Hydrochemical and environmental isotope based investigation of the Masama Ntane Sandstone Aquifer, Botswana
Date
2017
Authors
Mofokeng, Thelma
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Abstract
The Masama Sandstone Aquifer is located in a semi-arid region of south-eastern Botswana
where there are no perennial rivers. Groundwater is the main source of water supply for the
communities. Historically many water drilling programs have been carried out in this area and
the hydrogeological system has been conceptualized. An integrated approach coupling
environmental isotopes, radioisotopes and multivariate statistical analysis of the hydrochemical
variables was employed to study the origin, age, recharge conditions, rock-water interaction and
the hydrological link between the aquifer and geological structures. The major ions in this area
are Na+, Ca2+, Mg2+ and HCO-3. Groundwater in the Masama area fall in the transition from a
Na-HCO-3 –type through Ca-Na-HCO-3 to Ca-Mg-HCO-3 -type waters from the western to the
eastern part of the area. The water types are as a result of cation exchange, carbonate dissolution
and rock-weathering processes. The δ18O and δ2H values vary spatially depending on the source
of moisture, rainfall season, geology, topography and groundwater circulation depth. Deep
circulating groundwaters are isotopically depleted whilst shallow circulating groundwaters are
isotopically enriched with respect to winter rain. Low tritium values < 0.8TU and 14C values <
80pmc testifies for recharge. Recent rainfall amount in the area is not sufficient enough to make
a profound replenishment in the aquifer. Tritium, 14C and Chloride Mass Balance helped in
identifying recharge location and hydrologic connections between structures and the sandstone
aquifer elucidating that recharge zones are in the NE and NW of the study area. High recharge
rates occur in the north-eastern part and the Makhujwane fault act as a conduit for groundwater
recharge. This study provides a better understanding of the aquifer and the information contained
herein can be incorporated into future works for sustainable use of the groundwater resource.
Description
A research report submitted to the Faculty of Science school of Geosciences in partial fulfillment of the requirements for the degree of Masters of Science in Hydrogeology. Johannesburg, June 2017.
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Citation
Mofokeng, Thelma (2017) Hydrochemical and environmental isotope based investigation of the Masama Ntane Sandstone Aquifer, Botswana, University of the Witwatersrand, Johannesburg, <http://hdl.handle.net/10539/23565>