Abstract
Impermeable aquifer boundaries affect the flow of groundwater, transport of contaminants, and the drawdown of water levels in response to pumping. Hydraulic methods can detect the presence of such boundaries, but these methods are not suited for mapping complex, 3D geological bodies. Airborne electromagnetic (AEM) methods produce 3D geophysical images of the subsurface at depths relevant to most groundwater investigations. Interpreting a geophysical model requires supporting information, and hydraulic heads offer the most direct means of assessing the hydrostratigraphic function of interpreted geological units. This paper presents three examples of combined hydraulic and AEM analysis of impermeable boundaries in glacial deposits of eastern Nebraska, USA. Impermeable boundaries were detected in a long-term hydrograph from an observation well, a short-duration pumping test, and a water table map. AEM methods, including frequency-domain and time-domain AEM, successfully imaged the impermeable boundaries, providing additional details about the lateral extent of the geological bodies. Hydraulic head analysis can be used to verify the hydrostratigraphic interpretation of AEM, aid in the correlation of boundaries through areas of noisy AEM data, and inform the design of AEM surveys at local to regional scales.
Highlights
Identifying impermeable geologic boundaries within and adjacent to aquifers is a fundamental pursuit of hydrogeologic investigations
The examples given in this paper show that effective characterization of impermeable boundaries can be achieved by combining hydraulic head effective of impermeable boundaries candemonstrate be achieved by analysis characterization with Airborne electromagnetic (AEM)
Impermeable boundaries can be detected through the traditional analysis of hydraulic head data from monitoring wells, pumping tests, and water table maps
Summary
Identifying impermeable geologic boundaries (barriers) within and adjacent to aquifers is a fundamental pursuit of hydrogeologic investigations. Numerical groundwater models require accurate representation of impermeable boundaries for proper calibration and simulation of hydraulic heads. For these reasons and others, a variety of methods have been employed to detect the presence of impermeable boundaries [7] and to determine the effects of these boundaries on inter-aquifer connectivity [8]. The zone of high resistivity corresponds to an interval of very fine to coarse, gravelly sand encountered in the geologic test hole drilled at the site of the observation well. The conductive unit above the sand layer corresponds to clay-rich till overlain by loess. The conductive unit below the below the sand layer corresponds to silty clay with layers of very fine to coarse, gravelly sand. The bottom 7 m of bottom 7 m of the test hole penetrated shale bedrock
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