Mapping the Boundary of the High Plains Aquifer in Nebraska with AEM and Hydraulic Head Data

Abstract:

Throughout much of its extent, the boundary of the High Plains aquifer (HPA) corresponds closely to outcrops defining the erosional limits of the Ogallala Formation. But the northeastern boundary of the HPA in Nebraska exists entirely in the subsurface and may be juxtaposed against, cross-cut by, or overlapped with highly heterogeneous glacial sediment (including sand, gravel, clay-rich diamicton, glaciolacustrine silt, and loess). The complexity of this boundary is problematic for groundwater modeling and management planning. A decade of airborne electromagnetic (AEM) surveys in Nebraska provide new insights to the 3D architecture of hydrostratigraphic units at this boundary. We identify broad, west-east trending resistive units with flat (<1°) lower surfaces interpreted as pre-glacial valley-fill deposits. Conductive bodies overlying low-angle (2–3°) truncation surfaces with as much as 80m of relief are interpreted to be part of a regional push-moraine complex. Buried valleys, interpreted to have formed at the ice margin, are as much as 10 km wide and 60 m thick, and have variable fill successions. East-west trending valley-fills are cut by northwest-southeast trending valley-fills, and both groups of fills are truncated by glacial deposits to the east. Wedge-shaped resistive bodies abruptly juxtaposed against push-moraine features are interpreted as fluvial outwash fans. The geologic bodies observed in AEM are either bordered by, or function as, impermeable boundaries. These boundaries are verified using hydrologic evidence of hydraulic barriers or confining units, including: (1) characteristic patterns in groundwater-level hydrographs, (2) accelerated drawdowns in observation wells during pumping tests, and (3) abrupt changes in gradient and direction of the water table. The combination of AEM and hydrologic data provides a fresh look at the geologic controls on groundwater flow, the genesis of aquifers and confining units, and the hydrogeologic significance of the northeastern boundary of the HPA.