GRSG Conference 2022: Orbit to Outcrop
Title: ASTER mineral mapping for groundwater discharge investigations of springs in the Canning Basin, northern Western Australia.
Author: Robert Hewson
In remote regions of arid and semi-arid Australia, perennial springs are scarce and are generally fed by groundwater. However, such springs and their associated groundwater sources, may also be an important resource for the local pastoral/agricultural industry, as well as the mining industry. As well as the issues of these competing interests, climate change variations and their associated effects on groundwater recharge, show the importance of studying and quantifying the extent of such resources, and their possible changes over time.
Such challenges exist in this study area of the Walyarta Conservation Park (WCP), located in the West Canning Basin (WCB), of semi-arid north-western Australia. The WCP contains small, heritage listed perennial organic springs that support threatened ecological communities (English et. al. 2016). The imperative to know more about water resources feeding the springs is high as the extensive groundwater resources of the WCB are fully allocated for agricultural and mining purposes.
Generally, springs, are investigated through traditional hydrogeological studies (e.g. borehole hydraulics and hydrochemistry). However, these methods don’t provide information on spring persistence, and where drilling is sparse, the interpolation of results is difficult. In the WCB/WCP, this gap was bridged by using airborne geophysical data (electromagnetics TEMPEST AEM), which have been used to develop a three dimensional conceptual model of spring flow (Rutherford et. al. 2021). Our current understanding of the springs is they are located within or near the margins of large valley floor evaporite systems, but discharge fresh to brackish groundwater from deeper artesian aquifers, via geological faults.
Spring discharge rates are low, so an important question that remains is can these springs retain their fresh to brackish status under current groundwater extraction and a warmer climate (Rutherford et. al. 2020)? Or will spring discharge slow down and discharging groundwater become saline as it interacts with the underlying evaporite minerals (e.g. calcite, dolomite and gypsum) that formed under the last marine transgression? These studies require access to very localised samples and spring discharges, and thereby challenging in remote restricted locations such as in the WCP. To answer this question we verify past hydrochemical data with field sampling and analyses of shallow spring cores. ASTER data are then used to provide a spatially contiguous map of the evaporite systems.
In this study, eight different ASTER acquisitions were sourced from NASA’s LPDAAC portal, avoiding the wet monsoonal (December to April) between 2000 to 2007. The standard practice for multi-spectral image processing was applied here, using band ratios and band parameter algorithms for the targeting of spectral absorption features, e.g. Relative Band Depth (RBD) (Crowley et al., 1989). Similar band parameter algorithms are described in Cudahy (2012) for the ASTER Australia Geoscience Products. In particular, this study focussed on the generation of multi-temporal ASTER maps of Gypsum, MgOH-Carbonate and Ferrous Iron map products.
It was found that because of the extensive presence of fire scars and changes in even the arid vegetation cover, interpretation of the ASTER products was restricted to imagery of the same acquisition dates and orbit strips. Surface gypsum mapping from ASTER surface emissivity (AST_05) was successful in generating anomalies of interest within the WCP study area. This utilised the simple RBD band parameter applied to the ASTER thermal sensors’ surface emissivity bands. ASD data from shallow spring cores was also used to verify ASTER data.
Transect profiling of the ASTER gypsum product across springs, located along a major fault in the WCP, was compared with an area showing no documented occurrences of spring discharge. Cross sections reveal the different hydrogeology and hydrological processes operating in the WCB. Results indicate that the threat of discharging spring groundwater interacting with evaporites and becoming saline is low in the WCP. Major outcomes of the study to date is that ASTER imagery is useful for complex surface water – groundwater interaction studies through the verification of hydrochemical data, the identification of potential threats to spring water quality, and the mapping hydrological process boundaries that can be used to manage the WCP springs.
In addition to the hydrological applications of ASTER mapping, these and nearby studies showed that localised high gypsum occurrences appear to be at fault intersections that are evident in the DEM, that could propagate at depth, as suggested by seismic data (Zhan, 2019). This has implications for currently active exploration of Mississippi Valley-type lead–zinc deposits, associated with faulting controlled SEDEX systems. Some spring discharges have been shown to have elevated base metals – e.g. lead, zinc, nickel and copper