6 Historic Climatic Variability and Change: The Importance …
135
in northern Africa and Europe, significant groundwater replenishment took place
during the pluvial periods in the late glacial maxima, where general cooling of around
5 to 6 °C was estimated through the noble gas method (Edmunds 2005). Jones and
Mann (2004) indicated that in the Late Pleistocene, global temperatures were about
4 °C cooler than in the mid-twentieth Century, confirming unprecedented warming
at a global scale. Evidence also suggests the existence of ancient lakes in the Sahara
(Street and Grove 1979), which dried up due to high ambient temperature. This,
obviously, has implications for the absence of active groundwater recharge in the
region, as well as on extreme desertification, which impacted the availability of
freshwater.
In recent times, southern Africa experienced an El Niño event, which is characterized by high surface temperature and a decrease in rainfall amount (Kogan and
Gu 2017) resulting in increased evaporation, while causing tremendous pressure on
water resources management (Abiye 2016). Based on the available environmental
isotope record from South Africa and Botswana (Limpopo River Basin, Fig. 1), this
study tries to determine the ambient temperature during rainfall events that generated
recharge to groundwater and correlate them with
14 C Mean Residence Time (MRT)
to observe temperature change in line with long-term climate variability and change.
In order to estimate the groundwater MRT, radiocarbon (
14 C) with a half-life of
5,730 years (Clark and Fritz 1997) was used.
14 C is naturally produced in the atmosphere to form
14 CO 2 , which is assimilated into plants through plant respiration.
14 C
is measured in “percent Modern Carbon” (pMC), which corresponds with 95% of
the Oxalic acid standard for the
14 C activity of 1950 (Clark and Fritz 1997; Suckow
et al. 2013). In water bodies, CO 2 exchange occurs through carbonate formation that
retains
14 C in its molecular structure, where the infiltrating water picks up
14 CO 2
in the root zone from respiring roots and decomposition of dead organic matter.
According to Leaney and Allison (1986) and Suckow et al. (2013), water that was
recharged pre-1952 when the atmospheric
14 C was 100 pMC for deep groundwater,
is not affected if a closed system is assumed for carbonate dissolution. Values of
14 C
close to 100 pMC could suggest that the recharge rate is fast or it is restricted to
certain areas of the aquifer. Therefore, the temporal variation in stable isotopes has
been thoroughly assessed based on
14 C data in order to gain insight into long-term
climate variability in the Limpopo River Basin. With the current high rate of population growth and an increase in water demand, this analysis is intended to assist water
managers to plan and an implement intervention to protect groundwater depletion
due to recurrent drought.
2 Methodology
Three sites were identified within the Limpopo River Basin in South Africa and
Botswana (Fig. 1), where groundwater samples were collected for environmental
isotope analysis. These sites are located in southeast Botswana, Dendron (Limpopo
Province, South Africa) and the Johannesburg region (Gauteng Province, South
135
in northern Africa and Europe, significant groundwater replenishment took place
during the pluvial periods in the late glacial maxima, where general cooling of around
5 to 6 °C was estimated through the noble gas method (Edmunds 2005). Jones and
Mann (2004) indicated that in the Late Pleistocene, global temperatures were about
4 °C cooler than in the mid-twentieth Century, confirming unprecedented warming
at a global scale. Evidence also suggests the existence of ancient lakes in the Sahara
(Street and Grove 1979), which dried up due to high ambient temperature. This,
obviously, has implications for the absence of active groundwater recharge in the
region, as well as on extreme desertification, which impacted the availability of
freshwater.
In recent times, southern Africa experienced an El Niño event, which is characterized by high surface temperature and a decrease in rainfall amount (Kogan and
Gu 2017) resulting in increased evaporation, while causing tremendous pressure on
water resources management (Abiye 2016). Based on the available environmental
isotope record from South Africa and Botswana (Limpopo River Basin, Fig. 1), this
study tries to determine the ambient temperature during rainfall events that generated
recharge to groundwater and correlate them with
14 C Mean Residence Time (MRT)
to observe temperature change in line with long-term climate variability and change.
In order to estimate the groundwater MRT, radiocarbon (
14 C) with a half-life of
5,730 years (Clark and Fritz 1997) was used.
14 C is naturally produced in the atmosphere to form
14 CO 2 , which is assimilated into plants through plant respiration.
14 C
is measured in “percent Modern Carbon” (pMC), which corresponds with 95% of
the Oxalic acid standard for the
14 C activity of 1950 (Clark and Fritz 1997; Suckow
et al. 2013). In water bodies, CO 2 exchange occurs through carbonate formation that
retains
14 C in its molecular structure, where the infiltrating water picks up
14 CO 2
in the root zone from respiring roots and decomposition of dead organic matter.
According to Leaney and Allison (1986) and Suckow et al. (2013), water that was
recharged pre-1952 when the atmospheric
14 C was 100 pMC for deep groundwater,
is not affected if a closed system is assumed for carbonate dissolution. Values of
14 C
close to 100 pMC could suggest that the recharge rate is fast or it is restricted to
certain areas of the aquifer. Therefore, the temporal variation in stable isotopes has
been thoroughly assessed based on
14 C data in order to gain insight into long-term
climate variability in the Limpopo River Basin. With the current high rate of population growth and an increase in water demand, this analysis is intended to assist water
managers to plan and an implement intervention to protect groundwater depletion
due to recurrent drought.
2 Methodology
Three sites were identified within the Limpopo River Basin in South Africa and
Botswana (Fig. 1), where groundwater samples were collected for environmental
isotope analysis. These sites are located in southeast Botswana, Dendron (Limpopo
Province, South Africa) and the Johannesburg region (Gauteng Province, South