Author(s): Martin Hamper; Joseph Sobanski; Marcella Landis; Xin Song; Gaston Leone; Richard Bartelt
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Abstract: The Thornton Composite Reservoir is a 30 million cubic meter (7.9 billion gallon) storage reservoir which is part of the Metropolitan Water Reclamation District of Greater Chicago (MWRDGC) Tunnel and Reservoir Plan (TARP). The Thornton Composite Reservoir is intended to minimize waterway pollution by combined sewer overflows (CSOs) and provide an outlet for floodwater. A Groundwater Protection System (GPS) for the Thornton Composite Reservoir is required to protect against migration of stored water into the surrounding Silurian Dolomite. Proposed GPS alternatives included various configurations of a perimeter grout curtain. Groundwater modeling was performed to evaluate the effectiveness of each alternative to contain the CSO water and understand potential migration of stored water under anticipated operating conditions. The groundwater model for the Thornton Composite Reservoir was developed using the simulation software MODFLOW-SURFACT, which is functionally identical to the public-domain MODFLOW software but offers several enhancements for improved handling of unsaturated conditions and multi-layer pumping wells. These features are of particular importance for the development of the Thornton Composite Reservoir model where strong vertical gradients and re-wetting issues may otherwise lead to convergence problems with MODFLOW. The modeling analyses indicate that under all protection alternatives, groundwater migration is toward the Main Quarry Sump. As expected, a grout curtain with a permeability of 1 x 10-6 cm/s provides more containment than a grout curtain with a permeability of 1 x 10-5 cm/s. Increasing the thickness of the grout curtain is not a significant design improvement under the scenarios simulated. However, containment may be improved by moving the grout curtain further from the reservoir walls as easements allow taking advantage of the additional storage in the aquifer. Regardless, all stored water that migrates beyond the curtain is eventually collected by the Main Quarry Sump or the nearby Thorn Creek Dewatering tunnel.
Year: 2009