134
T. A. Abiye and K. C. Leketa
southern Africa have changed since the Last Glacial maximum, about 18,000 years
ago (Vogel 1983).
The stable isotope signature of groundwater retains the effect of climatic conditions through which rainfall occurred. Therefore, environmental isotopes are suitable
to characterize climatic factors that alter the isotopic composition of water. In groundwater, stable isotopes help to explore the presence or absence of evaporation, which
could be directly linked to ambient temperature. Groundwater that receives indirect
recharge from surface water systems through ponds, channels, wetlands, streams and
lakes tends to show more enrichment in stable isotopes relative to local rainfall. This
is because of exposure of water to evaporation prior to infiltration, whereby the lighter
isotopes preferentially evaporate, leaving heavier isotopes behind, which eventually
percolate into groundwater. On the other hand, water that is directly recharged into
groundwater from rainfall through fractures and pore spaces could have a similar
isotopic signature as the rainfall, with a very limited effect of evaporation. In most
cases, surface water acts as a buffer between precipitation and groundwater so that the
effect of ambient temperature can be traced within the groundwater archive through
the identification of the stable isotope signature. Varying rates of infiltration during
summer and winter seasons could also result in an isotopic shift in rainfall and/or
temperature, besides climatically induced changes in the plant cover. These effects
are superimposed on the climatic signal imprinted in regional precipitation (Rozanski
et al. 2000).
For several decades, stable isotopes have been widely used as environmental
tracers in groundwater studies, catchment water balance, infiltration through soil
horizon, surface water and groundwater interaction, and dam leakages across the
world. Particularly in arid and semi-arid regions that are characterized by high
temperature, the isotopic composition of groundwater is dependent on the extent
of evaporation that is affected by the occurrence of effective rainfall to replenish
aquifers and hence, maintain the signature of atmospheric effect.
The spatial and temporal variability in the δ
2 H and δ
18 O values of meteoric
water results from the isotope-fractionation effect that accompanies evaporation and
condensation (Yurtsever and Gat 1981; Clark and Fritz 1997; Edmunds et al. 2005).
Variability could be long term, based on the change in climatic variables. On a millennial time-scale, changes in the δ
18 O values of ice in the tropics represent large-scale
climatic variations (Thompson and Davis 2005; Hoffmann et al. 2005). Past rainfall stored as groundwater and ice provides evidence of former climatic conditions
(Rozanski et al. 1992; Edmunds 2005; Dragoni et al. 2008). The δ
18 O distribution in
the Holocene and Late Pleistocene in large sedimentary aquifers in North Africa has
been documented by Edmunds 2005, and revealed the presence of highly depleted
δ
18 O values within the Nubian sandstone aquifer in the Late Pleistocene as compared
to the West African region, which indicates a cooler climatic condition at the time
of recharge in North African region.
The Pleistocene climate is marked by major glacial and interglacial oscillations,
where the most recent glacial period culminated about 21,000 years ago (the last
glacial maximum) with vast ice sheets that extend into the mid-latitude of North
America and Europe (Jones and Mann 2004). In the Holocene and Late Pleistocene
T. A. Abiye and K. C. Leketa
southern Africa have changed since the Last Glacial maximum, about 18,000 years
ago (Vogel 1983).
The stable isotope signature of groundwater retains the effect of climatic conditions through which rainfall occurred. Therefore, environmental isotopes are suitable
to characterize climatic factors that alter the isotopic composition of water. In groundwater, stable isotopes help to explore the presence or absence of evaporation, which
could be directly linked to ambient temperature. Groundwater that receives indirect
recharge from surface water systems through ponds, channels, wetlands, streams and
lakes tends to show more enrichment in stable isotopes relative to local rainfall. This
is because of exposure of water to evaporation prior to infiltration, whereby the lighter
isotopes preferentially evaporate, leaving heavier isotopes behind, which eventually
percolate into groundwater. On the other hand, water that is directly recharged into
groundwater from rainfall through fractures and pore spaces could have a similar
isotopic signature as the rainfall, with a very limited effect of evaporation. In most
cases, surface water acts as a buffer between precipitation and groundwater so that the
effect of ambient temperature can be traced within the groundwater archive through
the identification of the stable isotope signature. Varying rates of infiltration during
summer and winter seasons could also result in an isotopic shift in rainfall and/or
temperature, besides climatically induced changes in the plant cover. These effects
are superimposed on the climatic signal imprinted in regional precipitation (Rozanski
et al. 2000).
For several decades, stable isotopes have been widely used as environmental
tracers in groundwater studies, catchment water balance, infiltration through soil
horizon, surface water and groundwater interaction, and dam leakages across the
world. Particularly in arid and semi-arid regions that are characterized by high
temperature, the isotopic composition of groundwater is dependent on the extent
of evaporation that is affected by the occurrence of effective rainfall to replenish
aquifers and hence, maintain the signature of atmospheric effect.
The spatial and temporal variability in the δ
2 H and δ
18 O values of meteoric
water results from the isotope-fractionation effect that accompanies evaporation and
condensation (Yurtsever and Gat 1981; Clark and Fritz 1997; Edmunds et al. 2005).
Variability could be long term, based on the change in climatic variables. On a millennial time-scale, changes in the δ
18 O values of ice in the tropics represent large-scale
climatic variations (Thompson and Davis 2005; Hoffmann et al. 2005). Past rainfall stored as groundwater and ice provides evidence of former climatic conditions
(Rozanski et al. 1992; Edmunds 2005; Dragoni et al. 2008). The δ
18 O distribution in
the Holocene and Late Pleistocene in large sedimentary aquifers in North Africa has
been documented by Edmunds 2005, and revealed the presence of highly depleted
δ
18 O values within the Nubian sandstone aquifer in the Late Pleistocene as compared
to the West African region, which indicates a cooler climatic condition at the time
of recharge in North African region.
The Pleistocene climate is marked by major glacial and interglacial oscillations,
where the most recent glacial period culminated about 21,000 years ago (the last
glacial maximum) with vast ice sheets that extend into the mid-latitude of North
America and Europe (Jones and Mann 2004). In the Holocene and Late Pleistocene