MULTI-AQUIFER SIMULATIONS AND PLANNING FOR SUSTAINABLE YIELD IN THE CAMBRIAN-ORDOVICIAN SANDSTONE AQUIFERS OF NORTHERN ILLINOIS

The Cambrian-Ordovician Sandstone Aquifer system of northern Illinois, principally the uppermost St. Peter Sandstone and deeper Ironton-Galesville sandstone, has over 150 years of documented withdrawals, with drawdown exceeding 1100 feet since predevelopment in the center of the cone of depression near Joliet, IL. Most modern high capacity wells are completed in the Ironton-Galesville Sandstone, a unit receiving minimal natural leakage throughout the most heavily developed areas, though it receives considerable artificial leakage through multi-aquifer wells open to the St. Peter Sandstone.

With available head in the multi-aquifer system rapidly dwindling, there has been an urgent need to revisit sustainable yield for the region. Using the Illinois Groundwater Flow Model, a head-specified simulation was developed preserving available head in the uppermost sandstone aquifer while enforcing the same head in other sandstone aquifers to maximize leakage rate while minimizing risk to the aquifers. Where this rate exceeded a 10% reduction in natural groundwater discharge to streams, it was capped at this value. Results suggest a sustainable yield of approximately 74 MGD could be achievable across the 9-county region including Chicago and surrounding communities, encompassing most of the current risk area. However, this optimized scenario necessitates severe restrictions on the majority of current sandstone users.

Alternate scenarios are being developed with input from communities, including a more equitable reduction across the region that preserves the current use distribution, as well as scenarios accounting for areas that continue to exceed their sustainable yield. Though these favor current users, they will be more realistic and representative during a transition period for water supply alternatives. In this talk, these scenarios will be discussed along with challenges for long-term planning, including accounting for the management of multi-aquifer wells.