AN UPDATED GEOLOGIC MAP TO IDENTIFY GROUNDWATER FLOW PATTERNS IN THE CENOZOIC VOLCANIC ROCKS OF THE PACIFIC NORTHWEST VOLCANIC AQUIFER SYSTEM STUDY AREA

A new four-part thematic geologic map for much of the United States Northwest volcanic province allows assessment of geologic controls on regional groundwater flow and geothermal heat flow. Earlier efforts have characterized groundwater systems in the Columbia Plateau, Snake River Plain, and most basins adjacent to the Cascade Range, but regional groundwater in the remainder of the province has not been systematically studied.. This understudied terrane, the Northwest Volcanic Aquifer Study Area (NVASA), is a region of high geothermal energy potential. Previous studies have established a correlation between age of volcanic rocks and permeability and between volcanic-rock age/composition and geothermal heat flow. The new thematic categorization of the NVASA allows examination of geologic controls on both heat and groundwater flow for parts of the state digital compilations of Oregon, California, Idaho, Nevada, Utah, and Washington (Ludington and others, 2005, USGS Open-File Report 2005-1305, 1:500,000–1:750,000). The four thematic categories are designed to identify similar rocks and deposits within the NVASA by (1) age, (2) composition, (3) inferred hydrogeologic grouping, and (4) lithologic pattern. Comparing the new categories with available hydrologic data shows that younger volcanogenic terranes (<2.58 Ma) tend to have higher primary permeability than older terranes, that reduced primary permeability with age is attributable to weathering and hydrothermal alteration of volcanogenic deposits, and that secondary permeability by open fractures results from dilation tendency of Quaternary faults. Overlaying geology and structure with groundwater flow rates and paths renders the NVASA into three broad hydrogeologic regions with differing permeability and flow rates of springs. Because primary permeability is tied to hydrothermal alteration of volcanic deposits, primary permeability diminishes rapidly with depth. Where temperatures are high enough for the generation of electricity (>150 °C), primary permeability is minimal, making regions with sufficient secondary permeability more likely geothermal energy targets.