136
T. A. Abiye and K. C. Leketa
Africa). Pertinent data representative of annual averages for stable isotopes was
systematically collated. Groundwater samples were analyzed for stable isotopes of
water molecules (
18 O and
2 H), and
14 C dating; 14 samples were taken from southeast Botswana, six from Dendron and 18 from the Johannesburg region. Analysis for
stable isotopes of water was conducted at the Hydrogeology Lab, University of the
Witwatersrand, South Africa, using an LGR Liquid Water Isotope Analyzer, while
14 C analysis was conducted at iThemba Labs in Gauteng, South Africa.
To estimate ambient temperature, regression equations from various published
studies were considered. Cuffey et al. (1995) proposed a linear regression for the
paleo-thermometer based on
18 O values as δ
18 O (‰) = 0.327T−24.8‰ for Greenland, using
18 O values of ice core and calibrated with borehole temperature. More
important, the pioneering work of Dansgaard (1964), based on the global isotopic
composition of precipitation and temperature, was applied. Based on the global
relationship, the
18 O values in relation to temperature at the time of rainfall event
was represented by δ
18 O‰ = (0.695xT annual −13.6‰). Rearranging the relationship
for temperature leads to: T annual = (δ
18 O‰ +13.6‰) /(0.695), which was used to
calculate the ambient temperature at the time of recharge in this study.
Based on the exponential function, groundwater mean residence time was estimated where: t(years) = −8267 ln (a
14
t C/a
14
0 C) (Clark and Fritz 1997), and a
14
t C is
the measured
14 C activity and a
14
0 C is the initial
14 C activity (100 pMC).
3 Results and Discussion
Data for the environmental isotopes of
18 O,
2 H and
14 C and the calculated ambient
temperatures are presented in Table 1. The stable isotopes of
18 O and
2 H have
been plotted in Fig. 2, which reveals at least three clusters. Results from southeast
Botswana indicate that the δ
18 O values fall between −8.22‰ and −3.95‰, with δ
2 H
values in the range of −38.8‰ to −27.3‰. In this region, the lowest
14 C recorded
is 1.4 ± 1.5 pMC (long Mean Residence Time) with the highest value of 96.5 ±
2.5 pMC (young water), and a corresponding
14 C age of 35,500 ± 50 years and 300
± 5 years, respectively (Table 1). Obviously, the highly enriched δ
18 O values of −
3.95‰ (BH9), −4.56‰ (BH12) and −4.61‰ (BH11) could suggest the occurrence
of recharge after evaporation (Fig. 2) where two distinct clusters of groundwater
are identified based on the
18 O and
2 H distribution: one cluster falling above the
GMWL (Cluster A), indicating recharge in a cold climatic condition affected by
the condensation process. The other cluster plotted below the GMWL line (cluster
B), from the Dendron area (Limpopo Province), indicates the impact of evaporation
prior to recharge. The highly depleted values in δ
18 O (Cluster A) could be related to
moisture that emanated from high latitude sources with minimal evaporation effect.
The cold south Atlantic air mass could also be a source for highly depleted moisture.
Large variation in stable isotope values could be attributed to variation in temperature during rainfall events that generate recharge to the groundwater. Calculated
temperature values during the rainfall event reveal variation in southeast Botswana
T. A. Abiye and K. C. Leketa
Africa). Pertinent data representative of annual averages for stable isotopes was
systematically collated. Groundwater samples were analyzed for stable isotopes of
water molecules (
18 O and
2 H), and
14 C dating; 14 samples were taken from southeast Botswana, six from Dendron and 18 from the Johannesburg region. Analysis for
stable isotopes of water was conducted at the Hydrogeology Lab, University of the
Witwatersrand, South Africa, using an LGR Liquid Water Isotope Analyzer, while
14 C analysis was conducted at iThemba Labs in Gauteng, South Africa.
To estimate ambient temperature, regression equations from various published
studies were considered. Cuffey et al. (1995) proposed a linear regression for the
paleo-thermometer based on
18 O values as δ
18 O (‰) = 0.327T−24.8‰ for Greenland, using
18 O values of ice core and calibrated with borehole temperature. More
important, the pioneering work of Dansgaard (1964), based on the global isotopic
composition of precipitation and temperature, was applied. Based on the global
relationship, the
18 O values in relation to temperature at the time of rainfall event
was represented by δ
18 O‰ = (0.695xT annual −13.6‰). Rearranging the relationship
for temperature leads to: T annual = (δ
18 O‰ +13.6‰) /(0.695), which was used to
calculate the ambient temperature at the time of recharge in this study.
Based on the exponential function, groundwater mean residence time was estimated where: t(years) = −8267 ln (a
14
t C/a
14
0 C) (Clark and Fritz 1997), and a
14
t C is
the measured
14 C activity and a
14
0 C is the initial
14 C activity (100 pMC).
3 Results and Discussion
Data for the environmental isotopes of
18 O,
2 H and
14 C and the calculated ambient
temperatures are presented in Table 1. The stable isotopes of
18 O and
2 H have
been plotted in Fig. 2, which reveals at least three clusters. Results from southeast
Botswana indicate that the δ
18 O values fall between −8.22‰ and −3.95‰, with δ
2 H
values in the range of −38.8‰ to −27.3‰. In this region, the lowest
14 C recorded
is 1.4 ± 1.5 pMC (long Mean Residence Time) with the highest value of 96.5 ±
2.5 pMC (young water), and a corresponding
14 C age of 35,500 ± 50 years and 300
± 5 years, respectively (Table 1). Obviously, the highly enriched δ
18 O values of −
3.95‰ (BH9), −4.56‰ (BH12) and −4.61‰ (BH11) could suggest the occurrence
of recharge after evaporation (Fig. 2) where two distinct clusters of groundwater
are identified based on the
18 O and
2 H distribution: one cluster falling above the
GMWL (Cluster A), indicating recharge in a cold climatic condition affected by
the condensation process. The other cluster plotted below the GMWL line (cluster
B), from the Dendron area (Limpopo Province), indicates the impact of evaporation
prior to recharge. The highly depleted values in δ
18 O (Cluster A) could be related to
moisture that emanated from high latitude sources with minimal evaporation effect.
The cold south Atlantic air mass could also be a source for highly depleted moisture.
Large variation in stable isotope values could be attributed to variation in temperature during rainfall events that generate recharge to the groundwater. Calculated
temperature values during the rainfall event reveal variation in southeast Botswana