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Sub-Surface Mapping & Aquifer Assessment
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CRBG Aquifer System and Lake Roosevelt

Lake Roosevelt, the reservoir impounded by Grand Coulee Dam, lies on the northern edge of the Columbia Basin and the basalt aquifer system hosted by the Columbia River Basalt.  This aquifer system characteristically consists of a series of individual water-bearing zones, or aquifers, that are confined between the widespread layered basalts that comprise the Columbia River basalt.  Each of these layers, which are stacked one a top the other, typically range from 50 to 150 feet thick, and the tops and bottoms of them consist of rubbly to vesicular rock that can host groundwater.  The zones between each basalt flow layer are known as interflow zones and they consist of the bottom few feet of one basalt flow layer and the upper few feet of the underlying layer. Figure 2 shows the extent and elevation of the top of the deeper aquifer system in GWMA, the Grande Ronde system.

Groundwater within this layered basalt system moves down slope (down dip) away from recharge areas.  Recharge generally occurs where an interflow zone is close to the ground surface and in contact with surface water, such as a stream, river, lake, or reservoir, or in high precipitation areas.  Within the Columbia Basin, high precipitation areas usually are at higher elevations around the edge of the Basin or on high ridges within the basin (where precipitation is significantly higher than in the low land central basin).  Surface water, where present, usually is found in the deep coulees and in areas where basalt interflow zones are exposed in major river canyons.

In and near the GWMA, the largest body of surface water is Lake Roosevelt.  Given that, the GWMA team looked at the physical relationship between Lake Roosevelt and the basalt aquifer system to evaluate the potential for recharge of GWMA basalt aquifers by the reservoir.  This was done using a combination of subsurface geologic maps produced through the subsurface geologic mapping effort and water level data previously collected by the Department of Ecology as part of their long term aquifer monitoring program in the Columbia Basin.  Based on this information, GWMA’s basic conclusion is that Lake Roosevelt is not a source of significant, if any, recharge to the Columbia River basalt aquifer system. 

Aquifer hosting interflow zones in the basalt aquifer system, with only one exception within the GMWA, simply do not come into physical contact with Lake Roosevelt.  Everywhere along the length of the reservoir within GWMA, basalt almost everywhere is found several hundred feet above the reservoir pool.  In fact, the basalt lies atop granite and other older pre-basalt rocks (basement) that are very impermeable.  This basement rock physically separates the basalt aquifer system from the water found in Lake Roosevelt.  There is no way for water within the reservoir to go up hill across the basement rocks above the reservoir and into the basalt interflow zones found along the crest of the ridges above the Reservoir.  Figures 3 and 4 illustrate this physical relationship.

This observation is confirmed by water level data in a a number of wells where Ecology has collected water level data over the past 20 to 30 years.  Water levels in these wells are several hundred feet higher than the pool elevation.  If the primary recharge mechanism for the basalt aquifer system being measured by these wells was the reservoir, water levels in them would be at or below the reservoir pool elevation.  This tells us that the primary influence for water level in these wells is related to recharge at elevations higher than the reservoir pool.  Figure 3 shows this relationship.

 

Figure 3 also shows the presence of a basement ridge buried beneath the basalt and separating the basalt aquifer system from the reservoir pool.  This ridge, which our mapping shows extends the full length of Lake Roosevelt on the northern edge of GWMA provides a further physical barrier to recharge of the basalt aquifer system by the reservoir. 

The one exception to this is at Hawk Creek north of Davenport, Washington.  In this area the basalt system does come into physical contact with the reservoir, and local basalt water wells suggests local recharge of basalt by the reservoir.  However, GWMA’s subsurface basalt mapping again shows that the buried basement surface may be highe enough south of the reservoir to greatly restrict, if not completely block, the southward movement of reservoir recharge through the basalt and into the regional basalt aquifer system.



Last Updated ( Thursday, 19 March 2009 )
 
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