Assessment of Groundwater Potential in Fractured Crystalline rocks of the North West Province, South Africa

Abstract:

Investigation of groundwater potential in crystalline rocks of the North West Province was carried out. The area is located close to the boundary between South Africa and Botswana. The main objective of the study is to investigate groundwater potential using multi-data integration approach. Geologically, the area comprises of volcano-sedimentary rocks of the Ventersdorp Supergroup, carbonates, arenaceous and argillaceous sedimentary rocks of the Transvaal Supergroup. The youngest rocks are the Karoo Supergroup and the Quaternary sedimentary cover.

Multivariate statistical modelling was carried out to produce groundwater prospectivity map of the area. In addition, groundwater quality was assessed using hydrochemical and environmental isotopes. The statistical modelling includes data-driven and knowledge-based approaches. The data-driven approach includes artificial neural networks (ANNs), weights of evidence (WoE) and logistic regression (LG), while the knowledge-based approach involves the use of fuzzy logic. The two statistical modellings were applied to five evidential themes that include geology, structures, geomorphology, land-use and soil types.

The data-driven approach provided nearly similar results showing lithologic units and fracture connectivity play a key role in controlling groundwater occurrence. The influence of lithology is significant in the southern part of the area, while fracture connectivity play important role in controlling groundwater occurrence within the crystalline rocks that are situated in the central and northern parts of the area. In general, the results of multivariate statistical modelling show number of groundwater potential zones varying from very good to very poo. The zone shown as very good and goodgroundwater potential covers ~17% and ~22% of the study area, respectively. These zones correspond to carbonate rocks located in the southern part of the area where percolating rainwater developed large cavities that can store and supply significant volume of water.

The integration of various data sets in the present study provided valuable information about the groundwater resource potential of the area. This approach can be used as a frame of reference for characterization of groundwater potential in similar hydrogeological setting.