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This study introduces a new analytical framework that employs the image‐well method to simulate the spatial and temporal temperature distribution in vertical borehole thermal energy storage (BTES) systems. The model accommodates complex boundary shapes and conditions, including insulation, convection, and constant temperature, without requiring iterative solutions at each time step. The model's accuracy and utility are demonstrated through an application to a borehole heat exchanger cluster arranged in an octagonal shape with insulating boundaries, based on a BTES site in Drake Landing (Canada). Model predictions are validated against a finite element model, showing a root‐mean‐square error of 0.012°C. A global sensitivity analysis highlights the influence of thermal parameters on system performance, identifying the heat flux of the borehole heat exchanger as the most sensitive parameter. Overall, this approach combines the advantages of analytical and numerical techniques to provide a clear and efficient tool for evaluating BTES systems, offering significant potential for advancing sustainable energy solutions. [ABSTRACT FROM AUTHOR]

Harmonic Earth tide components in well water levels have been used to estimate hydraulic and geomechanical subsurface properties. However, the robustness of various methods based on analytical solutions has not been established. First, we review the theory and examine the latest analytical solution used to relate well water levels to Earth tides. Second, we develop and verify a novel numerical model coupling hydraulics and geomechanics to Earth tide strains. Third, we assess subsurface conditions over depth for a range of realistic properties. Fourth, we simulate the well water level response to Earth tide strains within a 2D poroelastic layered aquifer system confined by a 100 m thick aquitard. We find that the non‐linear inversion of analytical solutions to match two observations (amplitudes and phases) to multiple unknown parameters is sensible to the initial guess. We reveal that undrained, confined conditions are necessary for the analytical solution to be valid. This occurs for the dominant M2 frequency at depths >50 m and requires specific storage at constant strain of Sϵ ≥ 10−6 m−1, hydraulic conductivity of the aquitard of kl ≤ 5 ⋅ 10−5 ms−1 and aquifer of ka ≥ 10−4 ms−1. We further illustrate that the analytical solution is valid in unconsolidated systems, whereas consolidated systems require additional consideration of the Biot modulus. Overall, a priori knowledge of the subsurface system supports interpretation of the groundwater response. Our results improve understanding of the effect of Earth tides on groundwater systems and its interpretation for subsurface properties. Plain Language Summary: Earth tide induced strains in the subsurface lead to well water level fluctuations in groundwater monitoring wells. This groundwater response has been interpreted with analytical solutions to estimate aquifer properties. However, analytical solutions are based on simplified assumptions whose accuracy have not yet been tested. We develop a new approach to simulate the influence of Earth tides on groundwater based on fundamental physical principles. We simulate realistic conditions and compare our results to those from analytical solution to determine the hydraulic and subsurface conditions under which simplified interpretations are valid. Our results improve understanding of the effects of Earth tides on groundwater systems and interpretation of subsurface properties from well water levels. Key Points: We develop and verify a numerical model for the well water level response to Earth tidesSubsurface property estimation requires undrained and confined conditions occurring at depths >50 mAmplitudes and phases from numerical and analytical solutions systematically diverge reflecting theory simplifications [ABSTRACT FROM AUTHOR]

Hydraulic properties of coastal, urban aquifers vary spatially and temporally with the complex dynamics of their hydrogeology and the heterogeneity of ocean-influenced hydraulic processes. Traditional aquifer characterisation methods are expensive, time-consuming and represent a snapshot in time. Tidal subsurface analysis (TSA) can passively characterise subsurface processes and establish hydro-geomechanical properties from groundwater head time-series but is typically applied to individual wells inland. Presented here, TSA is applied to a network of 116 groundwater boreholes to spatially characterise confinement and specific storage across a coastal aquifer at city-scale in Cardiff (UK) using a 23-year high-frequency time-series dataset. The dataset comprises Earth, atmospheric and oceanic signals, with the analysis conducted in the time domain, by calculating barometric response functions (BRFs), and in the frequency domain (TSA). By examining the damping and attenuation of groundwater response to ocean tides (OT) with distance from the coast/rivers, a multi-borehole comparison of TSA with BRF shows this combination of analyses facilitates disentangling the influence of tidal signals and estimation of spatially distributed aquifer properties for non-OT-influenced boreholes. The time-series analysed covers a period pre- and post-impoundment of Cardiff's rivers by a barrage, revealing the consequent reduction in subsurface OT signal propagation post-construction. The results indicate that a much higher degree of confined conditions exist across the aquifer than previously thought (specific storage = 2.3 × 10−6 to 7.9 × 10−5 m−1), with implications for understanding aquifer recharge, and informing the best strategies for utilising groundwater and shallow geothermal resources. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Groundwater is an important global resource and its sustainable use faces major challenges. New methods and advances in computational science could lead to much improved understanding of groundwater processes and subsurface properties. A closer look at current groundwater monitoring practice reveals the need for updates with a special focus on the benefits of high-frequency and high-resolution datasets. To future-proof hydrogeology, this technical note raises awareness about the necessity for improvement, provides initial recommendations and advocates for the development of universal guidelines. [ABSTRACT FROM AUTHOR]

Groundwater extraction is increasing rapidly in many areas of the world, causing serious impacts such as falling water tables, ground surface subsidence, water quality degradation, and reduction of stream baseflow on which many ecosystems depend. Methods for understanding and predicting the impacts of groundwater extraction generally lack detailed spatial and temporal knowledge of the subsurface hydrogeomechanical properties. This review provides a comprehensive understanding of Earth and atmospheric tides and their impact on subsurface pore pressure. First, we evaluate the global occurrence of Earth and atmospheric tides. Then, we illustrate their impact on the groundwater response and connect this with the theory of poroelasticity, which underpins quantitative analyses. Finally, we review methods that utilize these impacts to characterize groundwater systems and to quantify their hydrogeomechanical properties. We conclude by highlighting their potential as passive and low‐cost investigation techniques and by outlining the research and developments required to progress and make analyses readily available. Thus, hydrogeomechanical properties of subsurface systems could be obtained at unprecedented spatial and temporal resolution, adding additional value to commonly acquired groundwater and atmospheric pressure data. Key Points: Earth and atmospheric tides occur globally, are predictable or observable, and induce groundwater oscillations under semiconfined conditionsTides, in combination with poroelastic theory, enable groundwater system characterization and hydrogeomechanical property quantificationAnalyzing groundwater responses to Earth and atmospheric tides is an underutilized passive technique to quantify subsurface properties [ABSTRACT FROM AUTHOR]

Solute dispersion and mixing in beach aquifers is strongly influenced by highly transient flow induced from wave forcing. While transport at the groundwater-ocean interface has been modelled, little is known about the quantitative effect of wave forcing on solute dispersion and mixing in beach aquifers. We use a prototype-scale laboratory flume experiment to conduct tracer transport experiments at two locations within a wave-forced beach aquifer. For the first time we demonstrate by systematic laboratory experimentation that transient conditions in the re-circulation zone due to run-up and beach face infiltration as well as head oscillations caused by wave forcing strongly disperse and mix subsurface solute plumes. Wave forcing can increase the apparent dispersion by an order of magnitude depending on conditions, compared to solute transport without waves. Our findings illustrate that beach aquifer transport models need to consider the additional dispersion to correctly quantify mixing and biogeochemical processes in this highly dynamic zone. [ABSTRACT FROM AUTHOR]

Ephemeral and intermittent flow in dryland stream channels infiltrates into sediments, replenishes groundwater resources and underpins riparian ecosystems. However, the spatiotemporal complexity of the transitory flow processes that occur beneath such stream channels are poorly observed and understood. We develop a new approach to characterise the dynamics of surface water-groundwater interactions in dryland streams using pairs of temperature records measured at different depths within the streambed. The approach exploits the fact that the downward propagation of the diel temperature fluctuation from the surface depends on the sediment thermal diffusivity. This is controlled by time-varying fractions of air and water contained in streambed sediments causing a contrast in thermal properties. We demonstrate the usefulness of this method with multi-level temperature and pressure records of a flow event acquired using 12 streambed arrays deployed along a  ∼ 12 km dryland channel section. Thermal signatures clearly indicate the presence of water and characterise the vertical flow component as well as the occurrence of horizontal hyporheic flow. We jointly interpret thermal signatures as well as surface and groundwater levels to distinguish four different hydrological regimes: [A] dry channel, [B] surface run-off, [C] pool-riffle sequence, and [D] isolated pools. The occurrence and duration of the regimes depends on the rate at which the infiltrated water redistributes in the subsurface which, in turn, is controlled by the hydraulic properties of the variably saturated sediment. Our results have significant implications for understanding how transitory flows recharge alluvial sediments, influence water quality and underpin dryland ecosystems. [ABSTRACT FROM AUTHOR]

Soil moisture and temperature are some of the most important controls for a wide variety of geochemical and ecological processes in the vadose zone (VZ). Soil moisture is highly variable both spatially and temporally. The development of methods to measure it on various scales has been the subject of much activity. Recently, geoscientists have been increasingly interested in measuring temperature as a proxy for hydrologic properties and parameters, including soil moisture. Here, we discuss the motivation, primary concepts, equipment, and fundamental thermal and hydraulic models related to heat and water transport in variably saturated porous media. A large variety of methods for heat tracing, including both passive and active-heating methodologies, are detailed. Heat tracing methods offer the capacity to measure soil moisture on a scale from ~ 1 cm up to several km using temperature, a parameter whose measurement in VZ studies is often required anyway due to its effect on many subsurface processes. Furthermore, heat-tracing methods are not affected by high salinity pore water that can limit electromagnetic soil moisture methods. We also review coupled thermo-hydro VZ modelling software and VZ thermal regime studies and identify several knowledge gaps. With the intention to serve as an introduction to VZ heat-tracing, this review consolidates recent advances and outlines potential themes for future research. [ABSTRACT FROM AUTHOR]

The dynamics of groundwater at the beach face land–ocean boundary have important implications to the exchange of water, nutrients, and pollutants between the ocean and coastal aquifers, and more subtly, varying groundwater levels may induce differing morphological response at the beach face. As a component of the multi-institution Barrier Dynamics Experiment (BARDEX II), groundwater fluxes and flow paths within a prototype-scale sandy barrier are quantified and reported at the three fundamental spatio-temporal scales (individual waves, the beach face, and total barrier), under controlled wave and water level conditions. A particular feature of the experimental programme was the inclusion of a back-barrier ‘lagoon’, that via a pump system and an intermediate water reservoir enabled the forcing of contrasting hydraulic gradients across the barrier. It was observed that the groundwater level, flow paths, and fluxes within the beach face region of the sand barrier were predominantly controlled by the action of waves at the beach face, regardless of the overall seaward- or landward-directed barrier-scale hydraulic gradients. In the presence of waves, all tests undertaken to complete this study developed a seaward gradient in this zone under the influence of waves. As a further result of wave forcing at the beach face boundary, localised groundwater flow divides were observed to develop, further partitioning the circulation and flow paths of groundwater within the prototype-scale sand barrier. [ABSTRACT FROM AUTHOR]

While several studies explore cave climate and thermal regimes, little is known about the controls on cave drip water temperature. Yet water temperature significantly influences biogeochemical processes associated with cave drips. To identify the processes that control the cave drip water temperature, we measured the temperatures at multiple locations along a speleothem flow path and drip sources (stalactites) concurrently with the drip rates in Cathedral Cave, Wellington, Australia. We monitored long-term drip water temperature, drip rates, surface and cave climate and in-cave evaporation rates and conducted 3 infiltration experiments with different flow, temperature and isotopic conditions. Our results show that the drip water temperature is controlled by multiple superimposed heat transport mechanisms that act upon the infiltrating water in the epikarst, the water film after it enters the cave and before it becomes a drip. The two main heat sources/sinks for drip water are the cave air and the surrounding rock. The subsurface temperature is coupled to the surface temperature by conduction through the soil and rock mass, but the cave climate is also coupled to the surface climate by venting. On a regional scale, drip temperatures are mainly driven by the annual ground surface temperature signal but damped with depth and shifted in time compared to the surface. On a local scale, the drip water temperature can differ significantly from cave air and speleothem temperature due to the latent heat exchange of evaporation and localised water film convection. The main controls are ground surface temperature, subsurface depth, air density induced ventilation, distance from entry and drip rate. We present a conceptual model that explains drip water temperature signals and provide signal driven guidance on best type and location for speleothem sampling. We anticipate that our results will significantly improve the understanding of temperature-dependent paleoclimate signals from speleothem archives. [ABSTRACT FROM AUTHOR]

Abstract: The dynamic distribution of thermal conditions present in saturated near-surface sediments have been widely utilised to quantify the flow of water. A rapidly increasing number of papers demonstrate that heat as a tracer is becoming an integral part of the toolbox used to investigate water flow in the environment. We summarise the existing body of research investigating natural and induced heat transport, and analyse the progression in fundamental and natural process understanding through the qualitative and quantitative use of heat as a tracer. Heat transport research in engineering applications partly overlaps with heat tracing research in the earth sciences but is more advanced in the fundamental understanding. Combining the findings from both areas can enhance our knowledge of the heat transport processes and highlight where research is needed. Heat tracing relies upon the mathematical heat transport equation which is subject to certain assumptions that are often neglected. This review reveals that, despite the research efforts to date, the capability of the Fourier-model applied to conductive–convective heat transport in water saturated natural materials has not yet been thoroughly tested. However, this is a prerequisite for accurate and meaningful heat transport modelling with the purpose of increasing our understanding of flow processes at different scales. This review reveals several knowledge gaps that impose significant limitations on practical applications of heat as a tracer of water flow. The review can be used as a guide for further research directions on the fundamental as well as the practical aspects of heat transport on various scales from the lab to the field. [Copyright &y& Elsevier]

Turner, I.L., Rau, G.C., Andersen, M.S., Austin, M.J, Puleo, J. and Masselink, G. 2013. Coastal sand barrier hydrology - observations from the BARDEX II prototype-scale laboratory experiment The dynamics of groundwater at the land-ocean boundary has important implications to the exchange of water, nutrients and pollutants between the ocean and coastal aquifers, and varying groundwater levels may induce differing morphological response (erosion versus accretion) at the beachface. As a component of the rier namics periment BARDEX II (Hydrolab IV) completed during June-July 2012 in the Delta Flume (The Netherlands), an extensive test program was undertaken to acquire a comprehensive data set of groundwater levels, pressure fluctuations, flow paths and tracer dispersion beneath the beachface and across the width of a prototype-scale sand barrier ( D50 = 0.42mm). A particular feature of the experiment was the inclusion of a landward 'lagoon' and pump system that enabled the water levels on both the seaward and landward side of the barrier to be independently varied. The completion of these experiments complimented the previous BARDEX test program undertaken in the same test facility in 2008, where a gravel barrier ( D50 = 10mm) was subjected to similar wave and lagoon-sea water-level conditions. An overview of the barrier hydrology instrumentation deployed and data obtained is provided here. [ABSTRACT FROM AUTHOR]

Specific storage (SS) has considerable predictive importance in the modelling of groundwater systems, yet little is known about its statistical distribution and dependency on other hydrogeological characteristics. This study provides a comprehensive overview and compiles 430 values of SS from 183 individual studies, along with complementary hydrogeological information such as estimation methods, lithology, porosity, and formation compressibility. Further evaluation of different approaches to determine and utilize SS values for numerical groundwater modelling, along with the scale and source of uncertainty of different measurement methods, was carried out. Overall, SS values range across six orders of magnitude (from 3.2 × 10–9 to 6 × 10–3 m–1) with a geometric mean of 1.1 × 10–5 m–1 and the majority (> 67%) of values are in the order of 10–5 and 10–6 m–1. High SS values of ~10–4 m–1 were reported for glacial till and sandy lithologies, particularly for shallow and thin strata where leakage may obscure the estimation of SS. A parallel assessment of 45 transient regional-scale groundwater models reveals a disconnect between findings of this study and the way SS is treated in practice, and that there is a lack of foundational SS data to conduct quantitative uncertainty analysis. This study provides the first probability density functions of SS for a variety of lithology types based on the field and laboratory tests collated from the literature. Log transformed SS values follow a Gaussian/normal distribution which can be applied to evaluate uncertainties of modelling results and therefore enhance confidence in the groundwater models that support decision making. [ABSTRACT FROM AUTHOR]

Locating clandestine graves presents significant challenges to law enforcement agencies, necessitating the testing of grave detection techniques. This experimental study, conducted under Australian field conditions, assesses the effectiveness of time-lapse ground-penetrating radar (GPR) and electrical resistivity tomography (ERT) in detecting pig burials as simulated forensic cases. The research addresses two key questions: (1) observability of graves using GPR and ERT, and (2) changes in geophysical responses with reference to changing climatic conditions. The principal novelty of this research is its Australian focus—this is the first time-lapse GPR and ERT study used to locate clandestine graves in Australia. The results reveal that both GPR and ERT can detect graves; however, ERT demonstrates greater suitability in homogeneous soil and anomalously wet climate conditions, with the detectability affected by grave depth. This project also found that resistivity values are likely influenced by soil moisture and decomposition fluids; however, these parameters were not directly measured in this study. Contrastingly, although GPR successfully achieved 2 m penetration in each survey, the site's undeveloped soil likely resulted in inconsistent detectability. The findings underscore the significance of site-specific factors when employing GPR and/or ERT for grave detection, including soil homogeneity, climate conditions, water percolation, and body decomposition state. These findings offer practical insights into each technique's utility as a search tool for missing persons, aiding law enforcement agencies with homicide cases involving covert graves. [ABSTRACT FROM AUTHOR]

Realistic modelling of tightly coupled hydro-geomechanical processes is relevant for the assessment of many hydrological and geotechnical applications. Such processes occur in geologic formations and are influenced by natural heterogeneity. Current numerical libraries offer capabilities and physics couplings that have proven to be valuable in many geotechnical fields like gas storage, rock fracturing and Earth resources extraction. However, implementation and verification of the full heterogeneity of subsurface properties using high-resolution field data in coupled simulations has not been done before. We develop, verify and document RHEA (Real HEterogeneity App), an open-source, fully coupled, finite-element application capable of including element-resolution hydro-geomechanical properties in coupled simulations. To extend current modelling capabilities of the Multiphysics Object-Oriented Simulation Environment (MOOSE), we added new code that handles spatially distributed data of all hydro-geomechanical properties. We further propose a simple yet powerful workflow to facilitate the incorporation of such data to MOOSE. We then verify RHEA with analytical solutions in one and two dimensions and propose a benchmark semi-analytical problem to verify heterogeneous systems with sharp gradients. Finally, we demonstrate RHEA's capabilities with a comprehensive example including realistic properties. With this we demonstrate that RHEA is a verified open-source application able to include complex geology to perform scalable, fully coupled, hydro-geomechanical simulations. Our work is a valuable tool to assess challenging real-world hydro-geomechanical systems that may include different levels of complexity like heterogeneous geology and sharp gradients produced by contrasting subsurface properties. [ABSTRACT FROM AUTHOR]

• Experiments combined with literature data reveals dispersion grain size dependence. • When dispersion dominates, solute and heat have similar longitudinal dispersivities. • In the transition regime, thermal dispersion nonlinearly increases with flow velocity. • Higher nonlinearity with increased pore-scale heterogeneity for smaller particle size. The growing applications of heat transport in engineering and hydrogeology, including ground source heat pump systems and the assessment of water flow and sediment thermal properties, necessitate precise thermal dispersion modeling. Despite various factors like particle size, shape, and distribution impacting dispersion in porous media, a lack of experimental data and analysis has limited our grasp of the relationship between flow velocity and thermal dispersion. To address this gap, we conducted solute and heat tracer experiments using sands of different grain sizes, employing analytical models based on a universal power law relationship for dispersion. Additionally, we systematically compiled dispersion data from the scientific literature to assist in interpreting of the effects of particle size, shape, and pore-scale heterogeneity on dispersion. Our findings reveal that solute and thermal longitudinal dispersion align for high flow velocities in the dispersion-dominated flow regime (Pe > 5), where the velocity-dispersion relationship is linear. However, in the transition regime (advection approx. equal to diffusion, 0.1–0.3 < Pe < 5), the observed deviation from linearity ( R 2 < 0.9) is linked to increased pore-scale heterogeneity, particularly in smaller particle sizes. This study underscores the pivotal role of thermal dispersion and underscores the need for caution when estimating thermal dispersion based on solute dispersion, given the intricate nature of real-world conditions and the potential influence of factors such as grain size distribution and pore-scale heterogeneity in natural porous materials. [ABSTRACT FROM AUTHOR]

The groundwater pressure response to the ubiquitous Earth and atmospheric tides provides a largely untapped opportunity to passively characterize and quantify subsurface hydro-geomechanical properties. However, this requires reliable extraction of closely spaced harmonic components with relatively subtle amplitudes but well-known tidal periods from noisy measurements. The minimum requirements for the suitability of existing groundwater records for analysis are unknown. This work systematically tests and compares the ability of two common signal processing methods, the discrete Fourier transform (DFT) and harmonic least squares (HALS), to extract harmonic component properties. First, realistic conditions are simulated by analyzing a large number of synthetic data sets with variable sampling frequencies, record durations, sensor resolutions, noise levels and data gaps. Second, a model of two real-world data sets with different characteristics is validated. The results reveal that HALS outperforms the DFT in all aspects, including the ability to handle data gaps. While there is a clear trade-off between sampling frequency and record duration, sampling rates should not be less than six samples per day and records should not be shorter than 20 days when simultaneously extracting tidal constituents. The accuracy of detection is degraded by increasing noise levels and decreasing sensor resolution. However, a resolution of the same magnitude as the expected component amplitude is sufficient in the absence of excessive noise. The results provide a practical framework to determine the suitability of existing groundwater level records and can optimize future groundwater monitoring strategies to improve passive characterization using tidal signatures. [ABSTRACT FROM AUTHOR]

Field measurements of hydraulic heads and gradients underpin the quantification ofsubsurface flow processes and properties throughout the geosciences. Their quantification isgenerally assumed to be trivial and hence few practitioners receive comprehensivefield training, resulting in a high potential for error and misinterpretation. In thiscontribution we comprehensively review the four individual measurements required tocalculate hydraulic heads and gradients: (1) geo-spatial positioning, (2) manualwater depth, (3) automated pressure, (4) spatial reference point for the head. Wesystematically and concisely summarise the standard practice for each measurement andestimate the random error magnitudes from the literature, our own datasets and fieldexperience. For example, the minimum random error for hydraulic head differences whenapplying standard practice is approximately 12 mm. We compare and propagatemeasurement errors for horizontal and vertical hydraulic gradient calculations andevaluate the impact of the error uncertainty on the quantification of subsurface flowprocesses. Our systematic assessment reveals some surprising aspects and demonstratessimple best-practice rules that should be followed to achieve the highest possiblemeasurement precision. We categorise these recommendations in order of errormagnitudes from highest to lowest. The largest errors contributing to hydraulic headand gradient measurements originate from borehole inclination and manual waterdepth measurement, respectively. Our analysis demonstrates that resolving headgradients that are smaller than 10−2 for boreholes that are closer than 10 m requiresextraordinary effort. This work will provide fundamental information for students andpractitioners in fields where hydraulic head data is vital; we anticipate that oursynthesis will avoid common pitfalls as well as provide a starting point for thedevelopment of best-practice guidelines for hydraulic head and gradient measurements. [ABSTRACT FROM AUTHOR]

One of the largest challenges for groundwater management is reliable modelling of semi-confined and confined aquifer systems with variable degrees of vertical connectivity. There is often a lack of sufficient hydraulic and hydromechanical data for each strata in a complex aquifer system to enable robust and reliable numerical modelling. Currently, data are often spatially and temporally sparse due to the extensive labour and setup costs of aquifer pump test techniques, and whether aquifers are semi-confined or confined is poorly quantified. The analysis of Earth and atmospheric tides in standard monitoring bores (pressure gauged) offers significant utility to provide a preferential alternative to long term pump tests in confined aquifers. These novel techniques also enable unprecedented monitoring of changes of the degree of confinement of aquifers and variations in hydromechanical properties over time. We provide a synthesis of the current understanding and utilization of Earth and atmospheric tidal analysis of groundwater level data for characterization of subsurface systems. We also highlight uncertainties in techniques that are being developed, to encourage discussion and further research. [ABSTRACT FROM AUTHOR]

Knowledge of fracture properties and associated flow processes is important for geoscience applications such as nuclear waste disposal, geothermal energy and hydrocarbons. An important tool established in recent years are hydro-mechanical models which provide a useful alternative to experimental methods determining single fracture parameters such as hydraulic aperture. A crucial issue for meaningful numerical modeling is precise imaging of the fracture surfaces to capture geometrical information. Hence, we apply and compare three distinct fracture surface imaging methods: (1) handheld laser scanner (HLS), (2) mounted laser scanner (MLS) and (3) Structure from Motion (SfM) to a bedding plane fracture of sandstone. The imaging reveals that the resolution of the fracture surface obtained from handheld laser scanner (HLS) is insufficient for any numerical simulations, which was therefore rejected. The remaining surfaces are subsequently matched and the resulting fracture dataset is used for detailed fracture flow simulations. The resulting hydraulic aperture is calibrated with laboratory measurements using a handheld air permeameter. The air permeameter data provide a hydraulic aperture of 81 ± 1 µm. For calibration, mechanical aperture fields are calculated using stepwise increasing contact areas up to 15%. At 5% contact area, the average hydraulic aperture obtained by MLS (85 µm) is close to the measurement. For SfM, the measurements are fitted at 7% contact area (83 µm). The flow simulations reveal preferential flow through major channels that are structurally and geometrically predefined. Thus, this study illustrates that resolution and accuracy of the imaging device strongly affect the quality of fluid flow simulations and that SfM provides a promising low-cost method for fracture imaging on cores or even outcrops. Highlights: We evaluate a handheld laser scanner, a mounted laser scanner and the Structure from Motion method for imaging of fracture surfaces. We use the scanned geometries for flow simulations and calibrate the results with air permeameter measurements. Our study reveals that the handheld laser scanner is unsuitable for fracture surface imaging due to low resolution and accuracy. The simulations performed with geometries obtained by the mounted laser scanner and the Structure from Motion method provided comparable results. The Structure from Motion method seems to be promising for fracture investigations in field applications due to the high flexibility and low costs. [ABSTRACT FROM AUTHOR]

Characterizing groundwater responses to natural drivers is cost effective and offers great potential in hydrogeological investigations. However, there is a lack of method development and evaluation, for example by comparing results with those derived from using conventional methods. This paper presents a modified method to calculate the hydraulic conductivity (K) of confined aquifers using the well water response to atmospheric tides. The approach separates the Earth and atmospheric tide influences on filtered well water-level records in the time domain. The resulting ill-posed regression deconvolution problem can be overcome by constraining the well water response to atmospheric tides in order to follow a physically realistic semi-diurnal barometric response function (S2-BRF), or to follow directly a modified hydraulic model (BE-Hvorslev) similar to a slug test evaluation. An analysis with synthetic data shows that K up to 10-4 m/s can be estimated when pressure records with short sampling intervals are available. Application to a field dataset from Cambodia and Benin, with 20-minute to 60-minute sampling intervals, respectively, results in K values of 5.82∙10-7 m/s and 2.9·10-7 m/s. This agrees with results independently derived from pumping tests for both confined sediments and semi-confined hard-rock conditions. This method offers a promising and low-cost approach to derive K solely from monitoring datasets in confined aquifers. This is especially advantageous for low-conductivity formations where hydraulic testing takes time. [ABSTRACT FROM AUTHOR]

Thermal dispersion, caused by fluid velocity and temperature fluctuations in the pore space and the effects of hydrodynamic mixing on the temperature field, controls convective heat transport in saturated porous media. While the thermal dispersion coefficient, a governing parameter in the thermal equilibrium model (TEM), has been investigated for natural systems, the dependence of the thermal dispersion coefficient on particle size remains undetermined. Previous research found that the relationship between the thermal dispersion coefficient and flow velocity follows a power law and that there may be a temperature difference between the solid and fluid phase (thermal non-equilibrium). However, experiments are limited to discrete particle sizes and comparison of the dispersion-velocity relationship is impeded by different experimental approaches. We conducted a series of separate heat and solute transport experiments in a column filled with uniform porous media consisting of different sized glass spheres for a range of flow velocities. The thermal and solute dispersion coefficients obtained from experimental measurements were correlated with flow velocities through the thermal or solute Péclet number. Our results demonstrate that, while solute dispersion is independent of particle size, the dependence of the TEM based thermal dispersion coefficient on flow rates is influenced by the particle size. This is caused by the fact that, unlike solute transport, heat exchanges between fluid and particles and that this induces thermal non-equilibrium between both phases. The results have significant implications for quantifying forced convective heat transport in natural porous media because thermal non-equilibrium between the phases is not considered. The porous media particle size must be considered when selecting appropriate values for the thermal dispersion coefficient. [ABSTRACT FROM AUTHOR]

A novel semi-analytical model for the calculation of water saturation levels in the near subsurface using passive temperature measurements is derived. The amplitude and phase of dominant natural diel temperature variations are exploited, although the solution is general so that a cyclical temperature signal of any period could be used. The model is based on the first-principles advection-conduction-dispersion equation, which is fully general for porous media. It requires a single independent soil moisture estimate, but directly considers the spatially variable saturation dependency of thermal conductivity which has been avoided in previous studies. An established empirical model for the thermal conductivity of variably saturated porous media is incorporated and two solutions for saturation are derived. Using data from numerical models, a spatially sequential implementation of one of these solutions is shown to predict the vertical saturation profile to within 2% for a hydraulically stable case and to within the saturation range observed over a single day for percolation rates up to 10 cm/day. The developed model and methodology can aid in the analysis of archived temperature data from the vadose zone and will serve as a powerful tool in future heat-tracing experiments in variably saturated conditions. [ABSTRACT FROM AUTHOR]

• We conduct transport experiment to test local thermal equilibrium (LTE) assumption. • The LTE assumption can be violated under natural groundwater flow conditions. • Violation of LTE significantly impacts mechanical thermal dispersion. • Effects of thermal disequilibrium on velocity estimates were negligible. • Modelling heat transport requires consideration of LTE condition. When modelling heat transport in hydrogeological systems, a standard assumption is local thermal equilibrium (LTE), which implies that the porous medium temperature at the interface between the solid and fluid instantaneously reaches equilibrium. Few studies have investigated the validity of the LTE assumption and its violation, also known as local thermal non-equilibrium (LTNE), as well as its impact on the accuracy of heat transport models. While the theoretical conditions under which LTNE occurs in natural groundwater flow have recently been revealed, these have not yet been experimentally verified. We examined the applicability of the LTE assumption by conducting systematic laboratory experiments using eight distinct flow velocities (Reynolds number, Re < 0.37) through sand (0.76 mm grain size), injecting both heat and solute as tracers and observing the response at multiple points downstream. Theoretical heat transport parameters were calculated from solute tracer experiments by applying a retardation factor, and the results were subsequently compared to the actual parameters estimated from heat tracing. Our experimental results confirm that the LTE assumption can be violated under natural groundwater flow conditions, and that LTNE impacts thermal dispersion coefficients more significantly than thermal front velocities. The largest differences between the predicted and estimated thermal transport parameters were 112% and 13% for the dispersion coefficients and velocities, respectively. Our results demonstrate that LTNE effects need to be considered in heat transport modelling especially when analyzing thermal dispersion coefficients. [ABSTRACT FROM AUTHOR]

• We synthesise interdisciplinary knowledge about Australian non-perennial rivers. • Current research strengths include salinity, dryland ecology, droughts, and floods. • Lessons are relevant globally and four high-level research challenges are identified. Non-perennial rivers are valuable water resources that support millions of humans globally, as well as unique riparian ecosystems. In Australia, the Earth's driest inhabited continent, over 70% of rivers are non-perennial due to a combination of ancient landscape, dry climates, highly variable rainfall regimes, and human interventions that have altered riverine environments. Here, we review Australian non-perennial river research incorporating geomorphology, hydrology, biogeochemistry, ecology, and Indigenous knowledges. The dominant research themes in Australia were drought, floods, salinity, dryland ecology, and water management. Future research will likely follow these themes but must address emerging threats to river systems due to climate change and other anthropogenic impacts. Four high level opportunities for future research are identified, namely: (1) integrating Indigenous and western scientific knowledge; (2) quantifying climate change impacts on hydrological and biological function; (3) clarifying the meaning and measurement of "restoration" of non-perennial systems; and (4) understanding the role of groundwater. These challenges will require inter- and multi-disciplinary efforts supported by technological advances. The evolving body of knowledge about Australian rivers provides a foundation for comparison with other dryland areas globally where recognition of the importance of non-perennial rivers is expanding. [ABSTRACT FROM AUTHOR]

Groundwater responses to barometric pressure fluctuations are characterized using the concept of barometric efficiency (BE). For semiconfined and confined aquifers, BE values can be used to provide efficient, low‐cost estimates of specific storage. This study compares, for the first time, eight existing methods of BE estimation. Comparisons were undertaken using data from the Peel region of Western Australia. Fourier analysis and regression deconvolution methods were used to estimate aquifer confinement status. The former approach was found to be robust and provided a quantitative basis for spatial comparisons of the degree of confinement. The latter approach was confounded by the presence of diurnal and/or semidiurnal signals. For wells at which semiconfined or confined responses were identified, frequency and time domain methods were used to estimate BE values. Most BE estimation methods were similarly confounded by diurnal and/or semidiurnal signals, with the exception of the Acworth et al. (2016) method. Specific storage values calculated from BE values were order‐of‐magnitude consistent with the results of four historical pumping tests. The methods implemented in this research provide efficient, low‐cost alternatives to hydraulic testing for estimating aquifer confinement, as well as the BE and specific storage of semiconfined and confined aquifers. The frequency and duration of observations required by these methods are minimal; for example, typically requiring a minimum of four observations per day over a four month period. In some locations they may allow additional insights to be derived from existing groundwater hydrograph data. Article impact statement: Existing methods of quantifying barometric efficiency were compared and used to estimate aquifer specific storage. [ABSTRACT FROM AUTHOR]

Ephemeral and intermittent flow in dryland streams plays a vital role in supporting delicate ecosystems and recharging groundwater in underlying aquifers. Despite extensive research, accurately predicting surface runoff, streambed water infiltration, and groundwater recharge during ephemeral streamflow events remains challenging due to the complex physical processes involved. Such processes are logistically difficult to observe in the field and should not be assessed through numerical studies alone. To tackle this complexity, we utilized an 8 m long, 0.25 m wide, and 0.6 m deep laboratory flume filled with fine sand as a physical model for event flows along a dry stream. This flume allowed us to measure both streamflow and streambed infiltration volumetrically as the stream wetting front advanced along its length during 8 steady inflow rates. The flume was divided into separate compartments to measure water across the sediment surface (streamflow) and water exiting through the flume's bottom (groundwater recharge). To validate our physical tests, we compared them with simulated flows in an integrated surface–subsurface hydrological numerical model. The model results exhibited good agreement with the observed stream and groundwater dynamics. Through a parametric numerical investigation, we identified that hydraulic conductivity significantly controlled the rate of surface infiltration and the advancement of the stream wetting front. Interestingly, we found a linear relationship between surface discharge and stream wetting front location, as well as groundwater recharge, under steady-state conditions, in line with the governing equations. However, our simulations of variable streambed heterogeneity and elevation unveiled the crucial influence of these factors in determining the timing of downstream flow progression. This insight is essential for establishing ecological connectivity along non-perennial rivers and has significant implications for delivering environmental flows in arid regions where ephemeral rivers are prevalent. Our study underscores the value of flume experiments in elucidating the transient physical processes during surface water and groundwater interactions in intermittent streams. By employing a combination of physical modelling and numerical simulations, we advance our understanding of these vital hydrological interactions, contributing to sustainable water management practices in semi-arid and arid environments. • Flume experiments demonstrated stream and groundwater dynamics during ephemeral flood wave advancement. • Location of the stream wetting front is linearly proportional to stream discharge. • Stream wetting front arrival is sensitive to depression storage and sediment heterogeneity. • Parametric analysis identified hydraulic conductivity as the primary control on stream wetting front. [ABSTRACT FROM AUTHOR]

Measurement of glycerol dialkyl glycerol tetraethers (GDGTs) preserved in speleothems offers a potential proxy for past temperature but, in general, their origin is unknown. To understand the source of speleothem GDGTs, we undertook an irrigation experiment to activate drip sites within a hydrogeochemically well characterised cave. The cave drip water was analysed for GDGTs, inorganic elements (major ions and trace elements), stable isotopes and dissolved organic matter concentration and character. Published speleothem GDGT records from the site have been observed to be dominated by isoprenoid GDGTs and interpreted as deriving from in situ microbial communities within the cave or vadose zone. The drip water in our irrigation experiment had a GDGT distribution distinct from that of soil and speleothem samples, providing direct evidence that the distinctive GDGT signature in speleothems is derived from a subsurface source. Analysis of GDGTs in this context allowed further elucidation of their source and transport in cave systems, enhancing our understanding of how they might be used as a temperature proxy. [ABSTRACT FROM AUTHOR]

Oxygen isotope ratios in speleothems may be affected by external processes that are independent of climate, such as karst hydrology and kinetic fractionation. Consequently, there has been a shift towards characterising and understanding these processes through cave monitoring studies, particularly focussing on temperate zones where precipitation exceeds evapotranspiration. Here, we investigate oxygen isotope systematics at Wellington Caves in semi-arid, SE Australia, where evapotranspiration exceeds precipitation. We use a novel D 2 O isotopic tracer in a series of artificial irrigations, supplemented by pre-irrigation data comprised four years of drip monitoring and three years of stable isotope analysis of both drip waters and rainfall. This study reveals that: (1) evaporative processes in the unsaturated zone dominate the isotopic composition of drip waters; (2) significant soil zone ‘wetting up’ is required to overcome soil moisture deficits in order to achieve infiltration, which is highly dependent on antecedent hydro-climatic conditions; (3) lateral flow, preferential flow and sorption in the soil zone are important in redistributing subsurface zone water; (4) isotopic breakthrough curves suggest clear evidence of piston-flow at some drip sites where an older front of water discharged prior to artificial irrigation water; and (5) water residence times in a shallow vadose zone (<2 m) are highly variable and can exceed six months. Oxygen isotope speleothem records from semi-arid regions are therefore more likely to contain archives of alternating paleo-aridity and paleo-recharge, rather than paleo-rainfall e.g. the amount effect or mean annual. Speleothem-forming drip waters will be dominated by evaporative enrichment, up to ∼3‰ in the context of this study, relative to precipitation-weighted mean annual rainfall. The oxygen isotope variability of such coeval records may further be influenced by flow path and storage in the unsaturated zone that is not only drip specific but also influenced by internal cave climatic conditions, which may vary spatially in the cave. [ABSTRACT FROM AUTHOR]

Cathedral Cave, Wellington, Australia, is a natural laboratory for studying water movement and geochemical processes in the unsaturated zone by using artificial irrigation to activate drip sites within the cave. Water sampled from two drip sites activated by irrigations carried out in summer 2014 was analysed for dissolved inorganic ions and fluorescent organic matter. The analysis allowed the development of a conceptual flow path model for each drip site. DOM analysis was further complemented by liquid chromatography with organic carbon detection (LC-OCD), applied for the first time to karst drip waters, allowing the characterisation of six organic matter fractions. The differences in organic matter fractions at each drip site are interpreted as a signature of the proposed flow paths. LC-OCD was also compared with parallel factor analysis (PARAFAC) of the fluorescence and good correlations were observed for high molecular weight organic matter. Strong positive correlations were also observed for high molecular weight matter and Cu and Ni. This is suggestive of colloidal transport of Cu and Ni by organic matter with high molecular weight, while small molecular weight colloids were not efficient transporters. LC-OCD uniquely provides information on non-fluorescent organic matter and can be used to further quantify drip water organic matter composition. [ABSTRACT FROM AUTHOR]

Abstract: A series of four short-term infiltration experiments which revealed hydrochemical responses relevant to semi-arid karst environments were carried out above Cathedral Cave, Wellington, New South Wales (NSW), Australia. Dripwater samples were collected at two sites for trace element and organic matter analysis. Organic matter was characterised using fluorescence and interpreted using a PARAFAC model. Three components were isolated that represented unprocessed, soil-derived humic-like and fulvic-like material, processed humic/fulvic-like material and tryptophan-like fluorescence. Principal Component Analysis (PCA) performed on the entire dataset comprising trace element concentrations and PARAFAC scores revealed two dominant components that were identified as soil and limestone bedrock. The soil component was assigned based on significant contributions from the PARAFAC scores and additionally included Ba, Cu, Ni and Mg. The bedrock component included the expected elements of Ca, Mg and Sr as well as Si. The same elemental behaviour was observed in recent stalagmite growth collected from the site. Our experiments demonstrate that existing paleoclimate interpretations of speleothem Mg and Sr, developed in regions of positive water balance, are not readily applicable to water limited environments. We provide a new interpretation of trace element signatures unique to speleothems from water limited karst environments. [Copyright &y& Elsevier]