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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.

Study region The study area is located at Kapunda, South Australia with a semi-arid climate and comprises a fractured rock aquifer. Established in 1842, Kapunda was the first mining town in Australia and its open-pit mine was a key economic driver for the whole continent. The mine was closed in 1879 and the remnant low-grade copper was considered sub-economic, hence the mine has been developed into a tourist attraction. With the recent advances in In-Situ Recovery (ISR), the feasibility of recovering the remnant copper is under consideration again. Study focus A field trial involving lixiviant injection and extraction is under development. This study uses a combination of stochastic groundwater modeling and multiple geophysical techniques to investigate (1) the flow paths of injectant and whether it will reach the Light River, and (2) the injectant residence time in the aquifer. New hydrological insights for the region Depending on the effective porosity of the aquifer, the modeling suggests a probability of 1 %–5 % for the injectant to escape the site. There is a large uncertainty in the simulated injectant residence time, ranging from 200 to beyond 1000 days. These analyses are conservative and do not consider the biodegradability of the injectant, which can achieve a 90 % degradation over 28 days. Our modeling shows no evidence that the injectant can reach the Light River within this timeframe.

The ability to accurately and precisely measure dissolved gas concentrations in groundwater is crucial for environmental monitoring purposes. Unfortunately, the collection of dissolved gas samples in groundwater is challenging because of the loss of dissolved gas in the groundwater as a result of inadequate sampling methodologies and the need to avoid exposure of the samples to the atmosphere. In environments where water can contain large amounts of methane and carbon dioxide (e.g., coal basins), there is a high probability of separation between the water and gas phases during sample collection, making it impossible to accurately measure the gas concentrations in a groundwater sample. In this paper, we describe the design and development of a novel downhole sampler that is used to collect dissolved gas samples in groundwater at in situ pressure. The sampler was used to collect samples from 26 bores in a proposed coal seam gas field in regional New South Wales, Australia. Comparison of CH4 concentration data showed that the gas concentrations were higher using our methodology than when using alternate methodologies, especially when CH4 concentrations were >10000 μg L–1. Not surprisingly, results from the new methodology suggest that isotopic fractionation occurs when samples are collected using alternate methodologies.