COLUMBIA RIVER BASALT AQUIFER CHARACTERISTICS REVEALED BY STATEMAP MAPPING IN OREGON’S UMATILLA BASIN
The primarily agricultural Umatilla Basin of northeastern Oregon is underlain by a thick sequence of Columbia River Basalt Group lava flows. Parts of the Umatilla Basin are heavily irrigated and dependent upon irrigation wells that tap aquifers in the Columbia River Basalt Group. To best understand, and model, the complex aquifers found in the Columbia River Basalt Group it is important to identify the specific members of the Columbia River Basalt Group from which groundwater is now being extracted. We use detailed mapping of Columbia River Basalt Group units exposed along the Umatilla River combined with well-established geochemical sections based on water-well cuttings and reference sections to identify and correlate unique flows and units. . Field observations indicate that the uppermost flows exposed along the Umatilla River, i.e. the Sentinel Gap and Sand Hollow units of the Wanapum Basalt Formation, and the Sentinel Bluffs unit of the Grande Ronde Basalt Formation, interacted with meteoric water, forming grussy-weathering outcrops, and, in places, palagonite breccias and pillow basalts. Preliminary correlation of exposed flows to water well sections suggests that upper units form one or more distinct aquifers that are separated from lower aquifers by the thick, areally extensive Winter Water unit of the Grande Ronde Basalt Formation. The Winter Water unit is a dense 70 m-thick, hackly-jointed flow that in one place formed a littoral cone. Flows exposed below the Winter Water unit show less evidence for interaction with water and, perhaps coincidentally, appear to host more productive aquifers.