A BI-STATE AQUIFER STUDY: MODELING THE SPOKANE VALLEY-RATHDRUM PRAIRIE AQUIFER IN WASHINGTON AND IDAHO

The Spokane Valley-Rathdrum Prairie aquifer in Spokane County, Washington, and Bonner and Kootenai Counties, Idaho, is the sole source of drinking water for more than 500,000 residents. In response to concerns about impacts resulting from recent and projected urban growth and other changes, a bi-state aquifer study was initiated in 2004 by the Idaho Department of Water Resources, the Washington Department of Ecology, and the U.S. Geological Survey. A memorandum of understanding among the three agencies established the organizational framework to accomplish a scientifically defensible hydrologic study. A key objective of the study was to develop a ground-water flow model to serve as a tool for analyzing aquifer inflows and outflows, simulating the effects of future changes in groundwater withdrawals, and evaluating aquifer management strategies. The model was constructed by a modeling team consisting of staff and personnel working under contract with the three collaborating agencies. The transient, three-layer MODFLOW model simulated flow in the aquifer and interaction between ground water and the Spokane River, and was calibrated using 1,573 measurements of water levels and 313 measurements of streamflow gains and losses. During the entire modeling development period, hydrologic data, design documents and interim versions of the model were accessible to all modeling team members via a web accessible repository. Decisions on modeling approach, methodology, assumptions, and interpretations were reached by consensus. Although this team approach is costly and time consuming, the final model has the endorsement of all team members. The model was completed in 2007 and presented to stakeholders and the public at a two-day conference, which consisted of one day of technical presentations and a public meeting, followed by a day of water-management policy discussion.