Unmanaged Aquifer Recharge: Revisiting Sustainable Yield in Northeast Illinois

Water supply planners often use estimates of sustainable yield of an aquifer to make decisions on future best management practices. The influence of wells with long open intervals on the Cambrian-Ordovician sandstones has long been documented, though early estimates of sustainable yield did not separate components of flow beyond water entering from natural recharge sources and sources internal to the system. Calibrating a transient head-specified MODFLOW model for northern Illinois to documented pumping rates yields improved estimates for the sustainable flow from recharge sources, as well as the individual contributions from both shallow aquifers and the Mt. Simon. Preliminary results indicate that before World War II, much of the water withdrawn from sandstone originated from shallow aquifers via wells uncased through bedrock, primarily the Silurian dolomite. The rate of this artificial recharge appears to decline in the post-war period, coinciding with wells being sealed in Chicago. Simultaneously, declining heads in the sandstone aquifers induce dewatering of the overlying Galena-Platteville dolomite and uppermost St. Peter Sandstone, with water removed from storage reaching a maximum with the peak of pumpage in 1979. Communities began switching to Lake Michigan water and head recovery occurred between 1980 and 2000; concurrently, rapid growth of subdivisions added a new source of artificial recharge in the form of private wells bypassing the Maquoketa Shale. Model simulations indicate that sandstone groundwater usage exceeds flow from outside northeast Illinois by about 40%, the remainder satisfied by water removed from storage and artificial recharge via wells with long open intervals. There is some question whether this artificial recharge can be considered sustainable, owing both to the transient nature of these wells and the removal of water from overlying aquifers. Regardless, a sustainable rate of water use has not been achieved as heads continue to decline in the region.