Sustainable utilisation of Table Mountain Group aquifers

Autor: Duah, Anthony A.
Jazyk: angličtina
Rok vydání: 2010
Předmět:
Druh dokumentu: Thesis and dissertation
Popis: The Table Mountain Group (TMG) Formation is the lowest member of the Cape Supergroup which consists of sediments deposited from early Ordovician to early Carboniferous times, approximately between 500 and 340 million years ago. The Table Mountain Group (TMG) aquifer system is 
exposed along the west and south coasts of South Africa. It is a regional fractured rock aquifer that has become a major source of bulk water supply to 
meet the agricultural and urban water requirements of the Western and Eastern Cape Provinces of South Africa. The TMG aquifer system comprises of an approximately 4000 m thick sequence of quartz arenite and minor shale layers deposited in a shallow, but extensive, predominantly eastwest striking 
asin, changing to a northwest orientation at the west coast. The medium to coarse grain size and relative purity of some of the quartz arenites, 
together with their well indurated nature and fracturing due to folding and faulting in the fold belt, enhance both the quality of the groundwater and its 
exploitation potential for agricultural and domestic water supply purposes and its hot springs for recreation. The region is also home to some unique 
and indigenous floral species (fynbos) of worldwide importance. These and other groundwater dependent vegetation are found on the series of 
mountains, mountain slopes and valleys in the Cape Peninsula. The hydrogeology of the TMG consists of intermontane and coastal domains which 
have different properties but are interconnected. The former is characterized by direct recharge from rain and snow melt, deep groundwater circulation with hot springs and low conductivity groundwater. The coastal domain is characterized by shallow groundwater occurrence usually with moderate to 
poor quality, indirect recharge from rainfall of shallow circulation and where springs occur they are usually cold. The sustainable utilization of the TMG 
aquifer addressed the issues of the groundwater flow dynamics, recharge and discharge to and from the aquifer
challenges of climate change and climate variability and their potential impact on the aquifer system. The concept of safe yield, recharge and the capture principle and the integration of 
sustainable yield provided the basis for sustainable utilization with the adaptive management approach. Methodology used included the evaluation of 
recharge methods and estimates in the TMG aquifer and a GIS based water balance recharge estimation. The evaluation of natural discharges and 
artificial abstractions from the TMG aquifer system as well as its potential for future development. The Mann-Kendal trend analysis was used to test historical and present records of temperature and rainfall for significant trends as indication for climate variability and change. The determination of 
variability index of rainfall and standard precipitation index were additional analyses to investigate variability. The use of a case study from the Klein 
(Little) Karoo Rural Water Supply Scheme (KKRWSS) within the TMG study area was a test case to assess the sustainable utilization of TMG aquifers. 
Results show that recharge varies in time and space between 1% and 55% of MAP as a result of different hydrostratigraphic units of the TMG based on 
geology, hydrology, climate, soil, vegetation and landuse patterns however, the average recharge is from 1% to 5% of MAP. The TMG receives recharge 
mainly through its 37,000 km2 of outcrop largely exposed on mountainous terrain. Natural discharges from the TMG include 11 thermal and numerous 
cold spring discharges, baseflow to streams and reservoirs, and seepage to the ocean. Results from this study also show increasing temperature 
trend over the years while rainfall trend generally 
remain unchanged in the study area. Rainfall variability persists hence the potential for floodsand droughts in the region remain. Global and Regional Models predict about 10% to 25% reduction in rainfall and increase in variability in future. Impacts of 
his change in climate will affect the different types of aquifers in various ways. Increase in temperature and reduction in rainfall will increase 
evapotranspiration, reduce surface flows and eventually reduce shallow aquifer resources. Coastal aquifers risk upsurge in salinisation from sea level 
rise and increase in abstractions from dwindling surface water resources. While floods increase the risk of contamination to shallow aquifers droughts 
put pressure on all aquifers especially deep aquifers which are considered to be more reliable due to the fact that they are far removed from surface conditions. Future population growth and increase in freshwater demand will put more pressure on groundwater. Recharge to groundwater have been 
over-estimated in certain areas in the past leading to high abstraction rates from boreholes causing extensive groundwater storage depletion evident by high decline in groundwater levels in these areas and hampering sustainable management of the aquifer resources. Over-abstraction have resulted in 
loss of stream flow and baseflow reduction to streams during summer, complete loss of springs and reduction of flow to others. Flow to wetlands, 
riparian vegetation, and sometimes loss and shifts in dependent ecosystems have also resulted from over-abstraction. Sustainability has spatial and 
temporal implications due to changing climate and demand. The study recommends adaptive management practices in which several factors are 
considered in managing groundwater together with surface water resources in order to maintain ecological and environmental integrity. The KKRWSS 
and other groundwater supply schemes in the Western and Eastern Cape Provinces demonstrate the huge potential of the TMG to provide freshwatersupply for domestic and irrigation water needs however, the huge decline in groundwater levels due to over-abstraction in the KKRWSS and 
other groundwater schemes underscores the need for sustainable utilization of the TMG groundwater resources for present and future generations with 
minimal impacts on the quality, dependent hydrological and ecosystems as well as the environment.
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