A groundwater recharge experiment in krast - Wellington Caves, NSW

Groundwater recharge is a process which is inherently difficult to measure directly due to soil and sediment heterogeneity and the tendency for this to cause preferential flow. This often leads to groundwater recharge being estimated by indirect methods, such as remotely or by differences in the water balance, which frequently causes huge uncertainties in the estimates. Karst terrains with cave features, although notoriously heterogeneous, offer a rare opportunity to physically enter the subsurface and make direct observations. Cathedral Cave at Wellington Caves in NSW, Australia was used for a multiple tracer experiment to look a recharge processes as well as physical and chemical processes affecting speleothems formation. On four consecutive days, four water releases were applied to a 3 x 7 m soil patch 2-3 m above the cave ceiling. The applications varying from 800 to 1500 L corresponds to natural precipitation events of 40 to 70 mm, events which have been known to cause recharge into the caves in the past. The four different events were variably modified by adding deuterium (2H), a fluorescing tracer and ice to reduce the temperature. Inside the caves onset of recharge was measured by automatic drip rate monitoring and temperature loggers. In addition, discrete water samples were collected and analysed for stable water isotope composition, fluorescence and dissolved trace elements. A preliminary assessment of the cave observations suggest that. although ow was preferentially along fractures, considerable interaction with pore water in the rock matrix must have taken place to significantly dilute the tracer content of water sampled in the cave. To our knowledge this is the first time that is has been directly shown that water sampled during a recharge event is only fractionally water from that particular event. For the location of this study the result is surprising considering the short distance of 2-3 m between the soil surface and the cave ceiling. The results have significant implications for our understanding of karst hydrology as well as reconstruction of past recharge and climate conditions from cave speleothems.

Keywords

Groundwater recharge, Soils, Sediments, Water, Underground, Caves, New South Wales, Australia, Deuterium, Trace amounts, Hydrology

Citation

Andersen, M. S., Baker, A., Graham, P., Rutlidge, H., Mariethoz, G., Roshan, H., Rau, G., Markowska, M., & Cuthbert, M. O. (2013). A groundwater recharge experiment in karst - Wellington Caves, NSW. Presentation to the IAH 2013 Perth Australia : "solving the groundwater challenges of the 21st century", International Association of Hydrogeologists 40th International Conference, Perth, Western Australia, 15-20 September 2013, (pp. 75).