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Anomalous behaviour of specific electrical conductivity (SEC) was observed at a karst spring in Slovenia during 26 high-flow events in an 18-month monitoring period. A conceptual model explaining this anomalous SEC variability is presented and reproduced by numerical modelling, and the practical relevance for source protection zoning is discussed. After storm rainfall, discharge increases rapidly, which is typical for karst springs. SEC displays a first maximum during the rising limb of the spring hydrograph, followed by a minimum indicating the arrival of freshly infiltrated water, often confirmed by increased levels of total organic carbon (TOC). The anomalous behaviour starts after this SEC minimum, when SEC rises again and remains elevated during the entire high-flow period, typically 20–40 µS/cm above the baseflow value. This is explained by variable catchment boundaries: When the water level in the aquifer rises, the catchment expands, incorporating zones of groundwater with higher SEC, caused by higher unsaturated zone thickness and subtle lithologic changes. This conceptual model has been checked by numerical investigations. A generalized finite-difference model including high-conductivity cells representing the conduit network (“discrete-continuum approach”) was set up to simulate the observed behaviour of the karst system. The model reproduces the shifting groundwater divide and the nearly simultaneous increase of discharge and SEC during high-flow periods. The observed behaviour is relevant for groundwater source protection zoning, which requires reliable delineation of catchment areas. Anomalous behaviour of SEC can point to variable catchment boundaries that can be checked by tracer tests during different hydrologic conditions.
Taylor Creek Cave is formed within sediments of the Red Bluff Sand, a Pliocene unit overlain by Newer Volcanics. The cave consists of a single low chamber, 12m long and 5m wide, that has been excavated in friable sandstone under a resilient silcrete roof; it has formed by an unusual combination of piping and stream erosion. Taylor Creek initially exposed the silcrete surface, then piping below the silcrete caused tunnel formation in the sandstone. Collapse of overlying material into this tunnel captured Taylor Creek, causing it to flow beneath the silcrete and thereby enlarge the cave to its present size.
Geomorphological and Hydrological investigation of un-mapped limestone outcrops and enclosed depressions that occur between North Wiburds Bluff and the headwaters of Bindo Creek has confirmed the presence of significant karst well to the north of the boundary of the Jenolan Caves Reserve and that karst drainage could conceivably breach the Great Dividing Range. The limestone becomes progressively less well dissected northwards with the karst being very subdued at the northern end of the belt. Fluoroscein testing has shown that a streamsink at the southern end of this area drains directly to Central River in Mammoth Cave, and thence to imperial Cave and Blue Lake. This indicates that at least some of the limestone in this area is continuous beneath the surficial covers with the main Northern Limestone rather than being a discrete lens. The situation has important management implications in view of expanding forestry operations in the area since these have the potential to seriously increase the sediment load of waters that pass through wild caves and the Jenolan tourist caves complex.
The regional movement of shallow groundwater in the fractured rock aquifer is examined through a conceptual-deterministic modeling approach. The computer program FRACNET represents the fracture zones as straight laminar flow conductors in connection to regional constant head boundaries within an impermeable rock matrix. Regional scale fracture zones are projected onto the horizontal plane, invoking the Dupuit-Forchheimer assumption for flow. The steady state flow solution for the two dimensional case is achieved by requiring nodal flow balances using a Gauss-Seidel iteration. Computer experiments based on statistically generated fracture networks demonstrate the emergence of preferred flow paths due to connectivity of fractures to sources or sinks of water, even in networks of uniformly distributed fractures of constant length and aperture. The implication is that discrete flow, often associated with the local scale, may maintain itself even at a regional scale. The distribution of uniform areal recharge is computed using the Analytic Element Method, and then coupled to the network flow solver to complete the regional water balance. The areal recharge weakens the development of preferential flow pathways. The possible replacement of a discrete fracture network by an equivalent porous medium is also investigated. A Mohr's circle analysis is presented to characterize the tensor relationship between the discharge vector and the piezometric gradient vector, even at scales below the representative elementary volume (REV). A consistent permeability tensor is sought in order to establish the REV scale and justify replacement of the discrete fracture network by an equivalent porous medium. Finally, hydrological factors influencing the chemical dissolution and initiation of conduits in carbonate (karst) terrain are examined. Based on hydrological considerations, and given the appropriate geochemical and hydrogeological conditions, the preferred flow paths are expected to develop with time into caves.
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