1 Uncertainty of estimates of groundwater yield for the Cambrian-Ordovician Aquifer in northeastern Illinois

The aquifers underlying northeastern Illinois and southeastern Wisconsin are an important water resource for a rapidly growing region. These aquifers include the sandstone and dolomite formations collectively known as the Cambrian-Ordovician Aquifer. A series of studies conducted in the 1950’s and 1960’s determined that the best estimate of the ‘maximum practical sustained yield ’ of this aquifer in northeastern Illinois is 65 million gallons per day (mgd). However, this estimate is based on inferred conceptual models and parameters whose uncertainties may affect the estimate of the practical sustained yield. Simple bounding estimates suggest that the practical sustained yield of this aquifer in northeastern Illinois could range from 28 to 180 mgd as a consequence of parameter and conceptual uncertainties. The analysis also suggests revising the current approach to estimating groundwater availability