Incorporating the spatial variability of tritium in precipitation to constrain the distribution of groundwater tritium and modern recharge in South Africa

Tritium, the radioactive isotope of hydrogen, has been used to understand groundwater recharge processes for decades. The versatility of tritium as an isotopic tracer of recharge was most pronounced during the tritium "bomb peak" in the late 1960's and 70's, when the tritium activity in the atmosphere was several orders of magnitude higher than natural background levels. Since then, the activity of tritium in the atmosphere has decayed back to background levels, particularly in the southern hemisphere. Thus, the current variation of tritium in the atmosphere can be largely attributed to regional stratospheric production and fall out rates as well as global circulation phenomena. Unfortunately, more local controls on the variability in atmospheric tritium activity are poorly constrained and tritium activities in precipitation are often assumed to be uniform over both local and regional catchments and watersheds. This assumption can result in both over and under estimation of modern recharge within an aquifer when using tritium as the recharge proxy. In order to minimize the inherent prediction residuals associated with tritium based recharge investigations within a specific catchment, the variability of tritium activity in rainfall across that catchment must be better constrained. The variability of tritium in rainfall was modelled from 142 spatially distinct rainwater samples taken over a two year period, combined with a 77 rainwater sample group-set taken over a one year period in a single location. Rainfall events are traced backward in time, from the point of collection, using HYSPLIT modelling to ascertain the origins of moisture as well as the maximum altitudes reached along the particle track. It is evident that particles originating from lower latitudes, as well as those reaching higher altitudes, have generally elevated tritium signals. The variability of tritium, both spatially and temporally, was significantly higher than expected, confirming that assuming uniform tritium inputs to the groundwater system would result in inaccurate modern recharge estimates. Higher spatial resolution of tritium variation in rainfall for a particular region will improve our ability to relate tritium activities in groundwater to local precipitation. The distribution of tritium in groundwater alone cannot characterize the extent of modern recharge in areas where rainfall has anomalously variable tritium activities. It is evident that regions receiving both convective and stratiform rainfall, which originates from contrasting latitudes and altitudes, have elevated variability in tritium. The characterization of local variability of tritium activities in rainfall/recharge must now become an important step when investigating modern recharge using tritium as an isotopic tracer.