Groundwater salinity mapping using airborne electromagnetics and borehole data within the lower Balonne catchment, Queensland, Australia

Abstract

Salinity can be a major constraint on the potential use of groundwater. Airborne electromagnetics (AEM) was used to map potential groundwater salinity in the Lower Balonne catchment in Queensland, Australia. These maps identified areas for further investigation for groundwater resource development. Three methods were used:

• Estimation of unknown geophysical parameters using measured field and laboratory parameters, thence estimation of groundwater EC inversely from the AEM bulk conductivity in the saturated zone—this required a priori knowledge of the water table configuration and three-dimensional geology, and a poor fit was obtained between model predictions and observed values of conductivity, with high non-random residuals;

• Establishment of a statistical relationship between groundwater salinity at a known depth and the AEM signal at the same depth—the resulting regression model was applied to the grid-based AEM data to produce groundwater salinity maps. The AEM signal explained approximately 65% of the variance in groundwater salinity, with the remainder probably due to variations in porosity, matrix conductivity and other factors;

• AEM data were coupled with expert interpretation of conductivity logs to map class boundaries of groundwater salinity. The groundwater EC class boundaries derived from the interpretive approach provided a better basis to target subsequent exploration drilling, clearly defining the extent of the groundwater resource.

A resistive lobe depicted by the AEM most likely defines a recharge zone generated by leakage from the Maranoa River. This investigation has provided confidence for further groundwater resource development and has underpinned the expansion of irrigated agriculture in the region.

Introduction

We describe recent work in the lower Balonne catchment of southern Queensland to map groundwater resources by predicting groundwater salinity from calibrated depth images of bulk conductivity derived from airborne electromagnetics (AEM). The flight area covers the floodplain of the Balonne and Maranoa Rivers which merge near St. George to form part of the Lower Balonne River (Fig. 1). Over the western third of the flight area, the modern fluvial system overlies alluvium deposited within the Dirranbandi Trough (Senior, 1970, Exon, 1976, Pain, 2004). Over the eastern part, Quaternary and Tertiary sediments of variable thickness unconformably overly deeply weathered basement of the Cretaceous Griman Creek Formation; the marginal marine zone of this formation is present beneath the entire study area (Fig. 2).