Uncertainties in Estimating Groundwater Availability
Uncertainty of estimates of groundwater yield for the Cambrian-Ordovician Aquifer in northeastern Illinois (490k pdf), D.D. Walker, S.C. Meyer, and D. Winstanley, in: Proceedings of Probabilistic Approaches and Groundwater Modeling, Am. Soc. Civil Eng., Env. and Water Resour. Inst. Symposium, Philadelphia.
The population of northeastern Illinois is predicted to increase 15 percent by the year 2020, which is expected to increase the demand for water in the region. Limitations on water withdrawals from Lake Michigan and declining groundwater levels have prompted regional planning agencies and managers to ask the Illinois State Water Survey (ISWS) for estimates of the amount of groundwater available from the regional aquifers (NIPC, 2001). But simple, single-valued estimates of groundwater availability often are accompanied by assumptions and uncertainties that limit their usefulness, although these assumptions and uncertainties may not be explicitly stated. To illustrate the limitations of such estimates, the ISWS (Walker, et al. 2003) has examined the estimates of groundwater availability from the series of sandstone and dolomite formations underlying northeastern Illinois and southeastern Wisconsin, described by Suter et al. (1959) as the Cambrian-Ordovician Aquifer.
The availability of groundwater from the Cambrian-Ordovician Aquifer has been expressed as its practical sustained yield (PSY), defined as "the maximum amount of water that can be continuously withdrawn by a pumping array without exceeding recharge or causing water levels to decline below critical levels" (Suter et al., 1959, Zeizel et al., 1962). A series of studies conducted in the 1950's and 1960's determined that the best estimate of the PSY of this aquifer in northeastern Illinois for the 1958 configuration of pumping centers was 46 million gallons per day (mgd) (Suter et al., 1959). Walton (1964) estimated that maximum PSY with an ideal pumping configuration is 65 million gallons per day mgd.
In this paper, a simple scoping analysis using published estimates for parameters and recharge areas suggests that maximum PSY may lay between 28 and 180 mgd. This level of uncertainty could affect regional planning because withdrawals from the Cambrian-Ordovician Aquifer are a large proportion of the water supply for the six collar counties surrounding Chicago (NIPC (2001) estimates show that approximately 41% of all groundwater for noncooling use in these counties came from this aquifer).
In addition to conceptual and parameter uncertainties, estimates of groundwater yield assume certain economic, legal, social, and climatic conditions, often implicitly. These conditions constrain well field design, land use, and other factors that indirectly limit the feasibility of groundwater development. As a consequence, changes in economic, legal, social, and climatic conditions can be expected to change the estimated yield.
Decisions to withdraw water from aquifers are based on consideration of many factors including science, engineering, economics, societal values, and politics. In the future, the ISWS will move away from providing single-value estimates such as the PSY and toward analyses of management alternatives that clearly state their assumptions, reflect parameter uncertainties, and evaluate the impacts on the surrounding environment. While such analyses will be more complex than single-value estimates, they will provide a richer basis for resource managers to use in making decisions about water withdrawals.