The Spokane Aquifer Joint Board (SAJB) works cooperatively on groundwater quality protection and aquifer resource management issues, and were interested in understanding whether changes in pumping operations could benefit the river. So GSI used the high-resolution groundwater flow model that it had previously developed for the SAJB – a model that encompasses the entire footprint of the Spokane Valley-Rathdrum Prairie (SVRP) aquifer in both Washington and Idaho – to look at different pumping and recharge scenarios.

Background

The SAJB is comprised of 21 water purveyors throughout the Spokane area (including the City of Spokane). Collectively they operate more than 100 wells that pull water from the SVRP aquifer to supply drinking water to more than 500,000 people in the area. The aquifer spans approximately 370 square miles in Washington and Idaho, and is the first aquifer in the nation to have been designated as a sole source aquifer by USEPA. With 500,000 people relying on this single source of water to meet water needs and the desire to maintain the health of the Spokane River, it has become critical to understand how surface water and groundwater interact, and the effect pumping has on the river.

GSI’s Solution

The first step in understanding the groundwater/surface water interconnection was to develop a groundwater flow model. In 2012, GSI helped the SAJB create a new high-resolution groundwater flow model that encompassed the entire footprint of the SVRP aquifer. The model features include:

  • A flexible mesh that allows groundwater managers to efficiently simulate the groundwater flow and groundwater/surface water interconnection at both a regional and local scale – all in one model simulation.
  • A very high spatial resolution, containing tens – and in some locations, hundreds – of individual nodes that are only ¼-mile by ¼-mile in area. Within these nodes groundwater elevations and budget terms are calculated.
  • The aquifer is further subdivided into three separate layers to provide representation of the detailed 3-D nature of groundwater flow, which is particularly important near the Spokane River.
  • It was built to be flexible; if changes are made, the stored data are retained and extrapolated to any new nodes that are added to the model.
  • It has been calibrated to aquifer-wide groundwater level data and stream flow gain/loss estimates under two different seasonal hydrologic conditions (winter-time high river flows and low pumping; and summer-time low river flows and peak pumping).

To let groundwater managers examine how the seasonal redistribution of pumping might change the timing and magnitude of groundwater discharges to the Spokane River, the model was used to:

  • Examine specific redistribution scenarios for wells near gaining reaches in the river.
  • Compare the influences against a set of state-proposed in-stream flow standards that periodically exceed current low-flows during the summer and early fall of particularly dry years.
  • Develop groundwater quality protection programs by analyzing the areas and depths from which each well obtains water in the aquifer, and running operational scenarios to identify the portions of the aquifer contributing groundwater to each well.

Following a November 2014 presentation on the model for the Spokane River Forum, GSI and fellow presenter, Ralston Hydrologic Services, were praised by the Forum’s Executive Director, Andy Dunau:

“To the masters of geekdom, sultans of water and CDA Press pinup,
Myself and the Spokane River Forum Board wish to thank you for the presentations you made at the conference. One of the questions we always ask ourselves is ‘that beyond the good conversations, has the ball been moved toward better, more informed decision making?’ Your modeling work has clearly done just that. And the professionalism with which you shared the data and supported each other’s efforts is a model of what ‘best science”’ is supposed to look like. The attendance, interest and excitement speak for themselves.
Again, our sincerest thanks.”