Application of 87Sr/86Sr ratios to discerning groundwaters from different lithostratigraphic units in the Southwest coastal region of South Africa
Date
2022-04
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: South Africa has been significantly affected by climate change and has experienced
reoccurring drought conditions induced by low precipitation levels and resulting in
declining surface freshwater levels. This is further exacerbated by a rapid rise in
population and increasing urbanization and demand for freshwater supply. The
Western Cape Province is one of the most affected regions in the country to the point
where in 2018, The City of Cape Town was close to “Day Zero” when all water reserves
were set to run out. This led to a search for alternative water supply options and
groundwater was identified as a major source of freshwater for residents. Western
Cape groundwater is sourced from fractured lithostratigraphic rock units and the
principal aim of this study was to constrain the 87Sr/86Sr ratio signatures of five of these
aquifers. These are namely, the Malmesbury, Cape Granite, Alluvial, Table Mountain
Group (TMG) and Bokkeveld Aquifers. A total of 83 borehole sites were sampled and
groundwater samples were analysed for EC, pH, alkalinity, hydrochemistry, 87Sr/86Sr
ratios and stable isotopes.
The TMG Aquifer had the lowest EC values ranging between 9.90 – 422 μS/cm with
an average of 199.55 μS/cm while the Malmesbury Aquifer had the highest average
EC 3223.73 μS/cm with values ranging between 129 – 8870 μS/cm. Alluvial
groundwaters had EC values ranging between 370 – 10810 μS/cm with an average of
2147.29 μS/cm. Cape Granite Aquifer samples had groundwater ranging between
43.40 – 5230 μS/cm with an average of 743.77 μS/cm. The Bokkeveld Aquifer had an
average EC of 470.20 μS/cm and a range between 268 - 919 μS/cm. In terms of pH,
TMG Aquifer groundwaters had the lowest pH with an average of 5.24 and values
ranging between 4.03 – 6.29. The Cape Granite Aquifer had values ranging between
5.83 – 8.06 and an average of 6.50. Malmesbury and Alluvial groundwaters had similar
average pH values of 6.74 mg/L and 6.40 mg/L respectively. However, the
Malmesbury Aquifer samples had a narrower pH range of 6.06 – 7.66 compared to the
wider range of 4.47 – 7.60 observed for the Alluvial Aquifer. Bokkeveld Aquifer
samples had a pH range of 6.29 – 6.88 and an average of 6.52.
Alluvial Aquifer alkalinity values ranged between 2.10 - 371.70 mg/L with an average
of 107.84 mg/L while Cape Granite Aquifer values ranged between 11.19 – 159.01
mg/L with an average of 57.54 mg/L. The Malmesbury Aquifer had the highest average
alkalinity at 143.66 mg/L with a range of 60 - 418 mg/L while the TMG Aquifer had the
lowest alkalinity values with a range of 1.80 – 34.20 mg/L and an average of 8.00
mg/L. The Bokkeveld Aquifer had an average alkalinity of 74.60 mg/L and values
varied between 31 - 108.10 mg/L. In general, the dominant cations and anions for the
groundwater samples are, Na+ < Ca2+ < Mg2+ < K+ and Cl-< HCO3-< SO42- < NO3- <
PO43-. Most of the groundwaters are saline with Na+ as the dominant cation and Cl- as
the dominant anion (NaCl-type). Malmesbury groundwaters generally had the highest
major ion concentrations, especially Na+, Ca2+, Mg2+, K+, Cl- , SO42-, NO3- and HCO3-
. This is followed by the Alluvial Aquifer which also has high concentrations of the
same ions. Groundwater from the TMG Aquifer had the lowest concentrations in Na+,
Ca2+, Mg2+, K+, Cl- , SO42-, NO3- and HCO3-. Cape Granite groundwaters did not exhibit
a distinct ionic composition but generally had higher concentrations of Na+ and Cl-
compared to Ca2+, Mg2+ and K+. The Cape Granite Aquifer also had higher
concentrations of HCO3- and SO42- compared to NO3-. The Bokkeveld groundwater
samples had low concentrations of Ca2+, Mg2+ and Cl- but slightly higher Na+ and K+
concentrations with low SO42- and NO3- concentrations.
The TMG and Bokkeveld Aquifers had similar δD ranges of -25.1 to -10.90 ‰ and -
29.4 to -12.8 ‰ respectively. The average δD for the TMG Aquifer was -20.3 ‰ and -
23.4 ‰ for the Bokkeveld Aquifer. The δ18O range of the TMG Aquifer was -5.33 to
-2.89 with an average of -4.40 ‰ and -5.23 ‰ to -2.55 ‰ with an average of -4.40 ‰
for the Bokkeveld Aquifer. The Malmesbury and Alluvial Aquifers had average δD
values of -17.4 ‰ and -15.4 ‰ respectively and the δD values had ranges between -
20.2 to -11.3 ‰ and -19.8 to -6.00 ‰ respectively. The δ18O values for Malmesbury
groundwaters ranged from -4.32 to -3.07 ‰ with an average of -3.74 ‰ while the
Alluvial Aquifer had an average δ18O value of -3.44 ‰ and values ranged between -
4.26 to -1.98 ‰. The Cape Granite Aquifer had a δD range of -27.3 to -12.3 ‰ and a
δ18O range of -4.44 to -2.96 ‰ with an average δD of -18.6 and an average δ18O of -
4.06 ‰.
Sixty samples were chosen for analysis of the 87Sr/86Sr ratio and 35 additional 87Sr/86Sr
ratios were obtained from Costaras (2019). The 87Sr/86Sr ratios for the aquifers were
found to have distinct ranges. The Cape Granite Aquifer had 87Sr/86Sr ratios ranging
between 0.71812-0.73939. The Alluvial and Malmesbury Aquifers had 87Sr/86Sr ratios
ranging between 0.71166 – 0.71988 and 0.70936 – 0.71852 respectively. The
Bokkeveld Aquifer had a 87Sr/86Sr ratio range between 0.71720 – 0.72214 while the
TMG Aquifer exhibited the most variability in 87Sr/86Sr with ranges between 0.71152
and 0.72955. The assigned 87Sr/86Sr ratio was 0.72620±0.0049 for the Cape Granite
Aquifer, 0.71510±0.0035 for the Malmesbury Aquifer, 0.71311±0.0029 for the Alluvial
Aquifer, 0.71580±0.0068 for the TMG Aquifer and 0.71892±0.0028 for the Bokkeveld
Aquifer.
Assessment of the spatial distribution of groundwaters showed that most coastal
samples are associated with the Alluvial Aquifer and have 87Sr/86Sr ratios close to
modern seawater. Further inland, groundwaters had progressively higher 87Sr/86Sr
ratios. A temporal variability study showed that the 87Sr/86Sr ratio remains largely
constant over shorter and longer term periods. The isotopic ratio for samples taken
three-hourly showed little variation while longer-term samples take monthly showed
more but not significant variation. This indicates that influence by recharge and dilution
processes is unlikely to significantly affect the 87Sr/86Sr ratio over long-periods. In
future studies, the study of the spatial distribution of 87Sr/86Sr ratios would require a
more well-distributed sampling area to get a better sense of the change in 87Sr/86Sr
ratios with geographical location. A challenge faced in this study was a lack of
boreholes in certain areas or the occurrence of boreholes where sampling could not
be done due to issues like lack of a take-off point or boreholes with no pumps. For the
longer-term temporal variability study, sampling would need to take place over a more
extensive period to further ensure the stability of the 87Sr/86Sr ratios.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Description
Thesis (MSc)--Stellenbosch University, 2022.
Keywords
Strontium -- Isotopes -- Environmental aspects, Hydrochemistry, Groundwater sustainability, Stable isotopes, Radioisotopes in hydrology -- South Africa -- Western Cape, Groundwater -- South Africa -- Western Cape, Isotope geology -- South Africa -- Western Cape, Water -- Quality, Climate changes, UCTD