Investigating modern recharge in drought prone South Africa through tritium variability in rainfall and groundwater radiocarbon/tritium mixing relationships.
Authors: van Rooyen Jared , Miller Jodie , Palcsu Laszlo 
Published: March 2020

&lt;p&gt;Stable and radioactive isotope tracers, for the purpose of recharge investigations, are well constrained in the literature and include a number of reviews describing the benefits of using isotope tracers for this purpose. Tritium (&lt;sup&gt;3&lt;/sup&gt;H), the radioactive isotope of hydrogen, has been used to better understand groundwater recharge processes for decades. During the 1960&amp;#8217;s and 1970&amp;#8217;s the popularity of &lt;sup&gt;3&lt;/sup&gt;H in hydrological investigations increased as a result of elevated &lt;sup&gt;3&lt;/sup&gt;H activities due to atmospheric testing of atomic bombs. Atmospheric &lt;sup&gt;3&lt;/sup&gt;H has subsequently decayed to background levels, and &lt;sup&gt;3&lt;/sup&gt;H research now focusses on the natural controls on &lt;sup&gt;3&lt;/sup&gt;H variability in rainfall and how this relates to &lt;sup&gt;3&lt;/sup&gt;H variability in groundwater. Because &lt;sup&gt;3&lt;/sup&gt;H (half-life = 12.43 yrs) is essentially an indicator of modern recharge, its relationship to the longer lived radiocarbon isotope (&lt;sup&gt;14&lt;/sup&gt;C half-life = 5730 yrs) can give insights into mixing relationships between fossil and modern groundwater (Jasechko et al., 2017). This relationship can be established by comparing the relative abundance of both &lt;sup&gt;3&lt;/sup&gt;H and &lt;sup&gt;14&lt;/sup&gt;C. Understanding this relationship is pivotal to constraining the sustainability of an aquifer as well as its vulnerability to modern contaminants. In this study, &lt;sup&gt;3&lt;/sup&gt;H precipitation data across South Africa was collected in three ways: (1) temporal data collected per rainfall event at a single location (96 samples, Western Cape); (2) storm event data collected at multiple locations for two single rainfall events (67 samples, Western Cape); and (3) opportunistic sampling of regional rainfall collected at different times and locations (57 samples, South Africa). This data was combined with historical groundwater &lt;sup&gt;3&lt;/sup&gt;H and &lt;sup&gt;14&lt;/sup&gt;C data (454 samples). HYSPLIT backward trajectory modelling was used to evaluate atmospheric residence time, altitude and water mass origin for rainfall from which &lt;sup&gt;3&lt;/sup&gt;H activities were obtained. Temporal variability of &lt;sup&gt;3&lt;/sup&gt;H in precipitation ranges ~3.5 T.U., where variability is largely controlled by seasonality as well as water mass origin. Spatial distributions of &lt;sup&gt;3&lt;/sup&gt;H in rainfall are predominantly controlled by the water mass origin and atmospheric residence time. However, single storm events also show significant variation with a similar range of ~3.3 T.U.. Regional groundwater samples across South Africa have a large range (~11.5 T.U.), and elevated &lt;sup&gt;3&lt;/sup&gt;H activities can be attributed to continental recycling of water as well as longer atmospheric residence times. Groundwater samples of &lt;sup&gt;3&lt;/sup&gt;H and &lt;sup&gt;14&lt;/sup&gt;C form a distinct decay relationship that shows significant mixing of modern and fossil groundwaters. This relationship becomes clearer when regional &lt;sup&gt;3&lt;/sup&gt;H variation in rainfall as well as potential carbonate dissolution is corrected for. By better constraining the potential activity of &lt;sup&gt;3&lt;/sup&gt;H in precipitation and the mixing relationship between modern precipitation and fossil groundwater using &lt;sup&gt;14&lt;/sup&gt;C, this study mitigated the potential to misinterpret the sustainability/vulnerability of an aquifer system.&lt;/p&gt;&lt;p&gt;Jasechko, S., Perrone, D., Befus, K.M., Bayani Cardenas, M., Ferguson, G., Gleeson, T., Luijendijk, E., McDonnell, J.J., Taylor, R.G., Wada, Y., Kirchner, J.W., 2017. Global aquifers dominated by fossil groundwaters but wells vulnerable to modern contamination. Nat. Geosci. 10, 425&amp;#8211;429. https://doi.org/10.1038/ngeo2943&lt;/p&gt;