R. I. Acworth

Evaporative cooling of speleothem drip water

This study describes the first use of concurrent high-precision temperature and drip rate monitoring to explore what controls the temperature of speleothem forming drip water. Two contrasting sites, one with fast transient and one with slow constant dripping, in a temperate semi-arid location (Wellington, NSW, Australia), exhibit drip water temperatures which deviate significantly from the cave air temperature. We confirm the hypothesis that evaporative cooling is the dominant, but so far unattributed, control causing significant disequilibrium between drip water and host rock/air temperatures. The amount of cooling is dependent on the drip rate, relative humidity and ventilation. Our results have implications for the interpretation of temperature-sensitive, speleothem climate proxies such as δ18O, cave microecology and the use of heat as a tracer in karst. Understanding the processes controlling the temperature of speleothem-forming cave drip waters is vital for assessing the reliability of such deposits as archives of climate change.

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.

Understanding connected surface-water/groundwater systems using Fourier analysis of daily and sub-daily head fluctuations

The long-term monitoring records of hydraulic heads frequently contain fluctuations originating from different cyclic drivers. Fourier analysis applied to these records can reveal connected surface-water/groundwater system characteristics. The various components of the atmospheric tides, the earth tides and the presence of diurnal responses to evapotranspiration are identified and isolated through band-pass filtering of data recorded from both vented and absolute gauge transducers. The signature of the different cyclic drivers is contained in amplitude and phase of the various signal components and can be used to determine the degree of system confinement. A methodology is described for the calculation of barometric efficiency in confined aquifers based upon the amplitude of the M2 and S2 components of the earth and atmospheric tides. It is demonstrated that Fourier analysis of water-level fluctuations is a simple but underused tool that can help to characterise shallow groundwater systems.RésuméLes enregistrements de suivi à long terme des charges hydrauliques renferment fréquemment des fluctuations ayant pour origine différents facteurs cycliques. L’analyse de Fourier appliquée à ces enregistrements peut révéler les caractéristiques de systèmes connectés eaux de surface/eaux souterraines. Les différentes composantes des marées atmosphériques, des marées terrestres et la présence de réponses diurnes à l’évapotranspiration sont identifiées et isolées par filtration des bandes passantes des données, enregistrées tant au moyen de capteurs absolus que compensés en pression atmosphérique. La signature des différents facteurs cycliques est contenue dans l’amplitude et la phase des différentes composantes du signal et peut être utilisée pour déterminer le degré de captivité du système. Une méthodologie est décrite pour le calcul de l’efficacité barométrique des aquifères captifs, basée sur l’amplitude des composantes M2 and S2 des marées terrestres et atmosphériques. Il est démontré que l’analyse de Fourier des fluctuations de niveaux d’eau est un outil simple mais sous-utilisé qui peut aider à caractériser les systèmes aquifères peu profonds.ResumenLos registros de monitoreo a largo plazo de las cargas hidráulicas frecuentemente contienen fluctuaciones procedentes de diferentes forzantes cíclicos. El análisis de Fourier aplicado a estos registros puede revelar las características de la conexión de los sistemas agua superficial / aguas subterráneas. Se identificaron y aislaron los distintos componentes de las mareas atmosféricas, las mareas terrestres y la presencia de respuestas diurnas a la evapotranspiración a través de filtros pasa banda de datos provenientes de sensores de presión ventilados y absolutos. Las peculiaridades de los diferentes forzantes cíclicos están contenidas en amplitud y fase de las distintas señales componentes y pueden ser usados para determinar el grado de confinamiento del sistema. Se describe una metodología para el cálculo de la eficiencia barométrica en acuíferos confinados basado en la amplitud de las componentes M2 y S2 de las mareas terrestre y atmosférica. Se demuestra que el análisis de Fourier de las fluctuaciones de los niveles de agua es una herramienta simple pero poco utilizada que puede ayudar a caracterizar sistemas de agua subterránea someros.摘要期水头监测记录经常包括不同循环驱动产生的波动。对这些记录进行傅里叶分析可揭示相互连接的地表水/地下水系统的特征。通过对排放的和绝对的计量传感器记录的资料进行带通滤波,确定了大气潮汐、地球潮汐各种各样的成分及存在着对蒸发蒸腾一日间的响应,并对每个因素进行了单独的分析。不同循环驱动的特征码包含在各种各样信号成分的幅相中,可用来确定系统限制的程度。根据地球潮汐和大气潮汐M2 和 S2成分的振幅,描述了计算承压含水层中气压效率的方法。研究表明,水位波动的傅里叶分析法是一个简单而又未充分利用的工具,可有助于描述浅层地下水系统的特征。ResumoOs registros de monitorização de níveis piezométricos a longo prazo contêm frequentemente oscilações causadas por fatores cíclicos. Uma análise de Fourier aplicada a esses registros pode revelar caraterísticas do sistema de interações água superficial/água subterrânea. As várias componentes das marés atmosféricas e terrestres e a presença de respostas diurnas à evapotranspiração são identificadas e isoladas por métodos de filtragem passa-banda de dados registados a partir de transdutores de pressão absoluta ou ventilados. A assinatura dos diferentes fatores cíclicos está incluída na amplitude e fase dos vários componentes de sinal e pode ser usada para determinar o grau de confinamento do sistema. É descrita uma metodologia para o cálculo da eficiência barométrica em aquíferos confinados, com base na amplitude das componentes M2 e S2 das marés atmosféricas e terrestres. Demonstra-se que a análise de Fourier das flutuações de nível de água é uma ferramenta simples, mas subutilizada, que pode ajudar a caraterizar sistemas hidrogeológicos subsuperficiais.

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.

Understanding and quantifying focused, indirect groundwater recharge from ephemeral streams using water table fluctuations

Understanding and managing groundwater resources in drylands is a challenging task, but one that is globally important. The dominant process for dryland groundwater recharge is thought to be as focused, indirect recharge from ephemeral stream losses. However, there is a global paucity of data for understanding and quantifying this process and transferable techniques for quantifying groundwater recharge in such contexts are lacking. Here we develop a generalized conceptual model for understanding water table and groundwater head fluctuations due to recharge from episodic events within ephemeral streams. By accounting for the recession characteristics of a groundwater hydrograph, we present a simple but powerful new water table fluctuation approach to quantify focused, indirect recharge over both long term and event time scales. The technique is demonstrated using a new, and globally unparalleled, set of groundwater observations from an ephemeral stream catchment located in NSW, Australia. We find that, following episodic streamflow events down a predominantly dry channel system, groundwater head fluctuations are controlled by pressure redistribution operating at three time scales from vertical flow (days to weeks), transverse flow perpendicular to the stream (weeks to months), and longitudinal flow parallel to the stream (years to decades). In relative terms, indirect recharge decreases almost linearly away from the mountain front, both in discrete monitored events as well as in the long-term average. In absolute terms, the estimated indirect recharge varies from 80 to 30 mm/a with the main uncertainty in these values stemming from uncertainty in the catchment-scale hydraulic properties.