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

Andrew Watson,Jodie A Miller,Anya Eilers

doi:10.3390/w12051362

Abstract

Groundwater recharge remains one of the most difficult 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 effects 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.

Highlights

Groundwater, which makes up the majority of freshwater globally, is an important reserve that humans and ecosystems can access to adapt to variations in precipitation [1,2,3,4]
Understanding groundwater recharge rates is important for the protection and management of global aquifer systems and in particular, for regions where climate change is likely to have significant impacts on precipitation
The chloride mass balance (CMB) is such a technique, the recycling of salts in groundwater hinders its application for semi-arid environments, especially coastal regions subject to dry deposition of aerosol salts

Summary

Introduction

Groundwater, which makes up the majority of freshwater globally, is an important reserve that humans and ecosystems can access to adapt to variations in precipitation [1,2,3,4]. Uncertainties in global climate models (GCMs) still limit our understanding of whether changes in precipitation will impact groundwater resources [5], the increased length of dry spells and shorter, more intense heavy rain events [6,7] will result in changes in surface runoff and soil moisture conditions. Whilst it is uncertain whether drought frequencies are increasing [8,9], the main consensus is that more areas will become susceptible to drought, with droughts establishing more quickly and with greater intensity [10]. Recharge in semi-arid environments is difficult to quantify due to a smaller and more variable recharge flux in comparison with humid areas, meaning that fewer recharge estimation techniques can be applied successfully [13,15]

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Conclusion