Michael Hatch

Geophysical methods in saline groundwater studies: locating perched water tables and fresh-water lenses

A fast and informative tool was required for the delineation of stratigraphy and hydrology and for monitoring salinisation effects around the Stockyard Plains Disposal Basin, which is used as a storage lake for salt water pumped from aquifers close to the Murray River near the town of Waikerie, South Australia. Such salinisation effects include the formation of a perched saline watertable close to the disposal basin and consequent threats to local fresh-water lenses. Ground penetrating radar, direct current (DC) resistivity, time-domain EM (TEM) and low induction number frequency-domain EM (FEM) were compared in terms of efficiency and effectiveness at two survey locations near the Stockyard Plains Disposal Basin. Low induction number FEM measurements were fast to implement but suffered from a nonlinear response with ground conductivity in high conductivity areas. Radar methods were ineffective due to high signal attenuation in the highly saline environment. Fast time-sampling TEM successfully depicted a perched saline water table related to leakage from the disposal basin, and was found to be the most useful technique for delineation of hydrogeology due to its high vertical resolution. DC resistivity was the slowest technique, but was found useful in imaging a fresh-water lens. Results from this study suggest that TEM and DC resistivity methods are useful tools for both fresh-water detection and hydrogeology monitoring in saline groundwater environments.

A comparative study of in-river geophysical techniques to define variations in riverbed salt load and aid managing river salinization

Increased interest in the character of sediments at the base of waterways, for the purpose of managing river salinization, has led to the application of several geophysical techniques for collecting information from this zone. These instream methods are based on established ground and airborne electrical and electromagnetic technologies, including towed transient electromagnetic systems, towed direct current resistivity array systems, and frequency-domain helicopter electromagnetic systems. Although these systems are individually successful, a systematic examination of their relative effectiveness for identifying variations in substrate conductivity for a common stretch of a river remains lacking. We have compared results obtained from data collected using three instream geophysical techniques for a common stretch of the Murray River in southeastern Australia. The Murray River is an important water resource for drinking and agricultural purposes. Data from these surveys were acquired tolocate areas of sig...

Groundwater flow underneath mound spring tufas from geophysical surveys in the southwestern Great Artesian Basin, Australia

ABSTRACT Artesian springs, which form carbonate mounds at the surface, occur in groups along the southwestern edge of the Great Artesian Basin (GAB), in northern South Australia. Their underground structure and relationship to faulting are not well understood. This study investigated four mound spring groups (Beresford Spring, Warburton Spring, the Bubbler Spring group, and Freeling Springs), which have different geological settings, using a range of geophysical techniques: self-potential (SP), magnetotellurics (MT) and time-domain electromagnetics (TEM). The results confirmed that despite generally similar mound morphologies at the surface, spring vents in different groups have different mechanisms for sourcing water from the GAB aquifer. SP measurements effectively showed where most of the vertical flow occurs at all groups, while MT and TEM models identified the location of faults, other geological structures and the thickness of the Bulldog Shale aquitard. Beresford and Warburton springs are similar in that the spring locations at both are directly related to points on a regional-scale fault, which appears to provide a low permeability pathway to the surface. At the Bubbler Springs group, a regionally aligned fault identified in MT data seems to be bounding an area where the aquifer is much closer to the surface (and the overlying aquitard has been thinned) where the spring abundance is highest. SP responses indicate that shallow flow paths feeding individual vents are unrelated to the fault. At Freeling Springs, the SP suggests that groundwater is flowing up within shallow aquifers, and that spring location is offset from this upward flow, instead located on range-front faults that provide the final conduit to the surface.

Ground penetrating radar as a non-invasive tool to better understand the population dynamics of a fossorial species: mapping the warrens of southern hairy-nosed wombats (Lasiorhinus latifrons)

Abstract Context. Management of wildlife that may simultaneously be of conservation concern and pose problems for humans is difficult, particularly when knowledge of their population dynamics is elusive. Culling of southern hairy-nosed wombats (Lasiorhinus latifrons) is often carried out in agricultural areas, with no understanding of potential impacts on the species as a whole. Monitoring fossorial species via non-invasive means (that do not adversely impact animals by damaging their burrows) has always represented a challenge for wildlife researchers. Aim. The aim of this research was to map the areal extent of different types of L. latifrons warrens to gain a better understanding of the relationship between the external warren signs and its subterranean structure. The findings will be used in the development of more accurate indices of population abundance to better inform management decisions. Methods. Ground penetrating radar (GPR) was used to map warrens at four locations in the western regions of South Australia. Radar data were collected using a Mala X3M GPR system with 250 MHz and 500 MHZ antennas. 3D models of each site were then produced using the ReflexW GPR software processing package. Key results. Subterranean warren structure varied from a mix of tunnel types in sandy-loam soil to a complex array of tunnels and caverns under sheet calcrete limestone. Conclusions. This was the first non-invasive mapping of wombat warrens and the first mapping of a warren under a layer of calcrete limestone. In sandy-loam soil, the size and extent of the external spoil mound provided some indication of warren complexity. However, there were no external signs of the extent of the calcrete warren. Implications. The lack of external cues regarding the extent of the calcrete limestone warren suggests that the current method of estimating population abundance based on a single index of wombats per active burrow is flawed. As a result, any management decisions in regard to culling may be based on inaccurate information. It is apparent that further research needs to be undertaken to develop a range of abundance indices that take into account local conditions such as soil type.

Riparian eucalypt biogeochemical expression of groundwater salinity, Murray River, South Australia

Salinisation of floodplains along the Murray River is a significant environmental and social problem in south-eastern Australia that can be expensive and time-consuming to monitor. The potential of plant biogeochemistry as an environmental monitoring tool, specifically its innovative application to groundwater salinity detection, is explored in this paper. Major and trace element biogeochemical data were compared to data from three geophysical surveys in the study area as well as field observations of the underlying stratigraphy. The result is an understanding of how groundwater chemistry can be characterised by plants, specifically river red gum (Eucalyptus camaldulensis) and black box (Eucalyptus largiflorens) leaves. From the survey it is evident that E. camaldulensis and E. largiflorens were both successful in expressing high salinity levels in the subsurface. Molybdenum was the most suitable pathfinder element for high salinity groundwater. The study has important implications for interpreting biogeochemical mineral exploration results; anomalous element concentrations need to be interpreted in relation to salinity levels.

Quantitative measure of salt interception using in-river transient electromagnetic geophysics

Abstract In the lower Murray-Darling Basin, most groundwater discharges to the floodplain of the River Murray. Most of the groundwater is of high salinity and therefore can transfer significant salt loads into the river. To mitigate saline groundwater intrusion into the river, salt interception schemes (SIS) have been commissioned since the early 1990s. The SIS intercept high-salinity groundwater flow adjacent to the river floodplain and the intercepted water is pumped to distant evaporation basins. The in-river transient electromagnetic (RTEM) geophysics technique can be used to infer saline groundwater discharge areas and to inform SIS locations. RTEM results have also been used, albeit qualitatively, in the monitoring and evaluation of the performance of SIS. A methodology for evaluating SIS performance has been developed based on the area above the cumulative frequency distribution (ACFD) of RTEM riverbed-only resistivities. In addition to RTEM maps and cross-sections, the ACFD characterises a river reach with a single number. Increases in ACFD, from pre-to post-SIS RTEM surveys, indicate the changing groundwater flow regime and the building of freshwater lenses.

Mapping Salt-loads of the Murray River, Australia, Using Airborne and In-river Electromagnetic Methods

This paper examines the potential of using the FDHEM RESOLVE system as basis for mapping reaches of the river that contribute to elevated salt loads in the Murray River to the south east of Mildura in Victoria. The advantages of the airborne systems become more apparent when data coverage and acquisition costs are considered, particularly in a situation where a parallel swath approached is employed. This entails the acquisition of adjacent lines of EM data along the centre and along the margins of the river. We suggest this approach provides for a better understanding of recharge and discharge processes and links between the floodplain and the main-river channel. Compared with data acquired along the river alone, this study demonstrated our ability to use Helicopter EM data to map losing and gaining (from a salt load perspective) stretches of the river and to provide insight into which parts of the groundwater-floodplain system were significant contributors to river salt loads. The rapid acquisition of airborne EM data makes these systems more suited to providing temporal snapshots of a river-floodplain environment during dramatic climatic events, such as flooding.

Fast-sampling EM Applied to the River Murray and Surrounding Floodplains in Australia

Accurate, high resolution images of the top 10 to 50 metres of the earth’s surface potentially provide insights into the underlying hydrogeology that help us to understand, for example, the distribution of saline groundwater in the subsurface, or subtle changes in the distribution of clays in the top ten metres. This paper reviews the use of a commercially available fast-sampling TEM system in use in the surrounds of the River Murray system in southeastern Australia. This system has been used to collect data in a fairly standard TEM mode, i.e. discrete stations at fixed intervals along a line. Additionally results are shown using the system in a modified configuration, with much smaller transmitting and receiving antennae towed behind a boat, for example, with data collected in conjunction with a GPS in a “continuous” mode. This type of data provides a continuous electrical cross section with data points as close together as five metres apart, to a depth of at least ten metres.

Instream NanoTEM: providing increased resolution to stream salinisation and floodplain processes along the River Murray, southeast Australia

Abstract Increasing salinity in the River Murray is well documented and is of concern environmentally, economically and socially. The Murray Darling Basin Commission and the Mallee Catchment Management Authority engaged the authors to collect base-line in-stream NanoTEM data within the River Murray from Lock 1 to Mallee Cliffs (675 km). This is a new application of a high resolution fast sampling Transient Electro-Magnetic (TEM) system, towed behind a boat, taking soundings every seven to ten metres along the river. The observed NanoTEM response was interpreted against the current understanding of the regional hydrogeology and groundwater processes in and around the river. This paper summarises some of the results from this investigation. The observed response correlates strongly with previously mapped major changes in underlying lithostratigraphy along the Murray River, and with gaining and losing reaches of the river. The extensive length of the survey provides an insight into potential interactions between the river, floodplain and groundwater, but does not replace the need for focussed ground-truthing programs to examine specific correlations. This rapid, portable technique should be applicable outside the Murray-Darling Basin as well as at additional locations within the Basin.

Methane variability associated with natural and anthropogenic sources in an Australian context

ABSTRACT Coal seam gas (CSG) has the potential to be a low-carbon transition fuel, but CSG fields may be a source of fugitive emissions of methane (CH4). We use mobile cavity ring-down spectroscopy (CRDS) measurements to attribute CH4 concentrations to sources in southeastern Australia including CSG fields. Our study shows higher CH4 concentration values associated with both natural and anthropogenic sources other than CSG. These include urban landfills (>320 ppm) and urban infrastructure (>17 ppm), agricultural activities (>20 ppm) and open-pit coal mines (>30 ppm). We confirm reports of increased concentration of CH4 downwind of some parts of CSG fields (<5 ppm), but elevated concentrations are not endemic and could not be separated from contributions of natural geological seeps (>16 ppm) that form part of the background levels. While CRDS allows direct determination of CH4 sources, repeat measurements show the strong influence of atmospheric conditions on concentration and highlight the need for methods that quantify flux.