth century from that of the nineteenth century. The United States is a relatively water-abundant country with moderate population growth; nonetheless, current trends are sufficient to strain water resources over time, especially on a regional basis. We have examined the areas of public water infrastructure, global climate effects, waterborne disease (including emerging and resurging pathogens), land use, groundwater, surface water, and the U.S. regulatory history and its horizon. These issues are integrally interrelated and cross all levels of public and private jurisdictions. We conclude that U.S. public drinking water supplies will face challenges in these areas in the next century and that solutions to at least some of them will require institutional changes.

[1] Starting early in 2005, the positions of GPS stations in the San Gabriel valley region of southern California showed statistically significant departures from their previous behavior. Station LONG moved up by about 47 mm, and nearby stations moved away from LONG by about 10 mm. These changes began during an extremely rainy season in southern California and coincided with a 16-m increase in water level at a nearby well in Baldwin Park and a regional uplift detected by interferometric synthetic aperture radar. No equivalent signals were seen in GPS station position time series elsewhere in southern California. Our preferred explanation, supported by the timing and by a hydrologic simulation, is deformation due to recharging of aquifers after near-record rainfall in [2004][2005]. We cannot rule out an aseismic slip event, but we consider such an event unlikely because it requires slip on multiple faults and predicts other signals that are not observed.

A new simulation optimization model for groundwater-surface water management identifies efficient strategies for meeting water demand and controlling a regional water quality problem, while accounting for varying availability of surface water supplies. Optimal values of model variables are computed for ranges of surface water availability. Expected groundwater levels are computed using a response function approach, which incorporates a probability for each hydraulic stress. Three objective functions are considered: (1) minimize need for supplemental water, (2) minimize imposed water use reductions, and (3) minimize changes from current pumping patterns. The model imposes constraints on expected groundwater levels, expected hydraulic gradients (surrogates for controlling contaminant transport), capacities of pipeline and recharge facilities, and water demand requirements. Capacity and demand constraints must be met every year for all possible surface water conditions. Application to the Santa Clara-Calleguas Basin in southern California indicates that a large quantity of supplemental water or significant pumpage reductions, particularly in the lower aquifer, may be required to control seawater intrusion with current facilities. Supplemental water would be delivered directly to users through pipelines and artificially recharged. Results also indicate that the current artificial-recharge program has been valuable and that construction of new artificial-recharge facilities might be beneficial. It should be noted that local agencies are currently developing several potential sources of supplemental water. Monte Carlo simulations yield estimates of the reliability of gradient constraints in controlling advective transport and the likelihood that water level constraints will be violated. 2845 2846 REICHARD:GROUNDWATER-SURFACE WATER MANAGEMENT In any given year there is a probability prob(j) that the actual river diversion will equal riv(j). Annual decisions on pumping, pipeline delivery, and artificial recharge will depend on the actual river diversion in that year. The approach taken to analyze this water management problem is to link a distributed-parameter groundwater simulation model with mathematical optimization techniques. Bredehoeft and Young [1970] were the first to combine groundwater simulation and optimization. Their approach involved iteration between simulation and optimization. Direct linkage of groundwater simulation and optimization was subsequently developed and advanced by Maddock [1972],Aguado and Remson [1974], and others (see review papers by Gorelick [1983] and Yeh [1992]).Several previous studies have applied deterministic simulation optimization methods to analyze regional groundwater management issues [Heidari Gharbi and Peralta, 1994]. Most of these studies focused strictly on quantity issues. Willis and Finney [1988], Lefkoff and Gorelick [1990a, b], and Gharbi and Peralta [1994] explicitly considered water quality issues. Willis and Finney linked nonlinear optimization with a sharp-interfa...

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