Deprecated: Function get_magic_quotes_gpc() is deprecated in /home/isthin5/public_html/addon-domains/speleogenesis.info/template/toolbar_right.php on line 7
Search KARSTBASE:
Search in KarstBase
|
|
Clastic sedimentary rocks are generally considered non-karstifiable and thus less vulnerable to pathogen contamination than karst aquifers. However, dissolution phenomena have been observed in clastic carbonate conglomerates of the Subalpine Molasse zone of the northern Alps and other regions of Europe, indicating karstification and high vulnerability, which is currently not considered for source protection zoning. Therefore, a research program was established at the Hochgrat site (Austria/Germany), as a demonstration that karst-like characteristics, flow behavior and high vulnerability to microbial contamination are possible in this type of aquifer. The study included geomorphologic mapping, comparative multi-tracer tests with fluorescent dyes and bacteria-sized fluorescent microspheres, and analyses of fecal indicator bacteria (FIB) in spring waters during different seasons. Results demonstrate that (i) flow velocities in carbonate conglomerates are similar as in typical karst aquifers, often exceeding 100 m/h; (ii) microbial contaminants are rapidly transported towards springs; and (iii) the magnitude and seasonal pattern of FIB variability depends on the land use in the spring catchment and its altitude. Different ground water protection strategies than currently applied are consequently required in regions formed by karstified carbonatic clastic rocks, taking into account their high degree of heterogeneity and vulnerability.
style=
The results are presented for one year of field measurement and analysis of water samples at the Junee resurgence, one of the largest karst risings in Tasmania. The water emerges from Junee Cave at an altitude of approximately 300m and forms the source of the Junee River at a point about 5km north-west of the township of Maydena. The resurgence drains a large area along the southern boundary of the Mt Field National Park and appears to be fed by a number of streamsinks, the nearest of which are at least 2km distant. The only underground drainage connection proved so far is with one of the largest of these stramsinks, Khazad-dum. This cave has been explored to a depth of 321m and is recorded as Australia's deepest cave system. The Junee area is located in central southern Tasmania and is centred on 146°40' East and 42°45' South. The Junee resurgence is the only significant rising in the area and is commonly thought to drain most of the Junee area. This opinion is based largely on the interpretation of the geological structure as shown in the geological sketch map of Hughes and Everard (Hughes 1957). However, a more detailed examination of the area on which Figure 1 is based, suggests that the western limit of underground drainage towards the Junee resurgence may be more or less coincident with the axis of the NNW plunging Nichols Spur anticline. Further mapping of the geological structure, and water tracing, will be required to confirm this.
This study was undertaken to gain a better understanding of karst hydrology. To do this, the present day hydrology and the paleohydrology were determined in three karst basins. The basins chosen were the Swago, Locust and Spring Creek basins in Pocahontas and Greenbrier Counties, West Virginia. A number of conventional field techniques were used successfully in this study, including the following: current meter and dye dilution gauging; dye and lycopodium stream tracing; geological and cave mapping; the setting up of stage recorders; geochemistry; and limestone erosion measurements. The climate of the region was investigated to obtain realistic precipitation, temperature and potential evaporation data over the study basins.
It was found that the mean precipitation over two of the basins was 30% higher than recorded data in the valleys. The karst development of the basins was found to take place in four major stages. These were: A) initial surficial flow, B) strike controlled drainage, C) major piracies from one sub-basin to another, and D) shortening of the flow routes. The major controls on the karst development were found to be: A) the Taggard shale, B) the strike direction, which controlled early basin development, and C) the hydraulic gradient from the sink to rising, which controlled later basin development.
To better assess the quantitative hydrology, and to assist in determining the type of unexplorable flow paths, a watershed model was developed. This modelled the streamflow from known climatic inputs using a number of measured or optimized parameters. The simulation model handled snowmelt, interception, infiltration, interflow, baseflow, overland flow, channel routing, and evaporation from the interception, soil water, ground water, snowpack and channel water. The modelled basin could be split up into 20 segments, each with different hydrological characteristics, but a maximum of 3 segments was used in this study.
A total of 29 parameters was used in the model although only 10 (other than those directly measurable) were found to be sensitive in the three basins. The simulated streamflow did not match the real flows very well due to errors in the data input and due to simplifications in the model. It was found, however, that as the proportion of the limestone in a segment increased the overland flow decreased, the interflow increased, the baseflow and interflow recessions were faster, the soil storages were smaller and the infiltration rate was higher, than in segments with a larger proportion of exposed clastics. The flow characteristics of the inaccessible conduits were inferred from the channel routing parameters and it was postulated that the majority of the underground flow in the karst basins was taking place under vadose conditions.
Work done at Bungonia since 1972 has filled gaps in our knowledge of this area. Water tracing has proven the earlier interference that the waters of all the major caves of the Lookdown Limestone and the uvula containing College Cave go to Efflux. Geological remapping shows that faulting allows these connections all to lie in limestone and accounts for the drainage of B4-5 away from the gorge. A 45m phreatic loop identified in Odyssey Cave, exceptionally large for south-eastern Australia, is also accounted for by the geological structure. Phoenix Cave has two successive cave levels similar to those of B4-5. The perched nature of the Efflux now finds a structural control in that the Folly Point Fault has interposed impervious beds between this spring and the gorge. Further analysis of the evidence relating the age of uplift and incision in the Shoalhaven and its tributaries strengthens the case for setting these in the lower Tertiary whereas most of the caves cannot be regarded as other than young. The geological remapping can partly account for the age discrepancy between underground and surface forms found at Bungonia.
|
|
|