Browsing by Author "Mahmud, K"

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    Climate and groundwater recharge: the story from Australian caves
    (National Centre for Groundwater Research And Training, 2017-07-11) Baker, AA; Treble, PC; Markowska, M; Andersen, MS; Wang, Z; Mahmud, K; Cuthbert, MO; Coleborn, K; Rau, GC
    Quantifying the timing and extent of diffuse groundwater recharge is crucial for our understanding of groundwater recharge processes. However, diffuse recharge is notably difficult to measure directly. Caves can be used as natural observatories of ongoing diffuse recharge processes, and speleothems (cave carbonate deposits such as stalagmites) as archives of past recharge. Cave records can improve our understanding of diffuse recharge in the context of climate change and past climate variability. A long-term, national monitoring program of infiltration into caves has been undertaken since 2010 using a network of over 200 automated loggers. This has been supplemented by artificial irrigation experiments at one semi-arid site. The timing of past recharge can be determined from the periods of past stalagmite growth. Recharge characteristics can be elucidated from oxygen isotope composition, with increased 18O likely caused by evaporative fractionation and increased 16O from high intensity/magnitude rainfall events. Automated logger data identify the diffuse recharge thresholds that vary with climate and geology. Both the logged data of natural events and the artificial irrigation experiments identify significant spatial heterogeneity in recharge in these karstified systems. Water infiltrating into the karst is often depleted in the lighter oxygen isotope due to soil and shallow subsurface evaporative fractionation. Speleothem deposition is more frequent during glacial periods, presumably because recharge thresholds are lower, and their isotopic composition provides evidence of the characteristics of the recharge process. Caves provide direct access into the unsaturated zone. Direct observation of groundwater recharge can be used to complement data from the saturated zone (boreholes) and models. The heterogeneity of recharge in karst aquifers can be directly observed and quantified. Speleothems preserve a record of groundwater recharge that can extend back for hundreds of thousands of years, providing a long-term view on the timing and variability of groundwater recharge in Australia.
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    Lidar investigation of infiltration water heterogeneity in the Tamala Limestone
    (American Geophysical Union, 2014-12-15) Mahmud, K; Mariethoz, G; Teble, PC; Baker, AA
    To better manage groundwater resources in carbonate areas and improve our understanding of speleothem archives, it is important to understand and predict unsaturated zone hydrology in karst. The high level of complexity and spatial heterogeneity of such systems is challenging and requires knowledge of the typical geometry of karstic features. We present an exhaustive characterization of Golgotha Cave, SW Western Australia, based on an extensive LIDAR measurement campaign. The cave is developed in Quaternary age aeolianite (dune limestone) and contains speleothem records. We collect 30 representative 3D scan images from this site using FARO Focus3D, a high-speed 3D laser scanner, to visualize, study and extract 2D and 3D information from various points of view and at different scales. In addition to LIDAR data, 32 automatic drip loggers are installed at this site to measure the distribution and volume of water flow. We perform mathematical morphological analyses on the cave ceiling, to determine statistical information regarding the stalactites widths, lengths and spatial distribution. We determine a relationship between stalactites diameter and length. We perform tests for randomness to investigate the relationship between stalactite distribution and ceiling features such as fractures and apply this to identify different types of possible flow patterns such as fracture flow, solution pipe flow, primary matrix flow etc. We also relate stalactites density variation with topography of the cave ceiling which shows hydraulic gradient deviations. Finally we use Image Quilting, one of the recently developed multiple-point geostatistics methods, with the training images derived from LIDAR data to create a larger cave system to represent not only the caves that are visible, but the entire system which is inaccessible. As a result, an integral geological model is generated which may allow other scientists, geologist, to work on two different levels, integrating different speleothem datasets: (1) a basic level based on the accurate and metric support provided by the laser scanner; and (2) an advanced level using the image-based modelling.
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    Roles of transpiration, forest bioproductivity and fire on a long-term dripwater hydrochemistry dataset from Golgotha Cave, SW Australia
    (American Geophysical Union (AGU), 2015-12-15) Treble, PC; Baker, AA; Fairchild, IJ; Bradley, C; Mahmud, K; Andersen, MS; Meredith, KT; Mariethoz, G
    Golgotha Cave is located in a forested catchment in SW Australia where evapotranspiration losses from the vadose-zone are high and forest biomass has been disturbed by fire. The cave has been continuously monitored since 2005 and this extensive dataset has been used to quantify key processes determining dripwater hydrology and chemistry (Mahmud et al., 2015; Treble et al., 2013; 2015). In this paper we present a synthesis of these findings and derive a conceptual model to illustrate the main hydrochemical processes that will impact cave dripwater in similar environments. We applied mass-balance techniques to quantify sources (water/rock interactions and aerosol) and sinks (prior calcite precipitation and biomass uptake). Mass-balance results suggest that transpiration and elemental sequestration into biomass modifies dripwater ion concentrations. The vegetation uptake impacts dripwater Mg, K and SO4, with the largest impact on SO4, estimated to be up to 60% at some drip sites. Overall, our findings suggest that varying amounts of transpiration by deeply-rooted trees contribute significantly to spatial and temporal variability in dripwater solute concentrations. This is in addition to the partitioning of infiltrating water between characteristic flow pathways. Applying principal components analysis, we identify a common long-term rising trend in dripwater Cl, Mg, K, Ca, Sr and Si. We assess whether the long-term trends in dripwater solutes are driven by post-fire biomass recovery and/or the impacts of a drying climate in SW Australia. References Mahmud et al. (2015), Terrestrial Lidar Survey and Morphological Analysis to Identify Infiltration Properties in the Tamala Limestone, Western Australia, doi:10.1109/JSTARS.2015.2451088. Treble et al. (2013), An isotopic and modelling study of flow paths and storage in Quaternary calcarenite, doi:10.1016/J.Quascirev.2012.12.015. Treble et al. (2015), Impacts of cave air ventilation and in-cave prior calcite precipitation on Golgotha Cave dripwater chemistry, doi:10.1016/J.Quascirev.2015.06.001.
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    Terrestrial LiDAR survey and morphological analysis to identify infiltration properties in the Tamala Limestone, Western Australia
    (IEEE, 2015-07-16) Mahmud, K; Mariethoz, G; Treble, PC; Baker, AA
    Caves are an ideal observatory of infiltration water in karstified limestone, and the application of remote sensing techniques can bring new insights toward flow patterns and processes. We present an exhaustive characterization of Golgotha Cave in SW Western Australia, based on a light detection and ranging (LiDAR) measurement campaign. The cave is developed in Quaternary age aeolianite (dune limestone) and its infiltration waters form speleothems. We collect ground-based LiDAR scans of the cave ceiling at three sites within the cave system. The resulting point-clouds are analyzed using mathematical morphology to determine statistical information on stalactite widths, lengths, and spatial distributions. We establish a relationship between stalactite diameter and length that is in agreement with the platonic ideal of stalactite shape. We relate stalactite density variation with topography of the cave ceiling and variations in hydraulic gradient. From this analysis, it appears that longer stalactites tend to occur in comparatively lower ceiling elevation, which, we hypothesize, represents greater mass of water in the limestone above the roof of the cave. We also investigate the relationship between stalactite distribution and ceiling features such as fractures. We apply this to identify different types of possible flow patterns such as matrix flow and fracture flow. This analysis demonstrates a spatial variability, with one site having linear groups of stalactites and another site mostly dominated by stalactite clusters. © 2015, IEEE.