A NEWLY DEVELOPED 3D HYDROGEOLOGIC MODEL OF SPRING AREAS FOR THE EAST SNAKE PLAIN AQUIFER IN SOUTHERN IDAHO BUILT ON 100 YEARS OF PREVIOUS WORK

The Eastern Snake Plain Aquifer (ESPA) is characterized by abundant spring discharge to the Snake River. The aquifer is approximately 10,000 square miles in area and is the largest aquifer in Idaho. Ancient canyon filling processes by lava flows have been identified from landmark work of Israel Russell with the U.S. Geological Survey (1902), Harold Stearns with the U.S.G.S. in the 1920s and 1930s; Howard Powers with the U.S.G.S. in the 1950s and 1960s; and expansive detailed understanding by Harold Malde with the U.S.G.S. nearly half a century ago in the 1960s and 1970s.

Since Malde, there has been little or no field work done to expand the understanding of those earlier studies. Still lacking is a detailed understanding of how canyon filling lavas of the Snake River Group (Quaternary) are deposited on the Idaho Group (Tertiary) sediment and rocks, and the relation to springs discharging from the East Snake Plain Aquifer in southern Idaho. The Q/T geologic contact forms a major hydrologic control for spring locations, flow rates, elevations and other characteristics. Typically, the underlying Tertiary age rocks have low hydraulic conductivity based on sediment composition (i.e. Clay or dense Basalts), outcrop evidence, and the absence of springs flowing from this underlying layer. The overlying Quaternary Group are typically Basalt dominated and have a high degree of fractures and conduits resulting in high hydraulic conductivity.

Groundwater velocities have been defined from 13 years of groundwater tracing which typically range from hundreds to thousands of feet per day (Farmer and Blew, 2008-2021). These two contrasting rock type characteristics combined with an ancient erosional unconformity surface, filled with more recent basalts, has created a dynamic undulatory geometry that appears to control groundwater flow paths to spring areas as well as the shape of the potentiometric water table surface.

A 3D model was produced of the spring areas and extending into the subsurface geology for several miles away from the spring areas. The Q/T Contact 3D model has provide a greater understanding of how groundwater is routed to springs and the foundation for determining the base of the ESPA in the near spring areas. The current work is still in progress and builds on previous works completed during the past century.