GROUND WATER FLOW MODEL OF BAINBRIDGE ISLAND, KITSAP COUNTY, WASHINGTON

The U.S. Geological Survey Washington Water Science Center, in cooperation with the City of Bainbridge Island, is developing a numerical model of ground-water flow for Bainbridge Island. Local stakeholders have raised concerns that residential development and increased domestic water usage might lower the water table of Bainbridge Island, and lead to seawater intrusion. This project focuses on the effects of groundwater withdrawals on the Island’s aquifers.

Data collection for the project included establishing a groundwater and surface-water monitoring network. The network allowed for determining base-flow stream discharge in 20 streams and monitoring monthly or continuous water levels in more than 70 domestic and unused wells. Drillers’ log data were interpreted and paired with a digital elevation model derived from LiDAR data to construct a hydrogeologic framework for the model.

The numerical flow model SEAWAT, which combines MODFLOW and MT3D, was used to simulate variable density groundwater flow. The model includes well withdrawals from domestic and public-supply systems, recharge modeled using the Deep Percolation Model and augmented by septic-return flows, stream and lake boundary conditions, and submarine discharge to Puget Sound. The model was calibrated to monthly transient conditions using the water levels collected as part of the monitoring network and parallel parameter estimation (PPEST). The model currently is being used to simulate hydrologic conditions that are expected to exist in the year 2035 under different management scenarios.