7. Wrap Up
Groundwater age refers to the elapsed time for a parcel of water to move along a flow path from recharge to the point of collection. Several environmental tracers can provide estimates of groundwater age, which in turn can highly constrain rates of groundwater movement and recharge. The use of [latex]_{}^{3}H,\ \frac{_{}^{3}H}{_{}^{3}{He}},\ {SF}_{6}[/latex], and CFCs for age dating groundwater that is less than about 60 years old are described in this book.
Except for [latex]_{}^{3}H[/latex], these dating methods involve dissolved gases and so the book also reviews basic concepts of gas dissolution in groundwater. [latex]_{}^{3}H[/latex] is one of the most widely used tracers of groundwater age and has been used to distinguish pre‑bomb—before the 1950s—from post‑bomb waters. Precise dating may be possible for waters recharged since about 1990 in the southern hemisphere and 2000 in the northern hemisphere. In the coming decades, [latex]_{}^{3}H[/latex] is likely to be used more frequently as a precise dating tool, especially for surface waters where gas exchange with the atmosphere is problematic for other techniques.
The [latex]\frac{_{}^{3}H}{_{}^{3}{He}}[/latex] dating method substantially extends the dating range of [latex]_{}^{3}H[/latex] alone and is one of the most robust of the young groundwater age‑dating methods. Although the basic age equation for [latex]\frac{_{}^{3}H}{_{}^{3}{He}}[/latex] is straightforward, its application is sometimes complicated by the fact that [latex]_{}^{3}{He}[/latex] is present in very small abundance and as a dissolved gas. The application of this method will improve as we better understand processes such as the formation of excess air and gas stripping. Analytical costs for [latex]\frac{_{}^{3}H}{_{}^{3}{He}}[/latex] are currently high but advances in portable mass spectrometry and gas separation techniques could substantially reduce costs.
The [latex]{SF}_{6}[/latex] dating method relies on its monotonic increase in the atmospheric concentration since about 1975. The nearly linear increase in atmospheric SF6 since 1980 results in water mixtures having an SF6 concentration that maps to approximately the average age of the mixture. Limitations in the SF6 method result from subsurface sources and the sensitivity of apparent ages to excess air. As atmospheric concentrations continue to increase, the SF6 method will become more robust, with diminished influence of subsurface (terrigenic) SF6 on young waters.
The CFC dating method also relies on a unique atmospheric concentration through time. This existed from about 1940 to 1990, but declining atmospheric concentrations result in non‑unique apparent ages for waters recharged since 1990. Nevertheless, it is still possible to date waters older than 1990 with CFCs and this method is still a powerful tool for distinguishing “old” from “modern” water.
While some researchers have argued against the quantitative use of groundwater age in favor of using raw tracer concentrations, calculated ages are useful for conceptualizing and understanding groundwater flow paths and turnover times. Because the tracer concentration (age) is related to the entire upstream flow field, the tracers discussed in this book are powerful tools for understanding groundwater flow systems. By integrating these tracer methods into numerical models of groundwater flow, we expect significant improvement in the uniqueness of such models and their ability to forecast future conditions.
However, molecular diffusion, hydrodynamic dispersion, and convergence of groundwater flow paths result in groundwater samples that contain a spectrum of ages rather than a single age. While many practical groundwater issues involve waters that were recharged within the past 60 years, the spectrum of ages in many aquifers exceeds the dating range of the tracers presented in this book. Emerging tracers such as radioactive [latex]_{}^{39}{Ar\ (t_{\frac{1}{2}} =}[/latex] 269 years) are likely to further our understanding of groundwater flow systems, their sustainability as sources of water supply, and strategies for protecting the quality of water in the flow systems.