Investigation ofδ18Oandδ2Hin the Namoi River catchment—elucidating recharge sources and the extent of sur-face water/groundwater interaction

Stable isotopes 18O and 2H were analysed in water samples from rainfall, surface water and groundwater within the semi-arid Namoi River catchment in NSW, Australia.The isotopic composition of rainfall events and groundwater samples plot along the Local Meteoric Water Line (LMWL). In contrast, the surface water samples of the Namoi River clearly show signs of evaporative enrichment and plot on a Local Evaporation Line (LEL) constructed for the area based on δ18O and δ2H time-series for surface waters of the Namoi River. The river samples have a distinctly lower slope than the LMWL which is due to evaporation. Shallow groundwater near the Namoi River shows considerable enrichment compared to average groundwater signatures and plots in between the LMWL and the LEL on a δ2H vs. δ18O graph. These results clearly indicate that the Namoi River is recharging the shallow aquifer system. Conversely, the isotopic composition of surface water in the tributaries of Maules and Horsearm creeks are similar to groundwater indicating that these creeks are receiving groundwater discharge. This study reveals many complex hydrological processes occurring in the catchment. It would not have been possible to elucidate these processes without the use of stable isotope data

Vertical hydraulic conductivity of a clayey-silt aquitard: accelerated fluid flow in a centrifuge permeameter compared with in situ conditions

This discussion paper is a preprint. It has been under review for the journal Hydrology and Earth System Sciences (HESS). The revised manuscript was not accepted.Evaluating the possibility of leakage through low permeability geological strata is critically important for sustainable water supplies, extraction of fuels from strata such as coal beds, and confinement of waste within the earth. Characterizing low or negligible flow rates and transport of solutes can require impractically long periods of field or laboratory testing, but is necessary for evaluations over regional areas and over multi-decadal timescales. The current work reports a custom designed centrifuge permeameter (CP) system, which can provide relatively rapid and reliable hydraulic conductivity (K) measurement compared to column permeameter tests at standard gravity (1g). Linear fluid velocity through a low K porous sample is linearly related to g-level during a CP flight unless consolidation or geochemical reactions occur. The CP module is designed to fit within a standard 2 m diameter, geotechnical centrifuge with a capacity for sample dimensions of 30 to 100 mm diameter and 30 to 200 mm in length. At maximum RPM the resultant centrifugal force is equivalent to 550g at base of sample or a total stress of ~2 MPa. K is calculated by measuring influent and effluent volumes. A custom designed mounting system allows minimal disturbance of drill core samples and a centrifugal force that represents realistic in situ stress conditions is applied. Formation fluids were used as influent to limit any shrink-swell phenomena which may alter the resultant K value. Vertical hydraulic conductivity (Kv) results from CP testing of core from the sites in the same clayey silt formation varied (10−7 to 10−9 m s−1, n = 14) but higher than 1g column permeameter tests of adjacent core using deionized water (10−9 to 10−11 m s−1, n = 7). Results at one site were similar to in situ Kv values (3 × 10−9 m s−1) from pore pressure responses within a 30 m clayey sequence in a homogenous area of the formation. Kv sensitivity to sample heterogeneity was observed, and anomalous flow via preferential pathways could be readily identified. Results demonstrate the utility of centrifuge testing for measuring minimum K values that can contribute to assessments of geological formations at large scale. The importance of using realistic stress conditions and influent geochemistry during hydraulic testing is also demonstrated.Australian Research CouncilNational Water Commissio

Accelerated gravity testing of aquitard core permeability and implications at formation and regional scale

Evaluating the possibility of leakage through low-permeability geological strata is critically important for sustainable water supplies, the extraction of fuels from coal and other strata, and the confinement of waste within the earth. The current work demonstrates that relatively rapid and realistic vertical hydraulic conductivity (Kv) measurements of aquitard cores using accelerated gravity can constrain and compliment larger-scale assessments of hydraulic connectivity. Steady-state fluid velocity through a low-K porous sample is linearly related to accelerated gravity (g level) in a centrifuge permeameter (CP) unless consolidation or geochemical reactions occur. A CP module was custom designed to fit a standard 2 m diameter geotechnical centrifuge (550 g maximum) with a capacity for sample dimensions up to 100 mm diameter and 200 mm length, and a total stress of  ∼  2 MPa at the base of the core. Formation fluids were used as influent to limit any shrink–swell phenomena, which may alter the permeability. Kv results from CP testing of minimally disturbed cores from three sites within a clayey-silt formation varied from 10−10 to 10−7  m s−1 (number of samples, n = 18). Additional tests were focussed on the Cattle Lane (CL) site, where Kv within the 99 % confidence interval (n = 9) was 1.1 × 10−9 to 2.0 × 10−9 m s−1. These Kv results were very similar to an independent in situ Kv method based on pore pressure propagation though the sequence. However, there was less certainty at two other core sites due to limited and variable Kv data. Blind standard 1 g column tests underestimated Kv compared to CP and in situ Kv data, possibly due to deionised water interactions with clay, and were more time-consuming than CP tests. Our Kv results were compared with the set-up of a flow model for the region, and considered in the context of heterogeneity and preferential flow paths at site and formation scale. Reasonable assessments of leakage and solute transport through aquitards over multi-decadal timescales can be achieved by accelerated core testing together with complimentary hydrogeological monitoring, analysis, and modelling

Terrestrial water load and groundwater fluctuation in the Bengal Basin

Groundwater-level fluctuations represent hydraulic responses to changes in groundwater storage due to aquifer recharge and drainage as well as to changes in stress that include water mass loading and unloading above the aquifer surface. The latter ‘poroelastic’ response of confined aquifers is a well-established phenomenon which has been demonstrated in diverse hydrogeological environments but is frequently ignored in assessments of groundwater resources. Here we present high-frequency groundwater measurements over a twelve-month period from the tropical, fluvio-deltaic Bengal Aquifer System (BAS), the largest aquifer in south Asia. The groundwater level fluctuations are dominated by the aquifer poroelastic response to changes in terrestrial water loading by processes acting over periods ranging from hours to months; the effects of groundwater flow are subordinate. Our measurements represent the first direct, quantitative identification of loading effects on groundwater levels in the BAS. Our analysis highlights the potential limitations of hydrogeological analyses which ignore loading effects in this environment. We also demonstrate the potential for employing poroelastic responses in the BAS and across other tropical fluvio-deltaic regions as a direct, in-situ measure of changes in terrestrial water storage to complement analyses from the Gravity and Climate Experiment (GRACE) mission but at much higher resolution

Propagation of pressure change through thick clay sequences: an example from Liverpool Plains, NSW, Australia

In-situ hydraulic conductivity and specific storage measurements are derived from an analysis of pore-water pressure changes in a nest of piezometers installed in a 40-m-thick succession of smectitic clay on the Liverpool Plains of northern New South Wales, Australia. The cumulative response to the rainfall events that typically occurs during winter or early spring is propagated through the clay with measurable loss of amplitude and increasing phase lag. Five major rainfall events occurred over the four years of detailed monitoring. The phase lag at the base of the clay varied between 49 and 72 days. Barometric efficiency (BE) measurements for the clay sequence (BE = 0.07) and the underlying confined aquifer (BE = 0.10) were used, with a known porosity of 0.567, to derive specific storage values of 3.7x10(-5) and 6.8x10(-6) m(-1) respectively. Vertical hydraulic conductivity (K-v) of the clay sequence derived from observed amplitude and phase changes, resulted in an average value of 2.8x10(-9) m/s. These in-situ-derived values indicate that previous estimates of vertical hydraulic conductivity of the clays, made on core samples, are unrealistically high. The instantaneous response to individual rainfall events transmitted through the clay succession (tidal efficiency of 0.93) is also described

A radioisotope tracer study of estuarine goundwater movement of the Eastern Australian coast

Groundwater discharge to coastal waters, and the associated transport of nutrients and contaminants, is believed to have a significant impact on coastal ecosystems. However, complex boundary conditions and rapidly changing short-term fluxes make net flows resulting from local or regional groundwater discharge difficult to quantify. Such boundary conditions include beach face wave runup and storm setup, sub-surface tidal forcing, frequent surface inundation and tidally driven surface/groundwater interactions in estuarine and coastal areas. Tracer techniques can complement hydrological and geochemical studies of such systems and help distinguish longer-term net fluxes from the highly variable short-term fluxes. At Hat Head, NSW, on the eastern Australian coast, a comprehensive study of hydrogeology and hydrogeochemistry has being conducted in an estuarine/coastal sand dune aquifer. A scheme currently under construction will dispose of treated sewage effluent from the small coastal community by injection into the sand dune aquifer. Geochemical and isotopic data from the site indicate a high degree of complexity showing evidence of regional groundwater flow occurring at depth and more localised and highly dynamic conditions in the top 10 m of the aquifer. Tritium data indicate that regional groundwater is modern and stable isotope ratios have been used to distinguish between salt flat and sand dune dominated systems where evaporative and seawater mixing processes are observed. Storm wave setup and beach wave runup have been shown to elevate the water table near the coast leading to flow reversal and potential discharge of effluent to the estuarine zone. A radioisotope tracer study of groundwater flow in response to tidal forcing was conducted adjacent to a tidal creek at Hat Head. Using the short-lived radioisotope conservative tracer, bromine-82, groundwater movement was tracked in-situ over a period of ∼5 days on two occasions encompassing both neap and spring tide conditions. The tracer was injected into a screened borehole and gross gamma counts monitored from an adjacent borehole using a variety of collimated NaI detectors as well as down-hole gamma spectrometry. This technique maps the path of the slow moving tracer without sampling and allows the net groundwater movement to be distinguished from short term tidally driven fluxes. Results from the environmental isotope and radiotracer studies will be presented. © The authors.https://inis.iaea.org/collection/NCLCollectionStore/_Public/34/051/34051744.pd

Constraining water fluxes through the streambed of a semi-arid losing stream using natural tracers: heat and radioisotopes

Natural physical and chemical tracers of flow have different advantages and shortfalls based on their properties and the uncertainty related to variability in their source concentration. Each tracer integrates over a characteristic spatial-temporal scale depending on its decay or production rate and the flow velocity of the system. For instance heat tracing using diurnal temperature fluctuations will, at best, provide information about flow in the upper 1-2 m of the streambed before the signal is dampened below measurement resolution (Constantz et al. 2003). Conversely, radioisotopes used as tracers will integrate over increasing spatio-temporal scales for decreasing decay constants. Radioisotopes with comparatively slow decay rates will be less sensitive for resolving flow conditions on short spatio-temporal scales. Therefore, it is difficult to use these tracers in the streambed of losing systems because the radioactive decay is not discernible against the variability. Consequently, employing a combination of different tracers provides information on different parts of a given flow system. Comparing flow velocities derived from tracers integrating over different scales allows for separating the local hyporheic exchange from the regional groundwater recharge. A field experiment was carried out in a perennial section of the mostly ephemeral Maules Creek in NSW, Australia. Streambed temperature profiles were monitored at three sites along a 400 m stretch of the perennial reach. Streambed temperatures were recorded at 4 depths within one meter below the streambed. Water samples were collected from surface water, streambed and groundwater and analysed for stable water isotopes (18O and 2H) and radioisotopes (222Rn and 3H). The streambed heat profiles provided time series of surface water/groundwater exchange. Using this method it was found that the conditions were losing at all three sites with recharge rates varying between 0 and 0.4 m/d. 222Rn measurements in the surface water along the perennial reach qualitatively identified losing and gaining sections of the stream with low and high 222Rn activities, respectively. One of the losing sections of the stream was instrumented with a transect of groundwater piezometers. In this transect, 3H levels of 1.3-1.5 TU were measured, comparable to surface waters, indicating recent groundwater recharge. However, the variations in 3H combined with the analysis uncertainty did not allow for a recharge estimate. 222Rn with its half-life of only 3.8 d proved more useful. A zone of low 222Rn activity was found as deep as 6-7 m below the stream, corroborating the 3H and temperature data. Regional groundwater 222Rn activities were used to estimate the secular equilibrium activity of Rn. Residence times of 1 to 7 days were calculated based on these estimates. Converted to Darcy velocities of 0.2-1.7 m/d these values generally agree with the velocities derived from the temperature data indicating that the measured fluxes from the temperature data represent recharge rates and not simply hyporheic exchange. © American Geophysical Union

Improved spatial delineation of streambed properties and water fluxes using distributed temperature sensing

A new method was developed for analysing and delineating streambed water fluxes, flow conditions, and hydraulic properties using coiled fibre-optic distributed temperature sensing (FO-DTS) or closely-spaced discrete temperature sensors. This method allows for a thorough treatment of the spatial information embedded in temperature data by creating a matrix visualisation of all possible sensor pairs. Application of the method to a 5-day field dataset reveals the complexity of shallow streambed thermal regimes. To understand how velocity estimates are affected by violations of assumptions of 1-D, saturated, homogeneous flow and to aid in the interpretation of field observations, the method was also applied to temperature data generated by numerical models of common field conditions: horizontal layering, presence of lateral flow and variable streambed saturation. The results show that each condition creates a distinct signature visible in the triangular matrices. The matrices are used to perform a comparison of the behaviour of 1-D analytical heat-tracing models. The results show that the amplitude ratio-based method of velocity calculation leads to the most reliable estimates. The minimum sensor spacing required to obtain reliable velocity estimates with discrete sensors is also investigated using field data. The developed method will aid future heat-tracing studies by providing a technique for visualizing and comparing results from FO-DTS installations and testing the robustness of analytical heat-tracing models

Heat as a tracer to quantify water flow in near-surface sediments

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