Glenn A. Harrington

The importance of silicate weathering of a sedimentary aquifer in arid Central Australia indicated by very high 87Sr/86Sr ratios

With the exception of brine systems and crystalline-rock aquifers, groundwater 87Sr/86Sr ratios rarely differ by more than ±0.01 from the Phanerozoic seawater value of 0.70923. Groundwater 87Sr/86Sr ratios from Central Australia (0.72562–0.76248) are the highest ever recorded for a low salinity, sedimentary aquifer. We propose a model for the evolution of these 87Sr/86Sr ratios and demonstrate how the lowest 87Sr/86Sr ratios reflect water–rock interactions with predominantly carbonate minerals, while the highest 87Sr/86Sr ratios are ultimately controlled by weathering of old primary silicate minerals. Detailed analysis of aquifer core material identified albite and microcline as the most prevalent primary silicates and where these minerals were abundant, relatively high Rb concentrations were measured. High Rb combined with long time scales since deposition has produced very high 87Sr/86Sr ratios in the rocks and subsequently the groundwater. Therefore, strontium isotopes can provide further constraints for chemical models of arid zone groundwater systems, particularly when silicate hydrolysis is believed to significantly contribute to dissolved solute compositions.

Chemistry of groundwater discharge inferred from longitudinal river sampling

We present an approach for identifying groundwater discharge chemistry and quantifying spatially distributed groundwater discharge into rivers based on longitudinal synoptic sampling and flow gauging of a river. The method is demonstrated using a 450 km reach of a tropical river in Australia. Results obtained from sampling for environmental tracers, major ions, and selected trace element chemistry were used to calibrate a steady state one-dimensional advective transport model of tracer distribution along the river. The model closely reproduced river discharge and environmental tracer and chemistry composition along the study length. It provided a detailed longitudinal profile of groundwater inflow chemistry and discharge rates, revealing that regional fractured mudstones in the central part of the catchment contributed up to 40% of all groundwater discharge. Detailed analysis of model calibration errors and modeled/measured groundwater ion ratios elucidated that groundwater discharging in the top of the catchment is a mixture of local groundwater and bank storage return flow, making the method potentially useful to differentiate between local and regional sourced groundwater discharge. As the error in tracer concentration induced by a flow event applies equally to any conservative tracer, we show that major ion ratios can still be resolved with minimal error when river samples are collected during transient flow conditions. The ability of the method to infer groundwater inflow chemistry from longitudinal river sampling is particularly attractive in remote areas where access to groundwater is limited or not possible, and for identification of actual fluxes of salts and/or specific contaminant sources.

Comparing vertical profiles of natural tracers in the Williston Basin to estimate the onset of deep aquifer activation

Comparing high-resolution depth profiles of different naturally occurring environmental tracers in aquitards should yield consistent and perhaps complementary information about solute transport mechanisms and the timing of major hydrogeological and climatological events. This study evaluated whether deep, continuous profiles of aquitard pore water chloride concentration could provide further insight into the paleohydrology of the Williston Basin, Canada, than possible using high-resolution depth profiles of stable H/O isotopes of water (δ18O, δ2H). Pore water samples were obtained from extracts of cores taken over 392 m of the thick Cretaceous shale aquitard. Water samples were also collected from wells installed in the underlying regional sandy aquifer (Mannville Group; 93 m thick) and from seepage inflows into potash mine shafts (to 825 m below ground). Numerical modeling of the 1-D vertical Cl− profile supported diffusion dominated solute transport in the shales. The modeling also showed a similar time frame for development of the Cl− profile prior to activation of the aquifer as determined from the δ18O profile (20–25 Ma); however, it provided a significantly longer and potentially better-constrained time frame for evolution of the profile during the activation phase of the aquifer (0.5–1 Ma). The dominant paleoevent reflected in present-day profiles of both tracers is the introduction of glaciogenic meteoric water to the Mannville aquifer underlying the shale during the Pleistocene. The source area of this water remains to be determined.

Influence of Seasonal Variations in Sea Level on the Salinity Regime of a Coastal Groundwater–Fed Wetland

Seasonal variations in sea level are often neglected in studies of coastal aquifers; however, they may have important controls on processes such as submarine groundwater discharge, sea water intrusion, and groundwater discharge to coastal springs and wetlands. We investigated seasonal variations in salinity in a groundwater-fed coastal wetland (the RAMSAR listed Piccaninnie Ponds in South Australia) and found that salinity peaked during winter, coincident with seasonal sea level peaks. Closer examination of salinity variations revealed a relationship between changes in sea level and changes in salinity, indicating that sea level-driven movement of the fresh water-sea water interface influences the salinity of discharging groundwater in the wetland. Moreover, the seasonal control of sea level on wetland salinity seems to override the influence of seasonal recharge. A two-dimensional variable density model helped validate this conceptual model of coastal groundwater discharge by showing that fluctuations in groundwater salinity in a coastal aquifer can be driven by a seasonal coastal boundary condition in spite of seasonal recharge/discharge dynamics. Because seasonal variations in sea level and coastal wetlands are ubiquitous throughout the world, these findings have important implications for monitoring and management of coastal groundwater-dependent ecosystems.

Estimating seepage flux from ephemeral stream channels using surface water and groundwater level data

Seepage flux from ephemeral streams can be an important component of the water balance in arid and semiarid regions. An emerging technique for quantifying this flux involves the measurement and simulation of a flood wave as it moves along an initially dry channel. This study investigates the usefulness of including surface water and groundwater data to improve model calibration when using this technique. We trialed this approach using a controlled flow event along a 1387 m reach of artificial stream channel. Observations were then simulated using a numerical model that combines the diffusion-wave approximation of the Saint-Venant equations for streamflow routing, with Philips infiltration equation and the groundwater flow equation. Model estimates of seepage flux for the upstream segments of the study reach, where streambed hydraulic conductivities were approximately 101 m d−1, were on the order of 10−4 m3 d−1 m−2. In the downstream segments, streambed hydraulic conductivities were generally much lower but highly variable (∼10−3 to 10−7 m d−1). A Latin Hypercube Monte Carlo sensitivity analysis showed that the flood front timing, surface water stage, groundwater heads, and the predicted streamflow seepage were most influenced by specific yield. Furthermore, inclusion of groundwater data resulted in a higher estimate of total seepage estimates than if the flood front timing were used alone.

Influence of Groundwater Hydraulic Gradient on Bank Storage Metrics

The hydraulic gradient between aquifers and rivers is one of the most variable properties in a river/aquifer system. Detailed process understanding of bank storage under hydraulic gradients is obtained from a two-dimensional numerical model of a variably saturated aquifer slice perpendicular to a river. Exchange between the river and the aquifer occurs first at the interface with the unsaturated zone. The proportion of total water exchanged through the river bank compared to the river bed is a function of aquifer hydraulic conductivity, partial penetration, and hydraulic gradient. Total exchange may be estimated to within 50% using existing analytical solutions provided that unsaturated zone processes do not strongly influence exchange. Model-calculated bank storage is at a maximum when no hydraulic gradient is present and increases as the hydraulic conductivity increases. However, in the presence of a hydraulic gradient, the largest exchange flux or distance of penetration does not necessarily correspond to the highest hydraulic conductivity, as high hydraulic conductivity increases the components of exchange both into and out of an aquifer. Flood wave characteristics do not influence ambient groundwater discharge, and so in large floods, hydraulic gradients must be high to reduce the volume of bank storage. Practical measurement of bank storage metrics is problematic due to the limitations of available measurement technologies and the nested processes of exchange that occur at the river-aquifer interface. Proxies, such as time series concentration data in rivers and groundwater, require further development to be representative and quantitative.

Constraining spatial variability in recharge and discharge in an arid environment through modeling carbon‐14 with improved boundary conditions

Carbon-14 (14C) has been widely used to estimate groundwater recharge rates in arid regions, and is increasingly being used as a tool to assist numerical model calibration. However, lack of knowledge on 14C inputs to groundwater potentially limits its reliability for constraining spatial variability in recharge. In this study, we use direct measurements of 14C in the unsaturated zone to develop a 14C input map for a regional scale unconfined aquifer in the Ti Tree Basin in central Australia. The map is used as a boundary condition for a 3-D groundwater flow and solute transport model for the basin. The model is calibrated to both groundwater 14C activity and groundwater level, and calibration is achieved by varying recharge rates in 18 hydrogeological zones. We test the sensitivity of the calibration to both the 14C boundary condition, and the number or recharge zones used. The calibrated recharge rates help resolve the conceptual model for the basin, and demonstrate that spatially distributed discharge (through evapotranspiration) is an important part of the water balance. This approach demonstrates the importance of boundary conditions for 14C transport modeling (14C input activity), for improving estimates of spatial variability in recharge and discharge.

Helium equilibrium between pore water and quartz: a clever, but limited tool

The partitioning of Fe in sediments and soils has traditionally been studied by applying sequential leaching methods. These are based on reductive dissolution and exploit differences in dissolution rates between different reactive Fe (oxyhydr)oxide minerals. We used lab-made ferrihydrite, goethite, hematite and magnetite spiked with 58Fe and leached two-mineral mixtures with both phases abundant in excess of the methods dissolution capacity. Leaching was performed with 1) hydroxylamine-HCl and 2) Na-dithionite as the reactive agent. Following Poulton & Canfield (2005) [1], the first dissolution is designed to selectively leach the most reactive Fe-phases, ferrihydrite and lepidocrocite, whereas the second dissolution is designed to leach goethite and hematite. Magnetite would then be dissolved in a third dissolution step with oxalic acid. First results show that the hydroxylamine-HCl method for ferrihydrite dissolves only insignificant amounts of goethite and hematite. However, magnetite-Fe constitutes about 10% of the total dissolved Fe. The Na-dithionite dissolved Fe from goethite-magnetite and hematite-magnetite mixtures contain about 30% of magnetite-Fe. We applied selective sequential leaching and Fe isotope analysis to fine-grained marine sediments from a depocenter in the North Sea, which contain abundant reactive Fe (oxyhydr)oxides and show evidence for Fe sulfide formation within the upper 10 cm. Fe isotopes of the hydroxylamine-HCl leach targeting ferrihydrite shows a downcore increase of !56Fe typical for sediments undergoing microbial reductive Fe dissolution, whereas Fe isotopes of the Na-dithionite leach (goethite and hematite) and oxalic acid leach (magnetite) are identical and show no downcore variation in !56Fe. This means, that only the most reactive Fe phases participate in the Fe redox cycle in this location. The similar isotopic composition of goethite + hematite and magnetite suggests a detrital source, which is not utilized possibly due to the abundant ferrihydrite and lepidocrocite present. [1] Poulton & Canfield (2005), Chemical Geology 214, 209– 221Seasonal Methane Fluxes and Sulfate Reduction Rates in a Eutrophied Baltic Estuarine System

REGIONAL, RECONNAISSANCE SCALE AEM SURVEYS TO BETTER DEFINE SURFACE WATER - GROUNDWATER PROCESSES BENEATH LARGE UNREGULATED RIVER SYSTEMS.

The groundwater resources in northern Australia offer the greatest potential for future irrigation development and are a potential source for water supply to other regions of Australia. The Fitzroy River with its associated alluvial aquifer in northern Western Australia has been considered in planning for Perth’s future water supply and for agricultural development but its potential needs to be informed by detailed understanding of groundwater-surface water interactions occurring along its extent, and in particular must consider and account for the consequences that might arise when extracting groundwater from shallow and deep aquifers linked to this river system. While the Fitzroy is one of Australia’s largest unregulated rivers characterised by braiding channels within a wide floodplain, knowledge concerning the extent and significance of its floodplain storage is limited. This paper considers results from the analysis and interpretation of a regional scale longitudinal transect (~274 line kms) of SkyTEM helicopter EM data to help elucidate river-bed processes occurring along its extent. The AEM data were acquired to provide an indication of the variation in groundwater quality and related aquifer characteristics associated with different parts of the river. Conductivity-depth sections derived from their inversion using a 1D spatially constrained inversion (SCI), were examined against available hydrochemical, environmental tracer (including 222 Rn and Cl-), and hydrogeological data sampled longitudinally. They provided further insight into the Fitzroy’s alluvial aquifer system and its links with the underlying Canning Basin sediments. The results demonstrate the value of regional, reconnaissance scale AEM surveys to better define groundwater processes beneath large unregulated river systems.