Groundwater age represents the time that has elapsed between water entering an aquifer and data collection. Determining groundwater ages within an aquifer system has the potential to better constrain estimates of groundwater recharge, flow rates and therefore to increase the reliability of groundwater models, including predictions. However, the age of groundwater cannot be directly measured but must be inferred from measured concentrations of selected individual or multiple environmental tracers such as tritium (3H), chlorofluorocarbons (CFCs) and carbon-14 (14C). Water samples analysed for environmental tracer concentrations represent mixtures of waters of different ages due to variations in flow paths, recharge and discharge patterns, and aquifer heterogeneity. In addition, environmental tracer concentrations may be impacted by various (bio)geochemical reactions. This can lead to significant discrepancies between the groundwater ages interpreted from different tracers, referred to as the apparent ages. Recent analysis of groundwaters sampled in WA’s Pilbara region show groundwater ages of < 50 years, based on measured CFC concentrations, while suggesting apparent ages of ~9,000 years, based on measured 14C concentrations (~30pmC).
This study uses numerical experiments to systematically study the transport of multiple environmental tracers to better understand which physical and geochemical processes can create these significant discrepancies in apparent ages. In the model simulations transport of environmental tracers in fractured rock is represented through a dual-domain approach, with the simulated mobile zones representing fractures and the model’s immobile zones representing the rock matrix. The model simulations suggest that age discrepancies can only be created for a narrow range of dual-domain characteristics. Therefore other potential influences on the observed age patterns such as (i) screen length of observation wells (ii) climate variability and (iii) previously unaccounted geochemical reactions are also investigated.