Bryce F. J. Kelly

A composite annual-resolution stalagmite record of North Atlantic climate over the last three millennia

Annually laminated stalagmites can be used to construct a precise chronology, and variations in laminae thickness provide an annual growth-rate record that can be used as a proxy for past climate and environmental change. Here, we present and analyse the first composite speleothem annual growth-rate record based on five stalagmites from the same cave system in northwest Scotland, where precipitation is sensitive to North Atlantic climate variability and the winter North Atlantic Oscillation (NAO). Our 3000-year record confirms persistently low growth-rates, reflective of positive NAO states, during the Medieval Climate Anomaly (MCA). Another persistently low growth period occurring at 290-550 CE coincides with the European Migration Period, and a subsequent period of sustained fast growth-rate (negative NAO) from 600-900 AD provides the climate context for the Viking Age in northern and western Europe.

Detection of subsurface soil cracks by vertical anisotropy profiles of apparent electrical resistivity

To date, an understanding of crack dynamics has been fundamentally hampered by the lack of available techniques to observe or monitor crack dynamics below the soil surface. A new technique relates the growth of soil cracks to a progressive increase in the electrical anisotropy of the soil. Although a single measurement of anisotropy is possible using a surface array of electrodes, the use of four strings of electrodes installed vertically at the corners of a square provides a valuable picture of the crack pattern at depth. In addition, time-lapse electrical surveys allow the growth of cracks to be clearly monitored. The electrical anisotropy is defined as the ratio of the α -to- β apparent resistivity for the square array and is determined for each coplanar set of four electrodesusing one electrode from each of the four vertical strings. In a laboratory, we measured the electrical anisotropy in a sand-filled lysimeter with a plastic sheet, introduced to represent an electrically insulating crack. Measurem...

Training Images from Process-Imitating Methods

The lack of a suitable training image is one of the main limitations of the application of multiple-point statistics (MPS) for the characterization of heterogeneity in real case studies. Process-imitating facies modeling techniques can potentially provide training images. However, the parameterization of these process-imitating techniques is not straightforward. Moreover, reproducing the resulting heterogeneous patterns with standard MPS can be challenging. Here the statistical properties of the paleoclimatic data set are used to select the best parameter sets for the process-imitating methods. The data set is composed of 278 lithological logs drilled in the lower Namoi catchment, New South Wales, Australia. A good understanding of the hydrogeological connectivity of this aquifer is needed to tackle groundwater management issues. The spatial variability of the facies within the lithological logs and calculated models is measured using fractal dimension, transition probability, and vertical facies proportion. To accommodate the vertical proportions trend of the data set, four different training images are simulated. The grain size is simulated alongside the lithological codes and used as an auxiliary variable in the direct sampling implementation of MPS. In this way, one can obtain conditional MPS simulations that preserve the quality and the realism of the training images simulated with the process-imitating method. The main outcome of this study is the possibility of obtaining MPS simulations that respect the statistical properties observed in the real data set and honor the observed conditioning data, while preserving the complex heterogeneity generated by the process-imitating method. In addition, it is demonstrated that an equilibrium of good fit among all the statistical properties of the data set should be considered when selecting a suitable set of parameters for the process-imitating simulations.

Bathymetry fusion using multiple-point geostatistics: Novelty and challenges in representing non-stationary bedforms

In large rivers, complex sediment dynamics cause rapid changes in the position and shape of bed deposits. Regular monitoring of changes in river bed geometry is essential for assessing the nature of morphological change and associated bed load during low, high, and medium flow conditions. We demonstrate the application of Direct Sampling (DS) for patching partial river morphological surveys to generate complete maps of the river morphology, by incorporating prior knowledge from bathymetry data collected in different seasons at collocated or adjacent reaches. This novel approach is based on multiple-point statistics (MPS), which uses a training image (TI) to provide prior statistical and architectural constraining data. In this study high and low resolution bathymetry data from a reach of the Mississippi river have been used. High-resolution measurements were conducted using Multi-beam-echo-sounder (MBES), which provides very detailed bed geometry at high spatial resolution. These measurements cannot be acquired at intervals frequent enough to characterize the rapid sedimentological processes. Low resolution bathymetry data can be obtained at frequent intervals but at sparse locations, by installing depth measuring sensors on boats passing the study reach several times a week. The DS method is used to simulate the high resolution bathymetry at the frequency of the low-resolution data. In the simulations, the method uses the bed geometry information contained in the MBES high-resolution surveys, the local information contained in the boat-borne low-resolution measurements, and provides an updated bathymetry map with quantified uncertainty.

Better placement of soil moisture point measurements guided by 2D resistivity tomography for improved irrigation scheduling

Soil moisture beneath irrigated crops has traditionally been determined using point measurement methods such as neutron probes or capacitance systems. These approaches cannot measure soil moisture at depths beyond the root-zone of plants and have limited lateral coverage. It is shown that surface two-dimensional electrical resistivity tomography (ERT) can be used to map the spatial heterogeneity in soil moisture throughout a field under irrigated cotton. The case study demonstrates that ERT provides a better understanding of the pathways of water migration, and provides spatial information on how water storage changes throughout the growing season. We conclude that ERT should be integrated into farm water management surveys to delineate zones of excessive water loss due to deep drainage and to improve the positioning of point measurement methods for measuring soil moisture, thereby improving irrigation scheduling.

Spatial variability of cave-air carbon dioxide and methane concentrations and isotopic compositions in a semi-arid karst environment

There is insufficient information on the movement of air in karst environments to constrain the uncertainty associated with quantifying sources and sinks of methane (CH4) and carbon dioxide (CO2) within caves for global carbon accounting. We analysed cave-air samples for their CO2 and CH4 concentrations ([CO2] and [CH4]) and carbon isotopic compositions from sampling campaigns in winter (August 2014) and summer (February 2015) at numerous heights and locations throughout Gaden and Cathedral caves, in a semi-arid environment, Wellington Caves, NSW, Australia. Ventilation is the dominant control on cave-air CO2 and CH4, with the highest cave-air CO2 concentrations ([CO2]cave) occurring in summer, in association with the lowest cave-air CH4 concentrations ([CH4]cave). Analyses show that the cave-air CO2 has both atmospheric and soil sources. Soil air and cave air in both caves undergo methanogenesis and methanotrophy, but we identify cave-specific differences in cave-air CH4 and CO2. [CH4]cave in Cathedral Cave shows an inverse relationship to [CO2]cave, particularly in areas separated from the main cave passage. In contrast, Gaden Cave has near-atmospheric [CH4]cave and isotopic ratios present at all locations sampled in winter. Where no ventilation is occurring in summer, [CH4]cave in Gaden Cave decreases, but remains reasonably high compared to Cathedral Cave. Our research shows adjacent caves vary in their ability to act as a net sink for CH4, and highlights the need for further studies before global generalisations can be made about the carbon budget of karst environments.

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.

3D Cross-hole resistivity tomography to monitor water percolation during irrigation on cracking soil

Irrigation water entering soil cracks can quickly move past the root-zone without being utilised by plants. To assess the efficiency of irrigation practices, reliable methods to monitor water percolation are needed. Three-dimensional (3D) cross-hole electrical resistivity tomography (ERT) was carried out during three irrigation events on soil with different surface-crack intensities. Changes in the resistivity distribution during the irrigation events were related to water movement. The propagation of resistivity change during irrigation events differed for different degrees of soil cracking. For surface cracks of <2 mm width before the irrigation, the resistivity change propagated evenly down-gradient, indicating matrix flow. During irrigation on soil with 30-mm-wide surface cracks, the resistivity change first occurred in the lower parts of the profile before propagating to the top. This suggests preferential flow filling cracks from the bottom up. The differences in initial soil moisture that resulted in these two flow behaviours were reflected in the pre-irrigation resistivity profile. Subsurface temperature changes during the irrigation confirmed the different flow behaviour. 3D Cross-hole ERT allows monitoring of percolation patterns as well as the pre-irrigation moisture states that cause these patterns. This makes 3D cross-hole ERT an excellent tool for researching irrigation management.

Assessing Connectivity Between an Overlying Aquifer and a Coal Seam Gas Resource Using Methane Isotopes, Dissolved Organic Carbon and Tritium

Coal seam gas (CSG) production can have an impact on groundwater quality and quantity in adjacent or overlying aquifers. To assess this impact we need to determine the background groundwater chemistry and to map geological pathways of hydraulic connectivity between aquifers. In south-east Queensland (Qld), Australia, a globally important CSG exploration and production province, we mapped hydraulic connectivity between the Walloon Coal Measures (WCM, the target formation for gas production) and the overlying Condamine River Alluvial Aquifer (CRAA), using groundwater methane (CH4) concentration and isotopic composition (δ13C-CH4), groundwater tritium (3H) and dissolved organic carbon (DOC) concentration. A continuous mobile CH4 survey adjacent to CSG developments was used to determine the source signature of CH4 derived from the WCM. Trends in groundwater δ13C-CH4 versus CH4 concentration, in association with DOC concentration and 3H analysis, identify locations where CH4 in the groundwater of the CRAA most likely originates from the WCM. The methodology is widely applicable in unconventional gas development regions worldwide for providing an early indicator of geological pathways of hydraulic connectivity.

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.