GROUNDWATER DEPLETION ALONG A REGIONAL FAULT ZONE IN NORTHERN ILLINOIS: IMPLICATIONS FOR NEAR AND LONG-TERM WATER QUALITY AND WATER SUPPLY

Large-scale withdrawals from the Cambrian-Ordovician sandstone system, a deep regional bedrock aquifer in the Midwestern US, has caused over 300 m of head decline in the suburbs of Chicago since predevelopment conditions. This extreme drawdown is centered along the Sandwich Fault Zone (SFZ), a 137-km long and up to 3-km wide high-angle fault zone that is poorly characterized in the subsurface. There is minimal offset of sandstone units in the areas with the largest groundwater demands, yet the SFZ behaves as a regional barrier to groundwater flow and limits recharge to the system. Continued groundwater depletion is now forcing numerous communities near the SFZ to seek alternate water supplies and build massive new water infrastructure within this decade. We present on 1) the results of a large-scale synoptic measurement of sandstone water levels in 2021 that includes the SFZ and 2) results of a geochemical and isotopic assessment of sandstone wells within and along the SFZ to determine vertical flow pathways. Understanding the flow behavior of the SFZ is critical for assessing risk to sandstone wells and for community water supply planning in Illinois. We also discuss the broader implications of unsustainable withdrawals from deep bedrock aquifers near a fault zone and the lessons that can be applied to other regional aquifer systems.