Review: Environmental tracers in arid-zone hydrology

Application of environmental tracers to arid-zone hydrology over the past several decades is reviewed, with particular reference to the Australian continent. Some notable successes in the application of stable and radio-isotopes include identifying arid-zone groundwater as palaeowaters, understanding the importance of episodicity and of large flood events to recharge, and delineating sources of water to vegetation. Estimating the rates of recharge and discharge have relied to a large extent on chloride and tritium profiles in the unsaturated zone, while radiocarbon and chlorine-36 are used to estimate horizontal flow rates. A number of new research opportunities are suggested. Improved understanding of processes that modify isotopic signatures at the interface zones such as the upper 5?m of the soil zone, the capillary zone, and the discharge zone, are needed to better quantify water fluxes across these zones. Furthermore, linkages between the atmosphere-soil-water-vegetation continuum although qualitatively understood, elude quantitative transfer to a scale commensurate with basin-scale groundwater management. The new generation of improved and more robust stable isotope and radiometric dating techniques, will be invaluable in advancing the science and its application to better management of meagre water resources in dry parts of the world.     

Local Determination of Weak Anisotropy Parameters from Walkaway VSP qP-Wave Data in the Java Sea Region

We apply the inversion scheme of Zheng and Peník (2002) to the walkaway VSP data of Horne and Leaney (2000) collected in the Java Sea region. The goal is a local determination of parameters of the medium surrounding the borehole receiver array. The inversion scheme is based on linearized equations expressing qP-wave slowness and polarization vectors in terms of weak anisotropy (WA) parameters. It thus represents an alternative approach to Horne and Leaney (2000), who based their procedure on inversion of the Christoffel equation using a global optimization method. The presented inversion scheme is independent of structural complexities in the overburden and of the orientation of the borehole. The inverson formula is local, and has therefore potential to separate effects of anisotropy from effects of inhomogeneity. The data used are components of the slowness vector along the receiver array and polarization vectors. The inversion is performed without any assumptions concerning the remaining components of the slowness vector. The inversion is made (a) assuming arbitrary anisotropy, i.e., without any assumptions about symmetry of the medium, (b) assuming transverse isotropy with a vertical axis of symmetry and (c) assuming isotropy of the medium. Inverted are the raw data as well as data, in which weighting is used to reduce the effect of outliers. It is found that the WA parameters _z, ₁₅ and ₃₅ are considerably more stable than the parameters _x and _x. The latter two parameters are also found to be strongly correlated. Weaker correlation is also found between the mentioned two parameters and _z. The results of inversion show clearly that the studied medium is not isotropic. They also seem to indicate that the studied medium does not possess the VTI symmetry.     

Contrasting mechanisms of salt delivery to the stream from three different landforms in South Eastern Australia

This study explores the pathways of salt and water movement from the landscape to the stream across major landforms, in dryland areas of south eastern Australia. It was conducted at the Livingstone Creek catchment (43 km²) a sub catchment of the Kyeamba catchment, NSW, Australia. An extensive stream salinity field monitoring network between major landforms was developed and data capture occurred from 2002 to 2004. Additional measurements of surface water isotopes were also taken to independently assess responses observed from the detailed monitoring network and assist in determining the sources of water. Flow and salt mass balances were calculated across four gauging stations for each event. The stream monitoring found patterns of salt delivery to streams were consistent during four monitored stream events. In the hill slope and colluvial fill, lower sloped, meta-sediment landforms, stream salinity responses showed the classical salinity response to an event: an initial increase of salinity at the beginning of an event (due to first flush) which then diminished as a consequence of dilution. The main difference between these landforms was that the colluvial fill lower sloped meta-sediments had sodic, low permeability soils near the stream edge. This lead to (1) less variation in stream salinities during event conditions and (2) during low base flow increases in stream salinity occurred as concentrated salts from the stream banks dissolved. For the flatter, alluvial landforms, the salinity response showed quite a different and contrasting temporal pattern: salinity continued to increase and vary directly with flow during events. For all the landforms, base flow salinity increases as flow diminished after a event although salinity responses were more lagged in the alluvial landform. This different salinity pattern in the alluvial landform is attributed to (1) for event flow, the increased contributions of more saline subsurface lateral flow of soil water from the alluvial landform compared to very fresh direct surface runoff sourced from hillslope landforms upstream and (2) for base flow, seepage of near stream alluvial groundwater through the stream banks that was less saline then the base flow water sourced upstream from the hillslope landforms. The stream water isotope values confirm the above findings by showing that, in the alluvial landforms soil water contributions are important during events and that direct surface runoff with little interaction of soil water occurs from the hill slope landforms during events. Conceptual models describing salt and water movement through the different landforms and under different antecedent catchment wetness conditions are presented. These conceptual models develop our understanding of water and solute (salt) pathways through the landscape to the stream. To date, this is one of the few experimental studies in Australia connecting landscape and stream salinisation.     

Monte-Carlo Bayesian look-ahead inversion of walkaway vertical seismic profiles

We describe a method to invert a walkaway vertical seismic profile (VSP) and predict elastic properties (P‐wave velocity, S‐wave velocity and density) in a layered model looking ahead of the deepest receiver. Starting from Bayes's rule, we define a posterior distribution of layered models that combines prior information (on the overall variability of and correlations among the elastic properties observed in well logs) with information provided by the VSP data. This posterior distribution of layered models is sampled by a Monte‐Carlo method. The sampled layered models agree with prior information and fit the VSP data, and their overall variability defines the uncertainty in the predicted elastic properties. We apply this technique first to a zero‐offset VSP data set, and show that uncertainty in the long‐wavelength P‐wave velocity structure results in a sizable uncertainty in the predicted elastic properties. We then use walkaway VSP data, which contain information on the long‐wavelength P‐wave velocity (in the reflection moveout) and on S‐wave velocity and density contrasts (in the change of reflectivity with offset). The uncertainty of the look‐ahead prediction is considerably decreased compared with the zero‐offset VSP, and the predicted elastic properties are in good agreement with well‐log measurements.     

Groundwater–surface water interactions in a large semi‐arid floodplain: implications for salinity management

Flow regulation and water diversion for irrigation have considerably impacted the exchange of surface water between the Murray River and its floodplains. However, the way in which river regulation has impacted groundwater–surface water interactions is not completely understood, especially in regards to the salinization and accompanying vegetation dieback currently occurring in many of the floodplains. Groundwater–surface water interactions were studied over a 2 year period in the riparian area of a large floodplain (Hattah–Kulkyne, Victoria) using a combination of piezometric surface monitoring and environmental tracers (Cl⁻, δ²H, and δ¹⁸O). Despite being located in a local and regional groundwater discharge zone, the Murray River is a losing stream under low flow conditions at Hattah–Kulkyne. The discharge zone for local groundwater, regional groundwater and bank recharge is in the floodplain within ∼1 km of the river and is probably driven by high rates of transpiration by the riparian Eucalyptus camaldulensis woodland. Environmental tracers data suggest that the origin of groundwater is principally bank recharge in the riparian zone and a combination of diffuse rainfall recharge and localized floodwater recharge elsewhere in the floodplain. Although the Murray River was losing under low flows, bank discharge occurred during some flood recession periods. The way in which the water table responded to changes in river level was a function of the type of stream bank present, with point bars providing a better connection to the alluvial aquifer than the more common clay‐lined banks. Understanding the spatial variability in the hydraulic connection with the river channel and in vertical recharge following inundations will be critical to design effective salinity remediation strategies for large semi‐arid floodplains. Copyright © 2005 John Wiley & Sons, Ltd.     

Anisotropic elastic full‐waveform inversion of walkaway vertical seismic profiling data from the Arabian Gulf

Borehole seismic addresses the need for high‐resolution images and elastic parameters of the subsurface. Full‐waveform inversion of vertical seismic profile data is a promising technology with the potential to recover quantitative information about elastic properties of the medium. Full‐waveform inversion has the capability to process the entire wavefield and to address the wave propagation effects contained in the borehole data—multi‐component measurements; anisotropic effects; compressional and shear waves; and transmitted, converted, and reflected waves and multiples. Full‐waveform inversion, therefore, has the potential to provide a more accurate result compared with conventional processing methods. We present a feasibility study with results of the application of high‐frequency (up to 60 Hz) anisotropic elastic full‐waveform inversion to a walkaway vertical seismic profile data from the Arabian Gulf. Full‐waveform inversion has reproduced the majority of the wave events and recovered a geologically plausible layered model with physically meaningful values of the medium.     

Salinization of a fresh palaeo-ground water resource by enhanced recharge

Deterioration of fresh ground water resources caused by salinization is a growing issue in many arid and semi-arid parts of the world. We discuss here the incipient salinization of a 10(4) km2 area of fresh ground water (<3,000 mg/L) in the semiarid Murray Basin of Australia caused by widespread changes in land use. Ground water 14C concentrations and unsaturated zone Cl soil water inventories indicate that the low salinity ground water originated mainly from palaeo-recharge during wet climatic periods more than 20,000 years ago. However, much of the soil water in the 20 to 60 m thick unsaturated zone throughout the area is generally saline (>15,000 mg/L) because of relatively high evapotranspiration during the predominantly semiarid climate of the last 20,000 years. Widespread clearing of native vegetation over the last 100 years and replacement with crops and pastures leads to enhancement of recharge rates that progressively displace the saline soil-water from the unsaturated zone into the ground water. To quantify the impact of this new hydrologic regime, a one-dimensional model that simulates projected ground water salinities as a function of depth to ground water, recharge rates, and soil water salt inventory was developed. Results from the model suggest that, in some areas, the ground water salinity within the top 10 m of the water table is likely to increase by a factor of 2 to 6 during the next 100 years. Ground water quality will therefore potentially degrade beyond the point of usefulness well before extraction of the ground water exhausts the resource.