Short-lived natural radionuclides as tracers in hydrogeological studies - A review

Fundamental approaches to the study of groundwater rely on investigating the spatial and temporal distribution of stable and radioactive isotopes and other anthropogenic compounds in natural waterbodies. The most often used tracers for estimating groundwater flow paths and residence times, groundwater/surface water interaction as well as tracing chemical (contamination) sources include stable isotopes of water (δ ¹⁸O and δ ²H), radiocarbon (¹⁴C; t_1/2 = 5730 a), tritium (³H; t_1/2 = 12.43 a) as well as unreactive fluorine-containing gases (e.g., chlorofluorocarbons CCl₃F or CFC-11; CCl₂F₃ or CFC-12; C₂Cl₃F₃ or CFC-113; and SF₆). While gas tracers are usually referred to as transient tracers and are appropriate for investigating modern flow systems, the isotopic tracers are often used to investigated paleo or regional flow systems. Stable isotopes of water can also be used to investigate groundwater/surface water interactions. Another, thus far been less frequently used group of groundwater tracers, are cosmo- and geo- genic short-lived radioisotopes. These isotopes are uniquely suited for studying a wide range of groundwater problems that have short time scales including high aquifer vulnerability to quantitative and qualitative impacts and groundwater discharge to surface waters. Here, we discuss and compare the applications of radio‑sulphur (³⁵S; half-life t_1/2 = 87 d), radio‑beryllium (⁷Be; t_1/2 = 53 d), radio‑phosphorus (^32/33P; combined t_1/2 = 33 d), natural tritium (³H; t_1/2 = 12.43 a), radon (²²²Rn; t_1/2 = 3.8 d) and short-lived radium (^224/223Ra; combined t_1/2 = 5.2 d). The paper discusses the principles of the individual tracer methods, focusing on the isotopes' input functions or values, on sampling techniques, and on methods of analyses. Case studies that applied a combined use of the tracers are referred to for readers who wish to learn more about the application of the so far underused cosmo- and geo- genic radioisotopes as aquatic tracers.