Evaluating groundwater residence time in arid aquifers: a crucial metric for monitoring sustainable water management

Assessing groundwater residence time in aquifers of arid regions by analyzing natural radionuclides is crucial for predicting water contaminants, understanding groundwater system dynamics and ensuring sustainable water utilization. This study focuses on the evaluation of groundwater residence time in northeastern Saudi Arabia using tritium as a tracer, where water samples were collected from monitoring wells. Due to the low levels of tritium, the water was reduced twenty to thirty- times from its initial values through the electrolysis process. The enriched samples were meticulously analyzed using a liquid scintillation counter under optimal conditions to determine tritium concentrations. Validation from two international commercial tritium laboratories further confirmed the estimated tritium concentrations. The tritium concentrations were then employed to estimate the groundwater age for each monitoring well. Comparative analysis with international datasets suggests that the collected groundwater in the studied areas is likely over a century old. Notably, groundwater ages display significant variations in different locations, even within the same aquifer. These disparities stem from differences in water flow dynamics, recharge rates, and the geological composition of the rocks and sediments through which the water travels. In contrast, water from selected monitoring wells exhibited an age exceeding a few hundred years, categorizing it as ‘dead water’ due to an extended residence time in the aquifer. This research contributes valuable insights into the longevity of groundwater resources, aiding in formulating sustainable water management strategies in arid regions.