Improving Aquifer Characterization through Integration of Airborne Electromagnetics (AEM) and Well Hydrographs

The objective of this study is to evaluate methods of hydrostratigraphic modeling using geophysics and well hydrographs at the eastern edge of the High Plains aquifer (HPA) in Platte and Colfax counties within Nebraska, USA. The HPA is very heterogeneous in the study area, being hosted by architecturally complex glacial sediments and having many irregular hydraulic boundaries. Further, the HPA exhibits local variations between unconfined and confined conditions. Pumping in such bounded aquifers can be unsustainable because of cost increases and lost agricultural productivity. Moreover, the large drawdowns typical of confined aquifers can contribute to well interference during heavy pumping. Mapping the HPA accurately at small (10’s of km2 ) to medium (100’s of km2 ) scales is vital to sustainable management. AEM modeling and well hydrograph interpretation methods were used to characterize the aquifer in the study area. A 2016 airborne electromagnetic (AEM) survey mapped the electrical resistivity of subsurface strata to depths of 300 m. This data was used in the present study to create 3D hydrostratigraphic models using cognitive-layer modeling and voxel-based geostatistical modeling approaches, both with their own advantages and disadvantages. Water-level hydrographs from piezometers near irrigated fields provide the basis for aquifer characterization at each site and for assessing the accuracy of the two AEM modeling approaches, which are applied commonly in Nebraska and elsewhere. The temporal pattern of water-level drawdown indicated possible boundaries and confinement. The existence of background displacement, size of displacement, and responses of nearby wells led to aquifer interpretations. Little correlation existed between the hydrograph interpretations and both of the modeling approaches, but the voxel model did show boundaries near many of the irrigation wells with bounded hydrograph signatures. Overall, the simple modeling approaches failed to adequately convert resistivity to accurate interpretations of subsurface stratigraphy, rendering both types of hydrostratigraphic models largely invalid here. Nevertheless, the results of this study lead to important future work recommendations: (1) modeling and quantifying uncertainty using more sophisticated methods, (2) applying different modeling approaches in different areas to fit hydrologic data, and (3) using hydrograph data and pumping tests to validate the results of hydrostratigraphic modeling. Advisor: Jesse Koru

Improving Aquifer Characterization through Integration of Airborne Electromagnetics (AEM) and Well Hydrographs

The objective of this study is to evaluate methods of hydrostratigraphic modeling using geophysics and well hydrographs at the eastern edge of the High Plains aquifer (HPA) in Platte and Colfax counties within Nebraska, USA. The HPA is very heterogeneous in the study area, being hosted by architecturally complex glacial sediments and having many irregular hydraulic boundaries. Further, the HPA exhibits local variations between unconfined and confined conditions. Pumping in such bounded aquifers can be unsustainable because of cost increases and lost agricultural productivity. Moreover, the large drawdowns typical of confined aquifers can contribute to well interference during heavy pumping. Mapping the HPA accurately at small (10’s of km2 ) to medium (100’s of km2 ) scales is vital to sustainable management. AEM modeling and well hydrograph interpretation methods were used to characterize the aquifer in the study area. A 2016 airborne electromagnetic (AEM) survey mapped the electrical resistivity of subsurface strata to depths of 300 m. This data was used in the present study to create 3D hydrostratigraphic models using cognitive-layer modeling and voxel-based geostatistical modeling approaches, both with their own advantages and disadvantages. Water-level hydrographs from piezometers near irrigated fields provide the basis for aquifer characterization at each site and for assessing the accuracy of the two AEM modeling approaches, which are applied commonly in Nebraska and elsewhere. The temporal pattern of water-level drawdown indicated possible boundaries and confinement. The existence of background displacement, size of displacement, and responses of nearby wells led to aquifer interpretations. Little correlation existed between the hydrograph interpretations and both of the modeling approaches, but the voxel model did show boundaries near many of the irrigation wells with bounded hydrograph signatures. Overall, the simple modeling approaches failed to adequately convert resistivity to accurate interpretations of subsurface stratigraphy, rendering both types of hydrostratigraphic models largely invalid here. Nevertheless, the results of this study lead to important future work recommendations: (1) modeling and quantifying uncertainty using more sophisticated methods, (2) applying different modeling approaches in different areas to fit hydrologic data, and (3) using hydrograph data and pumping tests to validate the results of hydrostratigraphic modeling. Advisor: Jesse Koru

Airborne electromagnetics supporting salinity and natural resource management decisions at the field scale in Australia

Airborne geophysics has been used at the catchment scale to map salt stores, conduits and soil variability, but few studies have evaluated its usefulness as a land management tool at the field scale. We respond to questions posed by land managers with: (1) comparison of airborne and ground-based electromagnetic surveys in the Lower Balonne catchment, Queensland, and (2) comparison with historical and anecdotal knowledge of landscape response in the country around Jamestown in mid-South Australia. In the Lower Balonne, direct comparison between ground electromagnetic survey (EM) and airborne electromagnetics (AEM) showed a strong relationship for both the absolute values and spatial patterns of conductivity. The penetration of AEM to greater than 100 m is valuable in defining hydrological barriers. In the Jamestown area, AEM conductivity corresponded well with specific outbreaks of salinity and observed variability in crop response; local inconsistencies at the ground surface could be resolved when sub-surface data were considered. AEM can provide valuable information at the field scale that is relevant to salinity management. Farmers can have confidence in any of these techniques (historical information, EM and AEM) and they may directly compare or integrate the results. (c) 2006 Elsevier B.V. All rights reserved

The saline Interface of a Shallow Unconfined Aquifer, Rangitikei Delta

The coastal communities of Tangimoana and Scott's Ferry have a long history of using shallow groundwater bores. The cumulative effect of pumping over decades could influence the saline interface given the close proximity of the communities to the seashore and river estuary. It is important to quantify the effects of pumping on both the shallow groundwater system and the dynamics of the saline interface. This is necessary to protect the groundwater system against saline intrusion especially given the increasing number of high volume groundwater consents to support dairying. Resistivity soundings and traverses, coupled with chemical analyses of groundwater samples, were found to be an effective method for defining the saline interface of the shallow groundwater aquifer under the Rangitikei delta. The saline interface extends from the salt marsh to beneath the farmland north of Tangimoana. The interface is a zone of diffusion with freshwater and brackish water mixing from the estuary. The interface is currently located on the outskirts of Tangimoana, and it is likely to extend beneath the township. The infiltration of brackish surface waters into sediments of the salt marsh form a surficial mixing zone that decreases with distance from the salt marsh. There is no indication of salinity in the area to the north of the Rangitikei delta. This area is most at risk of contamination from saline intrusion because of high volume groundwater abstractions, even though these abstractions are from deeper aquifers. The shallow groundwater beneath Tangimoana showed high concentrations of Ca and HCO3 ions. This may be a result of carbonate dissolution, which can occur when saline and freshwater mix. This creates groundwater that is under-saturated with calcium. The mixing water dissolves carbonates and increases the concentrations of Ca and HCO3. The major source of sodium and chloride was likely rainwater with evaporated solutes from seawater. The saline interface near Tangimoana appears to be relatively static, but the estuary and salt marsh are areas of low relief. There are preferential flows paths across the salt marsh to the farmland. These factors make the shallow groundwater in the Rangitikei delta vulnerable to saline intrusion