INVESTIGATION OF GROUND WATER POLLUTION ASSOCIATED WITH WASTE DISPOSAL: DEVELOPMENT OF AN ENVIRONMENTAL ISOTOPE APPROACH
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:
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:
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.