Evaluating risk in the depleted Cambrian-Ordovician sandstone aquifer system, Midwestern US: what can production wells tell us about sustainability of deep bedrock aquifers?

Deep aquifers are increasingly relied upon to meet water supply needs despite minimal recharge rates, increased costs of drilling and well maintenance, and water quality issues. Deep bedrock aquifers present challenges to groundwater flow modeling due to uncertainties associated with geologic barriers (fault zones), complex stratigraphy with varying permeabilities, and the presence of multi-aquifer wells that bypass aquitards and modify flow pathways. The Cambrian-Ordovician sandstone system, a regional bedrock aquifer in the Midwestern US, has over 320 m of head decline in the suburbs of Chicago since predevelopment conditions. Unsustainable withdrawals, a lack of groundwater governance, and changing well construction practice has lead to: 1) desaturation of the upper St. Peter sandstone, 2) head separation between sandstone units, and 3) accelerated drawdown due to loss of transmissivity as units are dewatered. The extreme drawdown is now forcing a large portion of the Chicago suburbs to seek alternate supplies and build massive new water infrastructure within this decade. We present on: 1) participatory modeling of the CO-system in partnership with affected communities using MODFLOW models, 2) the assessment of risk to communities out to 2070 based on demand projections and production well behavior data (specific capacity and pumping levels), and 3) scenarios of emergency groundwater use under drought and surface water supply disruptions. Production well data gathered from stakeholders are critical for evaluating risk to individual communities and long-term water supply management in the region. We also discuss the broader implications of unsustainable withdrawals from deep bedrock aquifers over a several-century timescale.