Spokane Valley-Rathdrum Prairie Hydrologic Project

The Spokane Valley-Rathdrum Prairie Hydrologic Project was initiated to develop a better understanding of water resources in the Spokane Valley-Rathdrum Prairie and to evaluate changes in regional and local ground water conditions and their interaction with surface water.

Introduction

The Spokane Valley-Rathdrum Prairie (SVRP) aquifer supplies water to over 400,000 residents in Spokane County, Washington, and Kootenai County, Idaho. The area includes the rapidly growing cities of Spokane, Washington and Coeur d'Alene and Post Falls, Idaho. The aquifer consists of thick deposits of coarse sediment, and was designated a "Sole Source Aquifer" by the Environmental Protection Agency in 1978 in response to local concerns about aquifer vulnerability to water quality degradation. Urban growth and increasing ground-water withdrawals and surface- water appropriations have raised concerns about water availability in the SVRP aquifer and declining streamflows in the Spokane and Little Spokane Rivers which may periodically receive water from the aquifer.

Water management of the SVRP aquifer occurs at federal, state, and local levels. The states of Washington and Idaho have primary responsibility for water allocation and water quality, however, local governments are increasingly being called upon to consider water supply and quality implications in land use planning. The potential influence of the SVRP aquifer on surface- water flows and quality further complicate aquifer management.

In order to deal with these issues, regional chambers of commerce and others prepared appropriations requests for Congress, which were granted in part in late 2003, with the appropriation of $500,000 to the U.S. Geological Survey (USGS) in consultation with the states of Washington and Idaho to accomplish the project to provide the tools that the states needed to address the hydrologic issues at hand for management purposes.

Purpose and Objectives

The purpose of this project is to provide a scientific foundation for management of the SVRP aquifer. The study will culminate in the development of a numerical ground-water model that Washington and Idaho can use to cooperatively manage the SVRP aquifer and adjacent rivers and lakes.

Specific objectives include the following:

1. Aquifer Characterization - Delineate the extent and thickness of the SVRP aquifer, determine aquifer boundary conditions, and document the spatial distribution of aquifer hydraulic properties.
2. Hydrologic Data Collection - Measure ground-water levels and streamflows, and quantify recharge rates to the aquifer from precipitation and evapotranspiration losses.
3. Water Use - Quantify the spatial and temporal distribution of aquifer and river withdrawals for municipal, commercial, industrial, domestic, and irrigation usage.
4. Water Quality - Define current ground and surface water quality characteristics.
5. Ground-Water Surface-Water Interactions - Delineate gaining and losing reaches of the Spokane and Little Spokane Rivers, and quantify the seasonal exchange of ground and surface water. Quantify losses from lakes that are in hydraulic connection with the aquifer.
6. Numerical Model - Construct a numerical model representing the current understanding of aquifer flow characteristics. Calibrate the model to steady-state and transient hydraulic conditions on the basis of current and historical data using automated parameter estimation methods that enable quantification of parameter uncertainty.
7. Model Application - Use the numerical model to (a) analyze aquifer inflows and outflows, (b) analyze alternative flow characteristic conceptualizations, (c) simulate responses to unit aquifer withdrawals, (d) simulate aggregate effects of future increased aquifer withdrawals, and (e) evaluate selected aquifer management scenarios.
8. Public Involvement - Convey information about regional water resources and knowledge gained from the study to federal, state and local decision-makers, the professional community, and the general public through published reports and public meetings.

Estimated Total Project Cost — $3,500,000

Memorandum of Understanding

Management of the SVRP aquifer is complicated because of the interstate, multi-jurisdictional responsibilities for the aquifer. The states of Washington and Idaho have primary responsibility for water allocation and water quality within their respective jursidictions. However, local governments are increasingly being called upon to consider water supply and quality implications in land use planning. Aquifer management is also complicated by the interconnection between ground water and surface water in the study area. Surface water flow rates and surface-water quality are influenced by ground water and vise versa.

The purpose of the Memorandum of Understanding is to establish the organizational framework to accomplish a scientifically defensible hydrologic study of the SVRP aquifer and to provide accountability. The study will provide the states with the data, the empirical models and the technical analysis they need to assist in water administration.

Primary areas covered by the Memorandum of Understanding include: