Improving the Hydrogeologic Conceptualization of a Remote and Arid Palaeovalley Groundwater System Using Airborne Electromagnetics and Seismic Refraction and Reflection

Abstract:

A hydrogeologic conceptualization is critical to understand, manage, protect, and sustain groundwater resources; especially in regions where data are sparse, and accessibility is difficult. Here, we used airborne electromagnetic (AEM) and shallow seismic reflection and refraction data to improve our understanding of an arid groundwater system that is influenced by palaeovalleys. In the current hydrogeologic model, it is unknown if the alluvial/colluvial fill in the palaeovalleys is connected to the underlying fractured bedrock. We focused on defining the thickness and hydrogeologic properties of saprolite, which is the layer of chemically altered rock separating the fractured rock from the palaeovalley fill. The AEM data provided an estimate of the top of saprolite but failed to effectively image the bottom particularly in the thickest parts of valley fill. In contrast, the seismic refraction and reflection data provided an estimate of the bottom of saprolite but failed to image the top. This unique geophysical combination of electrical and elastic data allowed us to map saprolite thickness in detail along a 1.7 km long transect that runs perpendicular the main trunk of a well-defined palaeovalley. We show that the palaeovalley is lined with a heterogenous layer of saprolite (5-80 m thick) that tends to be thickest near the palaeovalley edges. Despite the variability, only a small percentage of the fractured bedrock (8-17%) is in direct contact with the alluvial/colluvial palaeovalley sediments. Furthermore, the lack of an elastic boundary and the presence of a strong electrical boundary at the top of saprolite suggests that the porosity of the saprolite is similar to the palaeovalley sediments. However, the electrical change is likely caused by a pore structure, and solute content change associated with the difference of weathering insitu versus transported materials, resulting in a decrease in hydraulic conductivity in the saprolite. Our geophysical data suggest that the saprolite acts more like an aquitard and that there is limited groundwater exchange between the palaeovalley fill and the underlying fractured unweathered bedrock aquifer.