An Overview of Aquifer Physiognomies and the δ18O and δ2H Distribution in the South African Groundwaters

Abstract

A comprehensive assessment of the stable isotope distribution in the groundwater systems of South Africa was conducted in relation to the diversity in the aquifer lithology and corresponding hydraulic characteristics. The stable isotopes of oxygen (18O) and hydrogen (2H) in groundwater show distinct spatial variation owing to the recharge source and possibly mixing effect in the aquifers with the existing water, where aquifers are characterized by diverse hydraulic conductivity and transmissivity values. When the shallow aquifer that receives direct recharge from rainfall shows a similar isotopic signature, it implies less mixing effect, while in the case of deep groundwater interaction between recharging water and the resident water intensifies, which could change the isotope signature. As aquifer depth increases the effect of mixing tends to be minimal. In most cases, the isotopic composition of recharging water shows depletion in the interior areas and western arid zones which is attributed to the depleted isotopic composition of the moisture source. The variations in the stable isotope composition of groundwater in the region are primarily controlled by the isotope composition of the rainfall, which shows variable isotope composition as it was observed from the local meteoric water lines, in addition to the evaporation, recharge and mixing effects.

Groundwater Recharge and Circulation in Dolomitic Aquifer Located in Semi-Arid Region: Evidence from the δ18O and δ2H Record, South Africa

“…It is obvious that in order to utilize groundwater, aquifers have to be sustained with active recharge owing to the presence of favorable media for recharged water to circulate with ease. The Johannesburg region receives an average rainfall of about 710 mm/year, which falls sporadically between October and March [2], and the region has high potential evapotranspiration of 1600 mm/year [3] which does not guarantee substantial recharge to the crystalline aquifers [4].…”

“…For rainfall to reach the zone of saturation, it must be facilitated by the hydraulic parameters including porosity, hydraulic conductivity and transmissivity that regulate the recharging water and mixing process. The variability in the hydraulic characteristics could affect the effective rainfall to pass through the vadose zone and reach the aquifer, hencecausing the mixing of stable isotopes during the circulation process [2].…”