Characterization of Groundwater with Complementary Age Tracers

Groundwater age or residence time is the time water has resided in the subsurface since recharge. Depending on the application, this definition may or may not include travel through the unsaturated zone. The determination of groundwater age can aid understanding and characterization of groundwater resources, because it can provide information on e.g. groundwater mixing and flow, and volumes of groundwater and recharge. Groundwater age can be inferred from environmental tracers, such as SF₆ and tritium, that have a known input to groundwater and/or undergo known alteration processes in groundwater. The currently used age tracers face limitations regarding their application range and reliability. For example, some age tracers have local sources that can lead to contamination of groundwater. This contamination can result in misleading estimates of age. Other tracers have ambiguous inputs to groundwater, which can result in ambiguous age estimations. To reduce these limitations, it is now recognized that multiple tracers should be applied complementarily. There is also a need for new groundwater age tracers and/or new groundwater dating techniques to supplement the existing ones. Cost-effective and easily applicable tracers/techniques are preferred, since most established groundwater dating techniques are very costly and/or complex. Commonly measured hydrochemistry parameters , such as the concentrations of major ions and pH, have been suggested as cost-effective and easily determinable potential age tracers. To date, the use of commonly measured hydrochemistry parameters as independent age tracer has only been demonstrated for water recharged weeks to months ago relying on seasonal changes. Other studies applied commonly measured hydrochemistry complementarily to established age tracers to better constrain groundwater age and/or better understand and predict anthropogenic effects on groundwater quality. Further study is needed to assess the extent to which commonly measured hydrochemistry can be used to reduce uncertainty in tracer-inferred age as well as the extent to which commonly measured hydrochemistry can be used to extrapolate tracer-inferred age.  In addition to tracer specific limitations, quantification of uncertainty and ambiguity is not standard in age modelling. Although a few studies have attempted to quantify uncertainty in age modelling with the aid of probabilistic approaches, their methods are often relatively complex and not transferrable to the many cases with little available data. Uncertainties in the tracer’s recharge estimate and identification of appropriate model components, such as the objective function, have not been considered. Studies in other areas of hydrological modelling, where probabilistic approaches are more commonly used, have highlighted the need for careful identification of model components.