Investigation of potential high-yielding groundwater aquifer zones using remote sensing & GIS technology

Abstract

Groundwater exploration has been critical in considering groundwater as an alternative freshwater source in basin management. Groundwater exploration simulates the aquifer yield capacity, which is helpful for planning purposes and water resource assessments. However, understanding the dynamic flow conditions of groundwater affected by anthropogenic land cover, water use changes, and uncertain climatic variability, especially in a semi-arid region like South Africa (SA), has called for quantification and quality rating of the resource. The aim of this study was to assess the groundwater potential (GWP) and identify high-yielding groundwater aquifer zones in KwaZulu-Natal (KZN), South Africa. Groundwater potential zones were spatially mapped using Geographic Information Systems (GIS) and remote sensingbased multi-criteria analysis. Spatial thematic layers viz. geology, lineament density, slope, drainage density, rainfall, land use/land cover and evapotranspiration were processed and developed using GIS and weighted using Saaty’s Analytical Hierarchy Process (AHP). The thematic layers were subsequently aggregated using the GIS Weighted Overlay Method to develop a groundwater potential index map. Indices from the map were correlated with data from 113 boreholes using the Receiver Operating Characteristic Curve (ROC) and Area Under the Curve (AUC) to validate the results. Due to the widespread domestic use of groundwater in SA and the minimal available groundwater quality data in KZN, the groundwater quality data for parts of SA were analysed regarding concentrations of pollutants, inorganic chemicals and macropollutants. Groundwater quality data was obtained from available literature across SA and was compared with the South African National Standards (SANS) and World Health Organization’s (WHO) guidelines for drinking-water quality using the weighted arithmetic Water Quality Index (WQI). A Piper Plot was then used to graphically analyse the chemistry of the groundwater samples to compare their ionic compositions. The groundwater quality results revealed that most parameters were below or slightly above the maximum permissible limit, except fluoride, which exceeded the permissible limit in most studied locations and drastically affected the WQI values. The computed WQI values ranged from 37.92 – 436.06. Therefore, of the eleven groundwater data sets, four are classified as “good”, two as “poor”, one as “very poor”, and four as “unfit for drinking”. The results highlight the need to treat fluoride in South African groundwater, as it is a significant factor in categorising areas suitable for groundwater consumption. From the groundwater quality results it was deduced that the water quality data was either clustered as “magnesium-bicarbonate” or “sodium-chloride” type. The results of the groundwater potential mapping revealed that the AHP-based GWP map exhibited a strong correlation with borehole data (r=0.726, n=113), indicating the accuracy of the AHP as a rating method. The results computed that approximately 47.3 km2 (2%) of the total area falls under Excellent GWP, 24405.4 km2 (27.45%) under good GWP, 50950.5 km2 (57.3%) under moderate GWP, and the poor and very poor GWP zones constitute around 13380.8 km2 (15.1%) and 135.6 km2 (1%) of KZN respectively.