Hydrology and fluid geochemistry

    Recent research highlights

    Fluid and vein geochemistry in the Southern Alps

    Catriona Menzies has just been awarded a PhD for her thesis "Fluid flow associated with the Alpine Fault, South Island, New Zealand" at the University of Southampton. Catriona's work involved an isotopic and chemical investigation of hot springs, veins and alteration. Amongst the many discoveries, Catriona found that hangingwall fluids are dominated by meteroic water, even in veins from below the brittle-ductile transition. 87Sr/86Sr ratios of veins and warm springs in the hangingwall are within the range of their host rocks and show no evidence for interaction with highly radiogenic footwall rocks, supporting the notion that the Alpine Fault is a barrier to fluid flow.

    Hanging-wall permeability and groundwater infiltration

    Armed with equipment sponsored by the New Zealand Hydrological Society, Alex Sims  from the University of Otago Geography Department has been monitoring fluid-flow in the Tartare tunnels near Franz Josef. These tunnels were cut through ~300m of schist in the hangingwall of the Alpine Fault for alluvial gold mining. The flow of water from the tunnels comes only from infiltration through the rockmass, so provides and ideal place to study groundwater in the mountains. Under the supervision of Simon Cox and Sean Fitzsimmons, Alex has been assessing groundwater flow and recharge during storm events, trying to understand seepage through the rockmass and hangingwall permeability.   An aim is to determine the proportion of rainfall infiltration into the rockmass - which appears to be surprisingly high. Alex is now well into his research and hopes to submit his MSc in early 2013.

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    University of Otago MSc student Alex Sims  at work in the Tartare Tunnels near Franz Josef

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    Another view of Alex Sims at work underground

    Warm spring gas geochemistry

    Samuel Niedermann and Martin Zimmer (GFZ Potsdam) have completed their laboratory work on gases in the warm springs along the Alpine Fault as part of a project with Joerg Erzinger. They found that N2 is generally the most abundant gas, CO2 concs vary considerably (from <0.1% to >96%), and CH4 contributes up to 13%.  The 3He/4He ratios are highest close to the fault and decrease to atmospheric values within a few kilometres across the hangingwall (although Copland spring is an exception). Their preliminary conclusion is that mantle fluids penetrate the thick crust beneath the Southern Alps directly at the fault, and may be diverted away from it only where major passageways exist through the crust.P2240048.JPG

    Geochemical sampling underway deep in the Southern Alps

    Copland hot spring monitoring

    After two years collecting data, Simon Cox (GNS Science) has removed his temperature and rainfall monitoring equipment from Copland hot spring. The  simple experiment has been highly successful, demonstrating distinct cooling associated with both rainfall and distal earthquake shaking. A series of other investigations have been carried out at Copland spring in conjunction with Damon Teagle, Catriona Menzies, Delia Strong and others. For example, a series of impaction cores were collected through the travertine deposit formed by the spring, down into a debris flow deposit containing leaves c. 660 years old. There appears to have been stability of the spring and travertine deposition since c.1300AD, and the spring appears to have survivied the 1717 earthquake.  The thermal anomaly associated with the spring has been mapped out in the ground at 1 m depth.  About 1 million Joules of energy are released into the environment per second as the spring water cools to ambient temperature - equivalent to 1 MW of power. Waters have been analysed for stable isotopes and tritium.  The percentage of atmospheric CO2 is very low.  A mean residence time of 126±30 years is suggested for the water following some modelling of tritium data by Uwe Morgenstern.

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    Simon Cox (left) and Delia Strong generating a thermal map of the ground around the hot pool.

     

    For further details on these and other projects, contact Simon Cox, GNS Science, Dunedin, New Zealand, +64 (3) 479 9670.

     
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