Background/Objectives: A three-dimensional (3D) numerical groundwater flow model was constructed for the purpose of simulating remedial design scenarios and is an important component for understanding the hydraulic effects of the various remedial design components of the Gowanus Canal Superfund Site remedy.
The model will assist in anticipating potential negative hydraulic impacts and possible modifications to remedy design. The model utilizes an extensive dataset of hydrogeologic parameters collected during the Remedial Investigation and Pre-Design phases as inputs and calibration targets.
Approach/Activities: The current groundwater model for remedial design is an adaption of an earlier model constructed during the Feasibility Study. The 3D, finite difference model was constructed using MODFLOW with Groundwater Vistas as a pre/post processor. The model was calibrated under steady state conditions to tidally averaged Canal stage (as an equivalent freshwater head) and groundwater elevations in upland monitoring wells, vertical hydraulic gradients in Canal sediments, and interpolated groundwater to surface water discharge rates from ultrasonic seepage meter and vibrating wire piezometer (VWP) data. The model was verified under transient conditions utilizing measured transient tidal stage data over a short-term period and comparing model simulated in-canal vertical gradients and groundwater elevations in upland monitoring to measured data. Once calibrated and verified, remedial design simulations were conducted to evaluate groundwater level responses and in-canal seepage velocities to: 1) removal of soft sediments through dredging, 2) in-situ soil stabilization (ISS), 3) bulkhead improvements, and 4) capping.
Results/Lessons Learned: The model was successfully calibrated to long-term estimates of canal-wide groundwater discharge providing a higher level of model calibration than simply to groundwater elevations outside the canal. Hydraulic effects of various remedial components include changes in groundwater discharge rates through canal sediments, groundwater discharge rates through canal bulkheads, changes in groundwater elevations (mounding) and hydraulic gradients outside the canal. Through the modeling exercise we were able to better understand the most sensitive remedial components to changes in discharge, degree of groundwater mounding and changes in hydraulic gradients.