Poster Presentation Australasian Groundwater Conference 2017

Impact of fire on hydrological and chemical signatures in Karst Vadose Zone Water, Wombeyan Caves, New South Wales, Australia (#52)

Fang Bian 1 , Katie Coleborn 1 , Ingrid Flemons 1 , Pauline C Treble 2 , Andy Baker 1 , Andrew Baker 3
  1. Connected Waters Initiative Research Centre, UNSW, Kensington, New South Wales, Australia
  2. Australian Nuclear Science and Technology Organisation, Lucas Heights, New South Wales, Australia
  3. National Parks and Wildlife Service, Bathurst, New South Wales, Australia

Impact of wildfire on karst-vadose-zone hydrology and hydrogeochemistry is hard to evaluate owing to the complexity of subsurface environment. The aim of this study is to understand the variation of hydrogeochemical components and drip discharge in response to a moderate-intensity 10m x10m experimental fire above the shallow Wildman’s Cave at Wombeyan, Australia, in May, 2016. Water isotopes and cation analyses were conducted on drip waters collected pre- and post-fire. Ongoing drip water collection began in Dec, 2014. And drip rate has been monitored continuously using acoustic data loggers. Discharge into cave is discontinuous, indicative of limited soil and karst storage.  The post-fire drip data demonstrate decreased duration of recharge, with approximately x3 increase in peak discharge, which we hypothesize is caused by the decrease of soil-storage capacity. Water isotope compositions have significantly changed after fire, with d2H isotope composition up to ~56 per mil lower and d18O ~6.3 per mil lower in the week after fire. With time, isotopic values return to pre-fire values. We hypothesize that this temporary depletion in water isotopic composition reflects a combination of post-fire rainfall isotope composition, loss of pre-fire evaporatively enriched soil and shallow karst stored water.Drip water concentrations of bedrock-related elements (Calcium, Strontium) and soil-related elements (Zinc, Nickle) decreased after the fire. We hypothesize that these reflect the loss of soil and soil biological activity above the cave, and agree with a decrease of soil storage capacity. This research demonstrates that even in complex hydrogeological settings, understanding the impact of local wildfire on subsurface system can be improved through the combination of drip water hydrograph analysis and geochemical analysis. This will provide opportunities to broaden the insights into improved fire management in karst environments and a better understanding of the relationship between surface environment conditions and vadose zone hydrology.

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