Report No 311/1/01


Ground water is a resource which is becoming increasingly important as South Africa's fresh water supplies become overcommitted. At the same time, this resource is most prone to pollution from waste disposal. Unless clear guidelines are given and useful approaches to impact assessment developed, major damage could be inflicted on ground water resources which are being, or may have to be harnessed for domestic supply.

This study develops and evaluates in particular the use of environmental isotope hydrology in assessing ground water vulnerability to waste disposal. The original project objectives were:

  1. To determine the potential of isotope hydrological methodologies for waste disposal site identification and how they could complement and support geohydrological methods
  2. A feasibility study to select suitable areas; survey existing information; gather new geohydrological, chemical and environmental isotope data: collate and evaluate this data in terms of its applicability in site investigation
  3. With a positive outcome of the feasibility study to intensify the investigation with strategic drilling and pump testing, and to elaborate the developed methodologies to other areas and other geohydrological situations.

    The Olifantsfontein area of Clayville in Midrand was selected, as it is an area of some importance to existing and potential future potable water supplies. The area of study at Midrand is underlain by Transvaal dolomites. A waste disposal facility occupies a worked-out clay quarry in a deposit (outlier) of Karoo sediments.

The structure, geology and geohydrology of the area are described in some detail. Ground water flow is northwards and is constrained by a system of compartmentalising dykes. Considerable differences in ground water level and chemistry are found at different sides of some of the dykes. Geophysical observations delineate areas of weathering of the dolomites and possible channels of preferential ground water flow.

An initial hydrocensus provided extensive analytical data on hydrochemistry and environmental isotopes. Isotopic data showed that the entire area of study is prone to infiltration of surface water and is therefore vulnerable to surface pollution. Ground water mobility was found to decrease markedly with depth. No specific information could be obtained from dolomite below or near Karoo outliers, as existing information on and newly-drilled boreholes at such locations was found to be inadequate.

Correlations between data sets of various isotopes and chemistry revealed that there exists considerable contrast between the regional flow system and local recharge. The isotopic contrast is inferred to be due to the distinct isotope signal of the Rand Water mains supply. A significant correlation is seen between certain dissolved ions and isotopes, suggesting a possible distinction between urban/industrial and agricultural pollution. The generally low level of this pollution during the period of study in relation to that observed in the control area, gives rise to the concept of "incipient pollution", as it can be identified as such by its environmental isotope association.

Ground water flow relationships between compartments could only be partially established through the drilling of study-related boreholes and hydrochemical and isotopic analysis. No existing or specially drilled boreholes were adequately sited or constructed to establish the relationship between Karoo and dolomite ground water, or assess the behaviour of water in the Karoo. These questions were of primary importance in the project formulation.

In order to overcome this shortcoming, three boreholes were drilled, during the final phases of the study near a worked-out quarry containing effluent derived from Rand Water mains. Isotope measurement on the water extracted from the drilling material at different depths established the extreme heterogeneity of transmissivity of the Karoo and its intrinsic unsuitability as a competent sealing layer against leachate transport in sanitary landfills operated in such worked-out quarries.

Four smaller studies, which ran partially concurrently with the Midrand study, were undertaken at other waste disposal sites, to obtain information from different geological environments.

Boreholes drilled a decade earlier to assess pollution of shallow ground water at the Waterval landfill disposal site in western Johannesburg were found to have all collapsed. However, surface water transport from the site could be established through the presence of artificial tritium, probably derived from the site, and its possible influence on the surrounding ground water.

A survey of ground water at the Linbro Park landfill site, north-east of Johannesburg, established the inadequacy of the available ground water monitoring network. Interpretation of the (inadequate) information from the existing boreholes suggests that, in spite of the rapid turnover time of the ground water, there is at present no evidence of leachate pollution.

Isotopic and chemical observations on monitoring boreholes at the northern and southern landfills of the city of Bloemfontein, and on some private boreholes, give an indication of the leachate transport from these sites in a Karoo environment. At the northern site, there is evidence of increasing ground water pollution and leachate transport to ground water through surface drainage. At the southern site, the influence of leachate can be established at nearby farm boreholes, probably due to overland flow following major rainfall events. Artificial tritium, and possibly radiocarbon, are present in both sites and may act as definitive tracers of leachate transport.

The main conclusions from the study were:

  1. That environmental isotope hydrology is an indispensable component of any study assessing actual and potential ground water pollution.
  2. That isotope information, powerful though it is in transport studies, can only be fully exploited when seen in the context of the geohydrology and hydrochemistry of the system studied.
  3. In this holistic context, environmental isotopes are appropriate technology and an economical way of obtaining important and sometimes unique information in geohydrological studies.
  4. Isotope data can establish "incipient" pollution, a concept of great importance in the timeous identification and rehabilitation of pollution sources.
  5. In the Gauteng area, the Rand Water mains supply has an isotopic signal which differs substantially from that of most ground water in the area. This difference can be exploited for tracing anthropogenic inputs into ground water and any pollution it may carry. Isotopic contrasts between different recharge water sources could be exploited in other environments.
  6. At least some landfill sites contain artificial radionuclides which can be exploited as unique low-level tracers for leachates from such sites into surrounding water systems.
  7. At all the landfill sites studied, ground water monitoring networks were found to range from inadequate, to totally inoperative and in need of upgrading.
  8. Worked-out quarries in Karoo outliers in dolomitic terrain should not be used for purposes of sanitary landfilling, which should rather be placed on surface, to facilitate safer future management.
  9. The complexity of the Olifantsfontein geological environment has been established and has highlighted the need for extensive investigations following a site specific approach.

The recommendations as a result of this study are as follows:

The conclusions arrived at this study firmly establish the combined methodologies in terms of their potential in investigating pollution-related ground water problems. It is important to note that the approach to their application depends on the individual system investigated. Apart from enumerating the overall principles underlying the various methods, only general guidelines as to their application can be derived. As each area of investigation is different, each investigation will require an individual research approach in order to creatively employ the methods and obtain the maximum benefit from them. It is therefore advisable, and ultimately most economical, to engage the various specialists in the study from the onset.

For the information obtainable, environmental isotope data is highly cost effective. The costs involved in the analysis in a water sample of the full suite of five isotopic parameters, as discussed in this report, may be compared with the costs of a full ionic plus microbiological analysis in a pollution study.

Environmental isotope hydrology is appropriate technology. Isotope techniques are state of the art, are employed worldwide, in particular in geohydrology, and wide experience and expertise have been developed in both their application and interpretation. The specifications for many ground water development and evaluation projects internationally stipulate an environmental isotope input, along with geophysical and hydrochemical studies. South Africa has available local analytical facilities of world standard and internationally recognised expertise in the application of environmental isotope hydrology and is therefore favourably placed to conform to international practice.