Recharge estimation using CMB and environmental isotopes in the Verlorenvlei Estuarine System, South Africa and implications for groundwater sustainability in a semi-arid agricultural region
Date
2020-05-12
Journal Title
Journal ISSN
Volume Title
Publisher
MDPI
Abstract
Groundwater recharge remains one of the most di cult hydrogeological variables to
measure accurately, especially for semi-arid environments where the recharge flux is much smaller
than in humid conditions. In this study, groundwater recharge was estimated using chloride mass
balance (CMB) in the Verlorenvlei catchment, South Africa where the e ects of recent severe drought
conditions in an already semi-arid environment have impacted both agricultural activity as well as the
RAMSAR-listed Verlorenvlei estuarine system. Chloride, 18O and 2H tracers were used to improve
understanding of the groundwater flow patterns and allowed the fresh parts of the groundwater
system, defined by Ca2+-HCO3
����� groundwater types, to be separated from those where additional salts
were being introduced through groundwater mixing, and thus characterized as Na+-Cl����� groundwater
types. Recharge rates calculated from CMB in the fresh parts of the system were between 4.2–5.6%
and 11.4–15.1% of mean annual precipitation for the headwater valley and mountains of the Krom
Antonies and are largely consistent with previous studies. However, much lower recharge rates in
the valleys where agriculture is dominant contrasts with previous results, which were higher, since
groundwater-mixing zones were not recognised. Although the chloride concentration in precipitation
is based on only one year of data between 2015 and 2016, where 2015 had on average 28% less
precipitation than 2016, the results provide a snapshot of how the system will respond to increasing
drought frequency in the future. The results suggest that low rates of groundwater recharge under dry
spell conditions will impact on low flow generations which are required to sustain the Verlorenvlei
estuarine lake system. Overall, the study highlights the importance of combining hydrochemical
tracers such as bulk chloride and stable isotopes with numerical modelling in data-scarce catchments
to fully understand the nature of hydrological resilience.
Description
CITATION: Watson, Andrew, Anya Eilers, & Jodie A. Miller. 2020. Recharge estimation using CMB and environmental isotopes in the Verlorenvlei Estuarine system, South Africa and implications for groundwater sustainability in a semi-arid agricultural region. Water, 12(5):1362, doi.org/10.3390/w12051362.
The original publication is available at: https://www.mdpi.com
The original publication is available at: https://www.mdpi.com
Keywords
Groundwater recharge, Chloride mass balance, Verlorenvlei Estuarine System, Semi-arid environments
Citation
Recharge estimation using CMB and environmental isotopes in the Verlorenvlei Estuarine system, South Africa and implications for groundwater sustainability in a semi-arid agricultural region. Water, 12(5):1362, doi.org/10.3390/w12051362