Wendy Timms

Long-Term Reactive Solute Transport in an Aquitard Using a Centrifuge Model

Characterizing and predicting reactive solute transport in low hydraulic conductivity (K) clay-rich media is challenging because the very long transport time for solutes renders conventional column tests impractical. In this study, a centrifugation technique was developed to assess the transport of a simple aqueous solution (NaCl) by accelerating flow by centrifugal force through low K (1.1 x 10(-11) m/s) core samples. Duplicate cores (52-mm length x 33-mm diameter) were centrifuged at 330 xg for 90 d to model the migration of saline pore water (0.5 M NaCl) under in situ conditions through an approximately 17-m-thick clay prototype over approximately 24,000 years. A PHREEQC one-dimensional reactive solute transport code simulated effluent breakthrough of the NaCl during centrifugation, with best-fit cation exchange coefficients similar to batch tests. The calibrated code was used to predict solute profile development over the long term in the prototype or simulated field-scale conditions. Chromatographic separation of solutes due to ion exchange was evident over several meters in the simulated prototype and the field profile. The applicability of centrifugation methods to predict transport of more complex suites of reactive solutes over the long term is yet to be verified.

Shallow groundwater dynamics in smectite dominated clay on the Liverpool Plains of New South Wales

Dynamic shallow (<5 m) groundwater levels are an important indicator of water and salt fluxes in smectite-dominated clay on the Liverpool Plains in north-eastern New South Wales. Previous hydrogeological assessments of shallow groundwater related salinity risk have focused on regional scale distribution and interaction with rising pressure levels in confined aquifer systems. In this study, groundwater levels over a 7-year period for the saline Yarramanbah subcatchment are presented, along with data from 60 new and existing shallow piezometers and precise elevation surveying and intensive automated monitoring at selected sites. The shallow groundwater system is shown to respond to recharge; however, over the medium-term it is in hydrologic balance, with no evidence of increased water storage. A proportion of recharge is lost by discharge into deeply incised surface channels. Groundwater salinity in the banks of Warrah Creek indicate that flushing of salts from clay is related to increased flux of fresh water. Concern exists that there may be increased salt export from the catchment. If this is in fact occurring while the plains are in hydrologic equilibrium, then increased salt fluxes must be related to factors other than rising groundwater levels.

Evidence for connected water processes through smectite-dominated clays at Breeza, New South Wales

The identification of deep drainage beneath crops and pastures is a major challenge for determining the water balance beneath irrigated crops. This is particularly the case in smectite-dominated clay subject to swelling and cracking. This detailed study at an irrigation site applied hydrogeological and hydrochemical techniques to identify drainage associated with irrigation water sourced from a deep underlying aquifer. The hydrogeology of the site was characterised by permeability (K v) testing of cores, installation of nested piezometers (16, 34 and 54 m depth) and automated logging of groundwater levels, barometric pressure and fluid electrical conductivity (EC). In response to irrigation and rainfall of 390 mm at the surface, a 2.3 m rise in water pressure was observed in the aquifer at 16 m depth during the irrigation season, gradually subsiding thereafter. The barometric efficiency of the P16 piezometer was an estimated as 70 ± 25%, typical of confinement by a low permeability matrix (K v 1.3 × 10−10 to 5.3 × 10−9 m/s). Yet, evidence of fracture flow to a shallow pit and freshening of porewater in the aquifers at 16 and 34 m depth during the irrigation season (ΔEC 1.73 to 1.43 mS/cm and 2.0 to 1.58 mS/cm, respectively), indicated that rapid leakage had occurred. PHREEQC geochemical models indicated that a significant proportion of porewater in the shallow aquifer was replaced by recharge water during the irrigation season, with dissolution of NaCl, precipitation of carbonates, ion exchange and possibly evaporation all shown to be occurring. Clearly, these clays cannot be considered to isolate the aquifer from overlying irrigation and associated contamination.

A reassessment of the Lower Namoi Catchment aquifer architecture and hydraulic connectivity with reference to climate drivers

We demonstrate the need for better representations of aquifer architecture to understand hydraulic connectivity and manage groundwater allocations for the ∼140 m-thick alluvial sequences in the Lower Namoi Catchment, Australia. In the 1980s, an analysis of palynological and groundwater hydrograph data resulted in a simple three-layer stratigraphic/hydrostratigraphic representation for the aquifer system, consisting of an unconfined aquifer overlying two semi-confined aquifers. We present an analysis of 278 borehole lithological logs within the catchment and show that the stratigraphy is far more complex. The architectural features and the net-to-gross line-plot of the valley-filling sequence are best represented by a distributive fluvial system, where the avulsion frequency increases at a slower rate than the aggradation rate. We also show that an improved understanding of past climates contextualises the architectural features observable in the valley-filling sequence, and that the lithofacies distribution captures information about the impact of climate change during the Neogene and Quaternary. We demonstrate the correlation between climate and the vertical lithological succession by correlating the sediment net-to-gross ratio line-plot with the marine benthic oxygen isotope line-plot – a climate change proxy. Pollens indicate that there was a transition from a relatively wet climate in the mid–late Miocene to a drier climate in the Pleistocene, with a continuing drying trend until present. Groundwater is currently extracted from the sand and gravel belts associated with the high-energy wetter climate. However, some of these channel belts are disconnected from the modern river and flood zone. We show that the cutoff between the hydraulically well- and poorly connected portions of the valley-filling sequence matches the connectivity threshold expected from a fluvial system.

Aquifer heterogeneity and response time: the challenge for groundwater management

Abstract. Groundwater is an important contributor to irrigation water supplies. The time lag between withdrawal and the subsequent impacts on the river corridor presents a challenge for water management. We highlight aspects of this challenge by examining trends in the groundwater levels and changes in groundwater management goals for the Namoi Catchment, which is within the Murray–Darling Basin, Australia. The first high-volume irrigation bore was installed in the cotton-growing districts in the Namoi Catchment in 1966. The development of high-yielding bores made accessible a vast new water supply, enabling cotton growers to buffer the droughts. Prior to the development of a groundwater resource it is difficult to accurately predict how the water at the point of withdrawal is hydraulically connected to recharge zones and nearby surface-water features. This is due to the heterogeneity of the sediments from which the water is withdrawn. It can take years or decades for the impact of groundwater withdrawal to be transmitted kilometres through the aquifer system. We present the analysis of both historical and new groundwater level and streamflow data to quantify the impacts of extensive groundwater withdrawals on the watertable, hydraulic gradients within the semi-confined aquifers, and the movement of water between rivers and aquifers. The results highlight the need to monitor the impacts of irrigated agriculture at both the regional and local scales, and the need for additional research on how to optimise the conjunctive use of both surface-water and groundwater to sustain irrigated agriculture while minimising the impact on groundwater-dependent ecosystems.

Late Cenozoic paleovalley fill sequence from the Southern Liverpool Plains, New South Wales—implications for groundwater resource evaluation

The Liverpool Plains in northern New South Wales contain some of the best agricultural land in Australia and are underlain by extensive smectite clay-dominated soils sourced from weathering the alkali basalts of the Liverpool Ranges. It had been thought that a relatively simple geological model explained the underlying Cenozoic sequence with salt-rich clays of the Narrabri Formation overlying sands and gravel aquifers comprising the Gunnedah Formation. Extensive groundwater modelling based upon this simple conceptualisation has been used in management plans proposed by the mining and agricultural industries. A 31.5 m core has been recovered using minimally disturbed triple-tube coring methods at Cattle Lane (Latitude –31.52° S, Longitude 150.47° E) to resolve uncertainty concerning the aquitard status of the upper layer. Recovered core has been examined and tested to determine grainsize, cation-exchange capacity, X-ray diffraction, X-ray fluorescence and microscopic examination of granular components. These measurements complement surface and borehole geophysical techniques, hydrogeological data and hydrochemical analysis of water samples recovered from a series of specially constructed piezometers adjacent to the cored hole. The sequence overlies a Late Cretaceous channel cut into Permian bedrock at 91 m depth with sands and clays below 31.5 m considered to represent various alluvial fill events mostly occurring since the Early Pliocene. Erosion of Late Eocene alkali basalts on the Liverpool Ranges, with the formation of smectite clays, pedogenic carbonates and with the addition of quartz from both eolian sources and locally derived from adjacent Triassic sandstone hills, provides the great majority of the sediment recovered from the cores. Late Pleistocene (114 ka) to Holocene ages were determined for the core from three optically stimulated luminescence (OSL) measurements on fine sands (13, 23 and 29 m BG). Detailed examination has failed to detect any evidence of a boundary between Narrabri and Gunnedah formations revealing rather a gradual change in dominance of clays and silts over sands and gravels embedded in a clay-rich matrix. This result challenges the conceptualisation used to conduct groundwater modelling on the Liverpool Plains.

Origin, lithology and weathering characteristics of Upper Tertiary ‐ Quaternary clay aquitard units on the Lower Murrumbidgee alluvial fan

The Lower Murrumbidgee alluvial fan at the eastern edge of the Murray Basin is comprised of high‐yielding coarse‐grained aquifers and interlayered fine‐grained deposits that exert an important control on recharge and vertical leakage of contaminants such as salt. Concerns over increasingly saline shallow groundwater, particularly in irrigation areas, has focused investigations on the depositional origin and spatial distribution of these fine‐grained deposits (aquitard units), which may constitute both a source of leachable salt and a barrier to leakage. Detailed laboratory analysis of a minimally disturbed core to 83m depth, obtained from a drillhole adjacent to an irrigation bore, was augmented with geophysical investigations from this and other boreholes near the apex of the alluvial fan. Previously mapped clay units (aquitards) are redefined as clayey silts based on clay content variation between 10% and 30%. Mineralogical and lithostratigraphic evidence for three clayey silt units is presented: a lower unit (75–83m), a middle unit (45–64m) and an upper unit (0–16m). Electrical image surveys indicate that the upper unit is discontinuous, interrupted by large palaeodrainage features probably containing sands and gravels. These palaeodrainage channels are buried beneath a veneer of clay and significantly increase recharge and leakage. Some evidence suggests an aeolian component near the surface and within the middle clayey silt unit. However, mixing with fluvial deposits and subsequent weathering has also occurred. The clayey silt units are extensively weathered and oxidised, with the degree of oxidation increasing towards the surface and adjacent to aquifers saturated with oxygenated groundwater. Post‐depositional weathering of the middle and lower units may also have been associated with leaching of salts. No salt remains in the middle and lower units, but 10.2kg/m2 is stored within 15m of the surface at the Tubbo site. The upper clayey silt unit is a significant source of leachable salt, which is associated with increasing shallow groundwater salinity at some sites.

Optimising subsurface well design for coastal desalination water harvesting.

Subsurface techniques which make use of a natural sand filter are often a preferred means of extracting saline water in coastal environments, and beach-well systems (both vertical wells and radial lateral wells) have been successfully used with minimum pre-treatment for large desalination plants in Israel, Malta, Saudi Arabia and numerous islands. Through a case study, this paper details the limitations of standard vertical wells in coastal aquifers and provides the general design considerations for radial lateral wells with reference to international best-practice. A feasibility assessment for a proposed desalination source site on the Central Coast of New South Wales in eastern Australia is described. Further work, including intensive monitoring and 3D numerical flow modelling, is in progress for the purpose of detailed design.

Vertical groundwater storage properties and changes in confinement determined using hydraulic head response to atmospheric tides

Accurate determination of groundwater state of confinement and compressible storage properties at vertical resolution over depth is notoriously difficult. We use the hydraulic head response to atmospheric tides at 2 cpd frequency as a tracer to quantify barometric efficiency (BE) and specific storage (Ss) over depth. Records of synthesized Earth tides, atmospheric pressure, and hydraulic heads measured in nine piezometers completed at depths between 5 and 55 m into unconsolidated smectitic clay and silt, sand and gravel were examined in the frequency domain. The barometric efficiency increased over depth from ∼0.05 in silty clay to ∼0.15 in sands and gravels. BE for silty clay was confirmed by calculating the loading efficiency as 0.95 using rainfall at the surface. Specific storage was calculated using effective rather than total moisture. The differences in phase between atmospheric pressure and hydraulic heads at 2 cpd were ∼180° below 10 m indicating confined conditions despite the low BE. Heads in the sediment above a fine sand and silt layer at 12 m exhibited a time variable phase difference between 0° and 180° indicating varying confinement. Our results illustrate that the atmospheric tide at 2 cpd is a powerful natural tracer for quantifying groundwater state of confinement and compressible storage properties in layered formations from hydraulic heads and atmospheric pressure records without the need for externally induced hydraulic stress. This approach could significantly improve the development of conceptual hydrogeological model used for groundwater resource development and management.

Establishing Empirical Relationships for the Effects of Water Content on the Mechanical Behavior of Gosford Sandstone

List of symbols UCS Uniaxial compressive strength E Young’s modulus PLI Point-load index TS Tensile strength x Water content Mps Mass of rock sample under partially saturated condition Md Mass of dry rock sample r Radius of the sample H Hydraulic diffusivity k Permeability B Skempton’s coefficient K Bulk modulus g Fluid viscosity a Biot’s coefficient P Maximum applied load measured during the point loading D Distance between two pointers (conical platens) in diametral point-load test which is also the core sample diameter L Length of the core sample F Peak load measured during the Brazilian or indirect tensile test T Thickness of the sample measured at the center rn Nominal strength such as UCS, E, PLI, TS a, b and c Material constants in Hawkins and McConnell (1992) model