Volcanic Plateaus—Columbia River Basalts
The Columbia Plateau area of Washington, Oregon, and Idaho consists of a very thick sequence of Miocene-age basaltic lava flows that erupted from fissures. The lava was very fluid: individual flows are 150 to 500 ft (50 to 150 m) thick, and some can be traced for 125 mi (200 km). For the most part, the basalt flows are either flat lying or gently tilted. In some places, the basalts have been folded by later tectonic activity. The basalt flows of the Columbia River Group of east-central Washington are as thick as 10,000 ft (3000 m), although in most areas a thickness of 4600 ft (1400 m) is more typical (Luzier & Skrvian 1975). The basalt flows dip at angles of 1° to 2° from northwest to southeast. Water occurs in distinct zones, probably related to interflow boundaries.
A number of irrigation wells have been drilled in the uppermost 1000 ft (300 m) of the basalt flows. Typical well yields of 1000 to 2000 gal/min (60 to 120 L/s) are obtained from confined aquifers located at a depth of between 500 and 1000 ft (150 and 300 m). Shallower aquifers are not as productive, but have a hydraulic head up to 100 ft (30 m) greater than those of the deeper aquifers. Uncased well bores are draining water from the shallower aquifers into the deeper ones (Newcomb 1972). Recharge to the aquifer systems comes from precipitation and loss from ephemeral streams. Precipitation—hence, recharge— increases to the north and east. As these are also topographical high areas, regional ground-water flow is away from them, to the southwest. Age determinations based on carbon-14 dating of ground water of the basalts range from modern to as old as 32,000 years B.P. Age appears to increase with depth, but relationships are obscured by mixing of water of different ages coming from different aquifer zones (Newcomb 1972).
The classic conceptual model of a series of confined aquifers corresponding to individual basalt flows might not be valid in all parts of the Columbia Plateau. Vertical joints form in basalt as it shrinks during cooling. These can provide substantial vertical conductivity, producing unconfined aquifer conditions in thick sequences of lava flows (Foxworthy 1983). In Horse Heaven Plateau, Washington, there is reported to be an unconfined aquifer consisting of the Saddle Mountains Basalt and the upper part of the underlying Wanapum Basalt. A deeper basalt, the Grande Ronde, is confined by a saprolite layer, which formed by weathering of a basalt flow (Brown 1979).