Surface resistivity imaging was proposed to map the saline interface produced by the interaction of a tidal creek and a shallow fresh water aquifer. Forward modelling was used to determine the relative merits of using two and three (2D and 3D) dimensional electrode arrays. The results of the forward modelling indicate that under ideal circumstances the surface techniques can resolve the conceptual saline interface near the tidal creek.One 3D and four 2D surveys were completed on the bank of Korogoro Creek. The 2D surveys were orientated parallel (3) and perpendicular (1) to the creek. The apparent resistivity data was inverted using RES2DINV and RES3DINV.The resulting 2D images demonstrate that the conceptual model used in the forward modelling oversimplified the distribution of saline water in the subsurface. The 2D images indicate that freshwater is discharging into the creek via preferred pathways at several locations along the creek bank. The 3D image shows that the distribution of saline water is constrained to an area close to the creek. Neither the 2D or 3D images were able to clearly identify the thin saline wedge observed using downhole techniques (EM39).The results of the resistivity surveys indicate that the techniques are useful in identifying relatively large-scale saline patterns but are unable to identify relatively small anomalies. Most importantly the results highlight the critical need to control interpretation of high resolution surface resistivity data with downhole techniques.
The Liverpool Plains in northern New South Wales contain some of the best agricultural land in Australia and are underlain by extensive smectite clay-dominated soils sourced from weathering the alkali basalts of the Liverpool Ranges. It had been thought that a relatively simple geological model explained the underlying Cenozoic sequence with salt-rich clays of the Narrabri Formation overlying sands and gravel aquifers comprising the Gunnedah Formation. Extensive groundwater modelling based upon this simple conceptualisation has been used in management plans proposed by the mining and agricultural industries. A 31.5 m core has been recovered using minimally disturbed triple-tube coring methods at Cattle Lane (Latitude –31.52° S, Longitude 150.47° E) to resolve uncertainty concerning the aquitard status of the upper layer. Recovered core has been examined and tested to determine grainsize, cation-exchange capacity, X-ray diffraction, X-ray fluorescence and microscopic examination of granular components. The...
Shallow (5–13 m) and deep (35–65 m) groundwaters were evaluated for their ability to generate conditioning films which affect bacterial adhesion to natural (sandstone, shale, andesite) and man-made substrata (polypropylene, stainless steel). Water contact angles indicated that all water samples produced conditioning films. Most films modified retention of the nonmotile Gram-negative bacterium SW8, but attachment of the organism did not correlate with water contact angles. Each borewater produced conditioning films with a characteristic attachment profile of SW8. Films adsorbed from standing borewaters often retained SW8 in different numbers than coatings derived from pumped bores. Groundwater chemistry was very heterogeneous and microbiological data indicated the presence of a diverse aerobic and anaerobic microbial community. These results indicate that conditioning films derived from dissolved compounds may play a significant role in controlling the interaction of bacteria with substrata in the subsurface.
Abstract A groundwater recharge investigation in the arid zone of Australia is presented. The investigation used a wide range of hydrogeological techniques including geological mapping, surface and borehole geophysics, groundwater hydraulics, streambed temperature and pressure monitoring, and hydrogeochemical and environmental tracer sampling, and it was complemented by analysis of rainfall intensity from 18 tipping-bucked rain gauges, climate data and stream runoff measurements. Run-off and recharge from a 200-mm rainfall event in January 2015, the largest daily rainfall in the local 50-year record, were investigated in detail. While this major storm provided substantial run-off as a potential source for focused, indirect recharge, it only produced enough actual recharge to the shallow aquifer to temporarily halt a long-term groundwater recession. A series of smaller rainfall-runoff events in 2016 produced a similar recharge response. The results suggest that the total magnitude of a flood event is not the main control on indirect groundwater recharge at this location. A deeper aquifer shows no hydraulic response to surface-water flow events and is isolated from the shallow system, consistent with its Pleistocene groundwater age. This supports a growing body of evidence indicating that attributing or predicting generalised changes in recharge to changes in climate in dryland environments should not be attempted without first unravelling the dynamic processes governing groundwater recharge in the locality of interest. The results should prompt more detailed and long-term field investigation in other arid zone locations to further understand the episodic and nonlinear nature of recharge in such environments.
A novel centrifuge permeameter (CP) system was designed to characterize and model seepage and reactive solute transport through low permeability materials and geological strata.A new CP module for the Broadbent G18 geotechnical centrifuge (2.0 m diameter), provides new capability for hydraulic conductivity (K) and transport testing of materials including drill cores, mine tailings and engineered barriers.By designing centrifuge models that maintain chemical equilibrium, reactive transport of solutes can be assessed within a reasonable experiment time at accelerated gravity.The K of minimally disturbed drill cores or porous materials having a diameter of 65-100 mm and a length of 20-200 mm can be measured using steady state flow, with a K detection limit currently within the order of 10 -12 m/s.This paper presents K values as a function of depth below ground for drill core, and K as a function of effective porosity of reconstituted silica flour specimens.
To date, an understanding of crack dynamics has been fundamentally hampered by the lack of available techniques to observe or monitor crack dynamics below the soil surface. A new technique relates the growth of soil cracks to a progressive increase in the electrical anisotropy of the soil. Although a single measurement of anisotropy is possible using a surface array of electrodes, the use of four strings of electrodes installed vertically at the corners of a square provides a valuable picture of the crack pattern at depth. In addition, time-lapse electrical surveys allow the growth of cracks to be clearly monitored. The electrical anisotropy is defined as the ratio of the [Formula: see text]-to-[Formula: see text] apparent resistivity for the square array and is determined for each coplanar set of four electrodesusing one electrode from each of the four vertical strings. In a laboratory, we measured the electrical anisotropy in a sand-filled lysimeter with a plastic sheet, introduced to represent an electrically insulating crack. Measurements were then repeated in a cracking-soil-filled lysimeter. Finally, measurements were made in a field where a flood-irrigated sorghum crop was grown on cracking soil. Measurements under all three conditions demonstrate that the lateral and vertical extents of cracking in a soil profile strongly influence the electrical anisotropy. The larger the cumulative cracking volume, the higher the electrical anisotropy. Soil-moisture changes after crack closure have a minor influence on the measured anisotropy, as have sorghum roots. These experiments demonstrate that electrical-anisotropy profiles are a valuable tool for monitoring crack dynamics within a soil profile.
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