Identifying recharge under subtle ephemeral features in a flat-lying semi-arid region using a combined geophysical approach

Brady A Flinchum,Michael Hatch,Eddie Banks,Okke Batelaan,Luk J M Peeters,Sylvain Pasquet

doi:10.5194/hess-24-4353-2020

Abstract

Abstract. Identifying and quantifying recharge processes linked to ephemeral surface water features is challenging due to their episodic nature. We use a combination of well-established near-surface geophysical methods to provide evidence of a surface and groundwater connection under a small ephemeral recharge feature in a flat, semi-arid region near Adelaide, Australia. We use a seismic survey to obtain P-wave velocity through travel-time tomography and S-wave velocity through the multichannel analysis of surface waves. The ratios between P-wave and S-wave velocities are used to calculate Poisson's ratio, which allow us to infer the position of the water table. Separate geophysical surveys were used to obtain electrical conductivity measurements from time-domain electromagnetics and water contents from downhole nuclear magnetic resonance. The geophysical observations provide evidence to support a groundwater mound underneath a subtle ephemeral surface water feature. Our results suggest that recharge is localized and that small-scale ephemeral features may play an important role in groundwater recharge. Furthermore, we show that a combined geophysical approach can provide a perspective that helps shape the hydrogeological conceptualization of a semi-arid region.

Highlights

Understanding groundwater recharge mechanisms and surface-water–groundwater connectivity is crucial for sustainable groundwater management (Banks et al, 2011; Brunner et al, 2009)
In April 2018, the shallow drill hole was logged with a downhole nuclear magnetic resonance (NMR) system (Vista Clara Dart) and the water level was measured by hand
We argued for the existence of a 3 m thick partially saturated region above the water table based on water contents from the NMR data (Figure 5c)

Summary

Introduction

Understanding groundwater recharge mechanisms and surface-water–groundwater connectivity is crucial for sustainable groundwater management (Banks et al, 2011; Brunner et al, 2009). Many aquifers in semi-arid areas receive a significant portion of their recharge from adjacent mountain ranges (Bresciani et al, 2018; Earman et al, 2006; Wilson and Guan, 2004; Winograd et al, 1998). In this common scenario, recharge can occur via groundwater flow from the mountain range directly into the aquifer – implying a significant lateral groundwater connection with the adjacent mountain range (Markovich et al, 2019). Precipitation from the mountain range flows out and across the semi-arid basin as surface water and recharges the aquifer via river infiltration processes – implying a vertical connection between surface and groundwater (Bresciani et al, 2018; Brunner et al, 2009; Winter et al, 1998)

Objectives

Methods

Results

Discussion

Conclusion