Using Production Well Behavior to Evaluate Risk in the Depleted Cambrian‐Ordovician Sandstone Aquifer System, Midwestern USA

As shallow aquifers become depleted or contaminated worldwide, use of deep aquifers will likely increase to meet growing water demands despite expensive drilling costs and well maintenance, and limited recharge to many of these deep systems. We discuss depletion of the Cambrian‐Ordovician sandstone aquifer, a deep bedrock aquifer that has been a major source of water in the Midwestern US for over 150 years. Using a participatory groundwater modeling approach with active stakeholder engagement, we developed a conceptual framework to assess risk to different sandstone units of the aquifer based on simulated heads and production well behavior (pumping levels and specific capacities). Over 320 m of static head decline has occurred since predevelopment in the deepest sandstone unit. Shallower units are actively being dewatered. Drawdown and head separation between sandstone units is even greater under pumping conditions and is most acute near a regional fault zone. Current depletion is prompting massive infrastructure changes to mitigate anticipated water supply shortages. Our results show that even with a substantial reduction in water use by 35% in 2030, many areas of the aquifer remain at High or Severe risk out to 2070. Head decline, as opposed to storage loss, is a critical metric for understanding depletion of deep bedrock aquifers. The participatory modeling process made clear that pumping levels at production wells should not be ignored in evaluations of aquifer sustainability and risk. Our conceptual risk framework could be applied to other deep bedrock aquifers undergoing depletion. Key Points Over 320 m of head decline has occurred in the deep sandstone aquifer system underlying the United States’ third largest metropolitan area Participatory modeling allows assessment of risk to groundwater supplies under pumping conditions Assessment of deep aquifer sustainability requires understanding of multi‐aquifer wells, geologic barriers, and production well behavior