IV3 ENVIRONMENT PROBLEMS IN KARST REGION
R G Lyons, Dept. of Geography and Oceanography,
ADFA University of New South Wales , Australia
Radon is well known to accumulate to hazardous levels in some buildings and mines associated with uraniferous rocks but , because of the low radioacti- vity in most limestone rocks , dangerous levels were not anticipated in caves. The potential health hazard of radon in caves was first mooted in 1975 at the National Cave Management Symposium , New Mexico ,but has recently become a topic of considerable concern. Factors governing the accumulation of radon in caves are discussed. Preliminary measurements in some Australian caves show levels which vary by factors of 4 (seasonal) and 75 (diurnal) , with the upper levels approaching recommended maximum exposure levels for some tourist cave guides. A workshop comprising interested scientists and cave managers was held in Canberra in April 1992.This meeting outlined a 3-tier research program designed to assess the seriousness of this problem and agreed that funding should be sought to action it. The usefulness of gamma spectrometer measurements to estimate radon concentrations is also discussed.
V.M.Kutepov & V.N.Kozhevnikova
Institute of the Lithosphere ,USSR Academy of Sciences ,Moscow ,USSR
The paper examines the methodological aspects of estimation of rock mass stability in the conditions of covered karst ,of the prediction of land subsidence and sinks over weakened karst zones as a result of lowered groundwater head pressure and level;specific examples of stability prediction of karst areas are given.
Disturbances in the natural hydrogeologic environment as a result of intensive industrial development of karst areas lead to activation not only of karst processes proper but also of various filtration processes in insoluble rocks which overlie the karst sequences.This is often accompanied by land subsidence and sinks hazardous for structures and human lives. Prediction of the stability of karst areas ,its changes due to technogenic impacts is one of the most important problems of engineering-geologic exploration in karst areas. A solution of this problem is possible on the basis of multidisciplinary studies of conditions and factors of the development of technogenic processes and patterns of their distribution.
THE INFLUENCE OF KARST HYDROLOGY ON WATER QUALITY
MANAGEMENT IN THE SOUTH EAST OF SOUTH AUSTRALIA
A.J.Emmett and A.L.Telfer
Engineering & Water Supply Department , South Australia
European settlement in the South East of South Australia has had a major impact on the quality of groundwater resources due to the presence of extensive karst formation. The establishment of dairying and in particular the associated secondary industry of cheese manufacture was the earliest activity to have a marked impact on groundwater quality. Cheese factories were often sited close to karst features such as caves and sinkholes which provided convenient means of waste disposal. Although most have long since closed they have left a legacy of pollution plumes of varying sizes.
As development progressed the convenience of karst features for waste disposal became almost a tradition for the region.In Mount Gambier , the main regional centre , the presence of both exposed and subterranean karst features provided a perfect system' for the disposal of storm water. Prior to the provision of a sewerage system within Mount Gambier all toilet and household waste waters were disposed to ground , a proportion of which would have reached groundwater via karst features.
These activities and the subsequent problems which began emerging in the 1960s have lead to a concerted effort over the last twenty years to change the philosophy of waste disposal and to generate an understanding and responsibility by those who live in the region and depend on groundwater for the major part of their water supply.
Mount Gambier's water supply comes from the Blue Lake which is one of several lakes in a spectacular volcanic complex. Groundwater inflow provides 90% of the recharge to the Blue Lake. The Blue Lake is a high value resource in a high risk environment and consequently a great deal of study has been undertaken into the Lake and its sustainability as a primary water supply for Mount Gambier and the surrounding district. The Gambier Limestone has high transmissivities for a few kilometres around the Lake , modeling indicates T values of approximately 20,000 m2/day compared to an average of approximately 300 m2/day for this aquifer.
Currently a management plan is being developed for the Lake. This plan will concentrate on four specific areas , namely:
- to minimize direct entry of pollutants
- to stop existing polluted groundwater from entering the Lake
- to prevent additional groundwater pollution from occurring and to identify alternate supplies.
The principal indicator and pollutant is nitrate which has been relatively stable in the Lake in recent years but a marked increase was evident during the 1970s when a significant increase of withdrawal from the Lake took place. Currently a detailed study of nitrate concentrations in the Lake is being undertaken.
This paper will provide an overview of how karst formation has had an impact on groundwater and the consequent influence this has had on water quality management in the region.
GROUNDWATER AND THE REHABILITATION OF NAURU
G.Jacobson and P.J.Hill
Australian Geological Survey Organisation
Nauru , in the central Pacific Ocean, is a raised atoll capping a volcanic seamount arising from an ocean floor depth of 4300 m. The land area is 22km2 , and the island rises to 70 m above sea level. Drilling has proved dolomitized limestone of upper Miocene or younger age to a depth of at least 55m below sea level. Gravity and magnetic surveys indicate that the limestone probably overlies volcanic bedrock at a depth of about 500m. Reverse-circulation drilling and geoelectrical probes indicate that there is a discontinuous freshwater layer in the limestone , close to sea level and averaging 5 m thick. This is underlain by a mixing zone of brackish water , 60-70 m thick , which in turn is underlain by sea water. The exceptional thickness of the mixing zone is ascribed to high permeability of the karstified limestone. The forthcoming cessation of phosphate mining will mean a shortfall in water supply which will probably have to be met by the desalination of brackish water. Groundwater beneath the mined-out area , and the settled coastal terrace , is highly vulnerable to pollution , and waste disposal management needs to be considered in relation to groundwater protection.
SURFACE HYDROLOGY AND SOIL EROSION IN
AN ARID KARST: THE NULLARBOR PLAIN
Gillieson , David1; Cochrane , Anne1; Murray , Andrew2
1 Department of Geography & Oceanography , University College ,
University of New South Wales ,
2 CSIRO Division of Water Resources Research
The Nullarbor Plain is the largest karst area in Australia (220,000 km2)and one of the largest in the world. Its climate is arid (Koppen BWk and BWh)and the surface relief is less than 10 m. The landscape is divided into extensive closed karstic depressions separated by low rocky ridges , and the dominant vegetation is chenopod shrubland. Wind erosion is most noticeable around stock watering points where grazing and trampling by sheep and rabbits bares the soil. Sheetflow processes have been observed by speleologists but are rare.
This study integrates remotely sensed data , providing estimates of the extent of erosion , with erosion rate estimates using fallout radionuclides. The extent and severity of soil degradation has been assessed using MSS and TM imagery. GPS rectified images from 1973 , 1983 and 1991 have been compared for two sites on the Nullarbor. Over the 19 years the total extent of bare soil has reduced significantly , but some areas around water points have degraded. Years in which rain falls in the northern plain allow grazing of ephemeral growth and development of bare soil patches. There is also a shifting mosaic of disturbance due to fossorial wombats and rabbits which regenerates rapidly and may promote plant species diversity.
Estimates of potential water and wind erosion have been derived for this landscape. Surface soil sorptivity and hydraulic conductivity differ markedly between ridges and depressions; the ridges are clearly zones of groundwater recharge while ponding is evident in most depressions. Bothparameters vary with vegetation type , and both are influenced by soil surface condition. Intense rainfall occurs two to three times per decade and is associated with rain depressions derived from tropical cyclones. Sheetflow occurs under these conditions and moves water and sediment into the caves. Wind erosion estimates are based on drift potential and vegetative cover. Erosion potential is moderate and wind erosion is not likely to occur where vegetation cover exceeds 30%.
Preliminary data on the activities of fallout radionuclides in soils and cave sediments have been determined by high resolution r spectroscopy. From this we have identified areas of soil accumulation , stable soil and eroding soil relative to the regional influx from rainfall. The spatial correlation between soil erosion indices and fallout radionuclide concentrations can be integrated in a GIS. The resulting model of extent and rates of soil erosion will permit the targeting of land rehabilitation work and identify land units in which there is a potential high risk of erosion.