A review of the use of radiocarbon to estimate groundwater residence times in semi-arid and arid areas

Groundwater is an important resource in arid and semi-arid regions and determining its residence times is critical for sustainable use. Radiocarbon (C-14) is currently the primary geochemical tracer for determining residence times of regional groundwater systems. The analysis of C-14 contents of dissolved inorganic carbon (DIC) became more straightforward following the development of accelerator mass spectrometry, which has led to an increase in the number of studies using C-14. However, the interpretation of C-14 data is not always straightforward and many studies consider relatively few of the multiple processes that may affect the C-14 contents of DIC in groundwater. Commonly, studies have focussed on correcting C-14 contents for closed-system dissolution of C-14-free calcite, which is a near-ubiquitous process. However, especially in semi-arid and arid areas, uncertainties in the initial C-14 contents and delta C-13 values of recharge due to the presence of low( 14)C soil CO2 in the deep unsaturated zone, recharge by rivers, or open-system calcite dissolution pose problems for mass balance calculations. Additionally, processes such as methanogenesis and mineralisation of organic carbon may be locally important. Most studies also assume a constant atmospheric C-14 content and non-dispersive piston flow in aquifers, which results in residence times being underestimated and makes it difficult to compare the groundwater archive to other palaeoclimate or palaeoenvironment records. Additionally, mixing of water within aquifers, diffusion of C-14 between low and high permeability layers, and sampling from multiple units in long-screen wells may limit whether a meaningful residence time can be determined. Overall, while it is relatively straightforward to estimate broad ranges of residence times or determine general patterns of groundwater flow, the quest to quantify residence times, flow rates, and recharge remains a challenge. The use of multiple radioactive tracers, better characterisation of delta C-13 values and C-14 contents of the potential sources of DIC, and more critical assessment of flow systems will improve the utilisation of this important tracer.