Mosier Valley Ground-Water Sustainability Study

In Cooperation with the Wasco County Soil and Water Conservation District
and the Mosier Watershed Council

OBJECTIVES

The overall objective of the proposed study is to advance the scientific understanding of the hydrology of the basin and use that understanding to develop a set of tools that can be used to evaluate the sustainable yield of the ground-water resource.  Another objective of this project is to advance the understanding of Columbia River Basalt (CRB) aquifers.  Columbia River Basalt aquifers constitute some of the most productive aquifers in Oregon, Washington, and Idaho and have been heavily developed for agricultural, municipal, and domestic water supplies.  As a result, many other areas have experienced the effects of overdraft and are seeking means to achieve sustainable levels of ground-water development in the CRB aquifers. As part of this objective we will test and evaluate methods of characterizing CRB aquifers through advanced borehole geophysics and well testing.

Some of the key scientific questions that will be addressed include:

• What are the boundaries to the ground-water system?

• What are the hydrologic inputs and outputs to and from the ground-water system and how have they changed since development began?

• What was the nature of flow between basalt aquifers under natural conditions and how has that been affected by pumping? By co-mingling wells?

• To what extent can water-level declines be attributed to pumping? Co-mingling wells? Climatic variations?

• What methods are most effective for assessing the effects of wells that connect multiple aquifers?

The tools that will be developed to evaluate strategies for sustainability are (1) a GIS spatial database and (2) a computer simulation model of the ground-water system. The GIS will store various hydrologic, geologic, and geophysical data, and will facilitate the construction of the ground-water model and planning of sustainability strategies. The ground-water model will incorporate data from this and previous studies and represent the geologic complexity and important hydrologic processes of the ground-water flow system. The model will be used to simulate historical conditions and verify that the model adequately represents the system. The model will then be used as a tool to simulate future conditions using assumptions regarding how ground water will be managed. Results of the simulations will show how each strategy affects water levels and ground-water discharge to streams. The strategy that results in the greatest available ground water with the fewest undesirable impacts will define the sustainable yield of the system.

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