The chemical groundwater characteristics of the Sutherland area, Northern Cape
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The objectives of the project were to establish, identify, interpret and map the chemical groundwater composition of the area surrounding the town of sutherland' Processes that govern the groundwater chemistry of the area are identified and interpreted' The study area is underlain by the fractured rocks of the Abrahamskraal formation of the Beaufort Sequence, Jurassic dolerites and intrusive associated with the Satpeterkop Carbonatite Complex. The groundwaters naturally evolve from a Ca-HCO. to a Na-Cl type water with Na-HCOr, Ca-SOo, Na-SO. and ca-cl being the intermediate water types formed. Statistical, graphical and hydro chemical techniques are used to characterise the groundwater composition of the fractured aquifers in the Sutherland region. The statistical techniques, specifically descriptive, Pearson's correlation matrices and varimax rotated factor analyses were used on the hydrochemical data set. The statistical analyses aided in reducing the rather large data set to the more significant variables that impact on the groundwater composition. The results of the statistical analyses coupled with graphical methods and the stable isotopes (18O and 2H) suggested that topography, evapotranspiration, geology and anthropogenic influences are the major factors responsible for the groundwater composition of the area. A number of processes were identified that occur within the subsurface. Rainwater charged with biogenic CO2, infiltrates into the subsurface where it dissolves carbonate-containing minerals, mostly CaCO3. ln flatter areas, where infiltration is slow, the infiltrating water leaches evaporitic salts to the subsurface. Thus, in higher lying areas where salt leaching is absent, Ca-HCO3. type waters would form. Where salt leaching is predominant, Na-Cltype water may form. Na-Cltype waters also form because of natural hydrogeochemical evolution. Through the mechanism of cation exchange Na-HCO3, waters are formed where Ca is exchanged mainly by bound Na. Precipitation of calcite out of a solution results in the dissolution of gypsum (CaSO4.2H2O), fluorite (CaF2) and carbonate minerals (i.e. strontianite). Formation of Ca-SO4 waters is found in this manner. Exchangeable Na is released and Ca is taken up by the geological matrix forming a Na-SO4. type water. At higher salinities' the process of reverse cation exchange result in the Ca-Cl type water from Na-Cl water, whereby Na is replaced by bound Ca. The presence of Ga-Cl type water is indicative of mixing or dilution of a more saline and older water by a fresher, younger water dominated by Ca ions. Ebments that do not contribute significantly to the groundwater salinity such as Mg, K, Sr and Ba participate in cation exchange processes but are masked by the major ions Ca, Na, Cl, HCO3 and SO4. The processes of dissolution, precipitation and cation exchange coupled with the physical environment, result in the hydrogeochemical groundwater evolution of the area. Superimposed on the processes occurring in the subsurface are the effects of concentration by means of evapotranspiration and land use practices. irrigation return flows together with the high evaporation rates increases the formation of the already high natural soluble salts in the unsaturated zone. Nitrate pollution from animal wastes causes isolated instances of pollution of the fractured rock aquifers. ln order to supply potable groundwater to the area it is recommended that boreholes, if possible, be situated away from topographical flat areas. Monitoring programs should also be initiated to determine the changes of the groundwater over time.