Lower Elkhorn NRD - Hydrogeologic Framework Development Using AEM and Geologic Log Data for a Groundwater Model

Staff at LRE Water, in partnership with JEO Consulting Group and Long Spring Consulting, have been working with the Lower Elkhorn Natural Resources District (LENRD) since 2018 on a study that incorporates borehole lithology data with Airborne Electromagnetic (AEM) data to develop a refined conceptual model and numerical groundwater flow model.  The LENRD covers approximately 4,000 square miles. An initial pilot study was conducted as a proof of concept for the workflow and focused on a 300 square-mile area and included parts of four counties in northeastern Nebraska. Upon approval from project stakeholders, the project team proceeded in implementing the workflow to the entire NRD.

The objective of the study was to develop the hydrogeologic framework utilizing AEM results and geologic logs data, and construct a groundwater flow model to assist the NRD with water management decisions. AEM surveys characterize electrical properties of earth materials from the land surface downward using electromagnetic induction. Upon review of the AEM data, it was apparent that defining stratigraphic contacts from AEM data alone was difficult; therefore, borehole data needed to be used to define specific geologic surfaces or layers. The project team concluded that the borehole data was best suited to define the stratigraphy, particularly the top of bedrock surface, and to use the AEM data to define or constrain the hydrostratigraphy. Aqua Geo Frameworks (who flew and collected the AEM data) identified correlations between the AEM resistivity data and hydraulic conductivity.  Therefore, the AEM data was processed using Leapfrog (3D geologic modeling software) to define 3D solids of resistivity zones. The resistivity zones were evaluated against a 3D numerical groundwater flow model grid to define hydraulic conductivity values (or a range of values), and exported to Groundwater Vistas.

Due to the scope and geographic extent of the project, resulting analyses, and data requirements of LENRD and the Nebraska DNR, ArcGIS and Leapfrog was used to compile, manipulate, interpolate, and interpret the large geographic and geologic datasets.  Comparisons were made between AEM-only, borehole-only, and combined AEM/borehole datasets. A final deliverable was provided to LENRD and NeDNR in ESRI geodatabase format. This project illustrated that the workflow captures the benefits of both borehole data and AEM data when building a regional groundwater flow model.