Hydrogeological investigations of groundwater and surface water interactions in the Berg River catchment, Western Cape, South Africa

Abstract

It is well established that the quality of fresh water resources has been and still is deteriorating at an escalated rate globally affecting the chemical, physical and biological composition of water. As a result, fresh water has thus become a rare commodity which is crucial for the survival of any living organism on earth. Fresh water is found in groundwater aquifers and surface water resources such as rivers, streams, lakes and dams however, these resources only comprise 0.3% of fresh water that is available for human consumption out of the 71% water that constitute the earth. The remaining quantity of water found in oceans and seas requires expensive processes of desalination in order to become potable for human use. Therefore, the deteriorating quality of fresh water is escalating the already existing problem of water scarcity and as such, in the nearer future the demand will surpass supply of fresh water. Moreover, drinking water from surface water bodies have to be purified first to meet the drinking water standards before consumption. This nonetheless, does not eliminate that groundwater also have to meet drinking water standards.

Groundwater and surface water have been considered as isolated components of the hydrological cycle for centuries in the application of water resources management. This has therefore resulted in the lack of understanding of the two hydrological components. The lack of understanding therefore continues to create gaps in determining important information such as factors impacting on the quantity and quality of groundwater in particular. The complexities in determining the interactions between surface water and groundwater has thus led to surface water receiving much attention in poorer countries as it is readily available and accessible to study as opposed to groundwater.

In South Africa, much of the water used is obtained from surface water bodies like rivers, springs and earth dams. The dynamics of surface and groundwater chemical transfers has not been thoroughly studied and well understood in the country. Such a case is also observable from the Berg River catchment (BRC) wherein surface water quality have been severely studied, while on the other hand groundwater chemistry and quality in relation to the natural setting remains questionable. This study therefore provides an investigation of the interactions of surface and groundwater in the BRC. The study focused on three objectives as follows: to a) investigate the role that geology and soils play on water chemistry in the BRC, b) to identify BRC surface and groundwater chemical trends and c) to identify the geochemical processes controlling surface and groundwater chemistry in BRC. This study employed a combination of techniques (i.e. hydrochemical, environmental tracer analysis and hydrogeological mapping).

This study was carried out using three types of research designs namely i) Experimental research design; ii) Field research design and meta-analysis research design. Furthermore, the study made use of hydrochemical data ranging from 2003 to 2013 obtained from the National Water Monitoring Database owned and maintained by the Department of Water and Sanitation and data which was sampled in 2016 and analyzed using the ICP-MS Technique. This method was employed when analysing the water samples for the major cations and anions in the laboratory using Perkin Elmer ICP-OES Optima 5300 DV (to analyze cations) as well as ThermoFischer Scientific Gallery plus discreet analyser (to analyze anions). Ground Water Chart, Arc-GIS and Geosoft (Oasis Montaj) were further employed to model the data.

Groundwater Water Chart was used to model hydrochemical facies which displays various water types while Geosoft (Oasis Montaj) was used to grid the nitrates concentrations in both surface and groundwater data to establish the trends and correlations. The color intensity and scale were used to denote the concentration magnitude of analytes understudied. Finally, Arc GIS was used for creation of maps that were used to indicate direction of water flow, sampling points (location), correlation, trends as well as vulnerable areas which are more prone to contamination.

From the analysis and interpretation, the hydrochemical facies indicated that in the upper Berg River Catchment, there is very minimal interaction between surface and groundwater systems. Other water types that were found in the groundwater either than NaCl were barely found in surface water and vice versa as it was observed that, surface water samples have both NaCl and Mixed-CaMgCl while groundwater have three additional water types to those of surface including CaHCO3, Mixed CaNaHCO3 and CaCl. This may be due to the underlying consolidated hard rock formations (granitic rocks) having less geohydrological properties like fractures and voids.

The Middle Berg however, indicated a degree of interaction with sharing of constituents between the two water systems. Most of the water types found in borehole data were also found in the surface water. This can be explained with reference to the structural geology of the area in which a northwest-trending strike-slip faults of the Piketberg-Wellington faults occurred which gave rise to more permeability and movement of water.

Moreover, the Lower Berg indicated only NaCl water type. It is worth note taking that the Lower Berg is situated near the river mouth whereby there is mixing of river and sea water. This notion therefore further explains the NaCl being the sole water type in the area. In surface water this may have further been exacerbated by means of sea spray. With reference to groundwater, there may have been a possibility of sea water intrusion also enhanced by the faulting of rocks originated from the Colenso fault.