Abstract
South Africa’s electricity landscape is dominated by coal combustion. Coal fly ash (CFA) is a principal by-product of coal combustion, and when disposed of on land, it has deleterious impacts on groundwater resources. Once disposed of to land, CFA leaches toxic and harmful elements such as Mg2+, Na+, Cl-, SO42-, Mn2+, Ba, B, Cr, and Fe3+ into groundwater resources. Groundwater has been earmarked as an alternative water source to augment the dwindling surface water resources; however, anthropogenic activities such as the disposal of coal fly ash threaten groundwater's ability to be a sustainable and reliable water resource for the future. The groundwater resources around Eskom Matimba power station’s ash disposal facility are not exempted from these adverse impacts caused by coal fly ash leachate.
The overall aim of the study was to determine whether the Matimba coal fly ash dump is a source of groundwater pollution, to identify contaminants of concern, and to evaluate the suitability of groundwater for domestic and agricultural irrigation purposes using water quality assessment indices.
A quantitative study was undertaken by collecting and analysing CFA samples, seepage water, and groundwater samples and studying their spatial distribution. For the determination of groundwater’s fitness for household consumption or use, the SANS 241 (2015), WHO (2022), and SAWQG for domestic use were used. The weighted arithmetic water quality index (WAWQI) was used for the classification of groundwater for domestic and drinking purposes. The suitability of groundwater for irrigation purposes was done using the SAWQG for agricultural irrigation and the parameters such as sodium adsorption ratio (SAR), sodium percentage (Na%), Kelley’s ratio (KR), permeability index (PI) and magnesium adsorption ratio (MAR) were deployed to classify the water for irrigation purposes. The statistical approaches were used to determine the relationship between variables, determine the variations in groundwater chemistry data, and determine water facies based on the geochemical processes governing the chemical composition of groundwater. The spatial distribution maps were produced using QGIS.
The findings of this study have revealed that groundwater within the vicinity of the Matimba ash dump is contaminated by leachate plumes from the coal fly ash disposal facility. This was confirmed by saline water with dominance of NaCl and CaCl2 types. The groundwater facies for the study area are the Na+-K+-Cl--SO42- type, constituting
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about two-thirds and the Ca2+-Mg2+-Cl--SO42- type representing one-third of the groundwater sampling sites. Further, the groundwater chemistry data results revealed that contaminants such as TDS, EC, SO42-, Cl-, Na+, Mg2+, Mn2+, and Fe3+ were higher than the drinking water quality guidelines, SANS 241 (2015), WHO (2022), and SAWQG for domestic use.
The suitability for use analysis for domestic consumption using WAWQI indicated that only 12.5% of the boreholes contained excellent water and about 50% will require pre-treatment before consumption. The irrigation water analysis has revealed that parameters PI, SAR, and Na% are the most realistic indices for determination of suitability for irrigation in this study and the majority of boreholes using these indices contain water suitable for agricultural irrigation purposes.
The use of groundwater quality indices for both domestic use and for agricultural purposes will close the gap in the paucity of such data in the region as previous studies only looked at the groundwater chemistry data.