tainty arises regarding both the quantity and quality of ground water systems. Uncertainties related to ground water flow include insufficient or erroneous data from imprecise measurements and observations, sampling errors, or statistical errors; inappropriate model assumptions; and inadequate characterization of subsurface hydrology. Uncertainty regarding quality arises from lack of information on both the fate of the contaminants in the subsurface, and their health effects.
Additional uncertainties concern the role of ground water in providing ecological services. Ground water supports microbial habitats in the subsurface and surface flows that sustain riparian habitats. Connections between ground and surface waters are better defined in theory than in application.
Mathematical models of ground water systems have been under development for decades, but data are rarely if ever adequate to allow accurate prediction of subsurface dynamics in three dimensions. Model uncertainty stems from shortcomings in current theory or failure of models to incorporate the elements of current theory, scarcity of field data for model calibration, inadequacies of computer capacity for modeling complex systems, and failure to incorporate operational constraints into models (Anderson and Burt, 1985).
This review of hydrological concepts, ground water quality, the influence of societal activities on ground water quantity and quality, and ground water treatment scenarios suggests the following conclusions regarding implications for ground water valuation.
Decision-makers should proceed very cautiously with any actions that might lead to an irreversible situation regarding ground water use and management. Ground water depletion, for instance, may often be irreversible. Some aquifers (e.g., the southern edge of the Ogallala) do not recharge in useful time scales, and thus any extractions constitute a form of mining. In other cases the length of time needed for natural recharge of deep aquifers where ground water removal rates are high leads to a continual reduction in stock that will not be replenished in short time frames. Moreover, overdrafting can sometimes lead to a collapse of the formation permanently reducing the aquifer's storage capacity.
Decision-makers should also be cautious regarding contamination of ground water. Restoration of contaminated aquifers, even when feasible, is resource intensive and time consuming. Restoration methods are uncertain and unlikely to improve significantly in the near future. As a result, it is almost always less expensive to prevent ground water contamination than to clean up the water.
Ground water often makes significant contributions to valuable ecological services. For example, in the Southwest, many flowing streams have