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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5045

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Introduction

Surface water in the Yakima River Basin, in south-central Washington (fig. 1) is under adjudication and the amount of surface water available for appropriation is unknown, but there are increasing demands for water for municipal, fisheries, agricultural, industrial, and recreational uses. These demands must be met by ground-water withdrawals and/or by changes in the way water resources are allocated and used. On-going activities in the basin for enhancement of fisheries and obtaining additional water for agriculture may be affected by ground-water withdrawals and by rules implemented under the Endangered Species Act for salmonids that have been either listed or were proposed for listing in the late 1990s. An integrated understanding of the ground-water flow system and its relation to the surface-water resources is needed in order to implement most water-resources management strategies in the basin. In order to obtain this understanding, a study of the Yakima River Basin aquifer system began in June 2000. The study is a cooperative effort of the U.S. Geological Survey (USGS), Bureau of Reclamation (Reclamation), the Yakama Nation (YN), and the Washington State Department of Ecology (WaDOE).

The overall objectives of the study are to fully describe the ground-water flow system and its interaction with and relation to surface water, and to provide baseline information for a management tool—a numerical model. The conceptual model of the flow system and the results of the study will be used to guide and support actions taken by management agencies with respect to ground-water availability and to provide information to other stakeholders and interested parties. The numerical model will be developed as an integrated tool to assess short-term to long-term management activities, including the testing of potential management strategies.

The study includes three phases. The first phase includes (1) project planning and coordination, (2) compiling, documenting, and assessing available data, and (3) initial data collection. The second phase consists of data collection to support the following phase 2 work elements: (1) mapping of hydrogeologic units, (2) estimating ground-water pumpage, (3) developing estimates of ground-water recharge, (4) assessing ground water-surface water interchanges, and (5) constructing maps of ground-water levels. Together, these five elements provide the information needed to describe the ground-water flow system, the conceptual model, and provide the building blocks for the hydrogeologic framework. In the third phase, six structural basin models and one regional model of the ground-water flow system will be constructed in order to integrate the available information. The numerical models will be used to gain a further understanding of the flow system and its relation to surface water, and to test management strategies. The results from selected work elements will be described in a series of reports. This report defines the hydrogeologic framework of the three major basalt units, and the two major interbed units as described under the phase 2 work element, mapping of the hydrogeologic units.

Purpose and Scope

This report describes the depth to top of selected basalt and interbed hydrogeologic units that compose part of the Yakima River Basin aquifer system. This and other study information will be used to develop a conceptual model describing local and regional ground-water flow systems. Maps presented in this report provide the extents and the depth to the top of three basalt and two interbed hydrogeologic units within the study area. The three basalt units are nearly equivalent to the Grande Ronde, Wanapum, and Saddle Mountains Formations of the Columbia River Basalt Group (CRBG). The two major interbed units are between the Grande Ronde and Wanapum Formations and the Wanapum and Saddle Mountains Formations.

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