DARCY LECTURE: INERT GAS TRACERS IN GROUND WATER

Inert gases are present in all ground water as dissolved species, but the concentration and origin of a specific gas can be highly dependent on physical processes within and at the boundaries of a flow system. As a result, dissolved gas tracers (DGTs) have been used to evaluate a variety of processes and flow systems including recharge temperature and elevation in mountainous regions, spatial variations in recharge rates, solute transport in diffusion controlled systems, and as partitioning tracers in the vadose zone. A particular strength of DGTs is that multiple tracers having independent sources can often be measured. For example, noble gases such as Ne, Kr, and Xe are of atmospheric origin and their concentrations in ground water depend on temperature, elevation, and the magnitude of water level fluctuations in recharge areas. Helium-3 is produced within an aquifer due to radioactive decay, and thus depends on travel times. Helium-4 is produced both within and external to aquifers and depends on flow rates and boundary fluxes. By combining tracers that are produced at the top, bottom and within a flow system a “tomographic” approach for unraveling ground water flow paths and rates can be taken. The presentation will illustrate the basic concepts of using inert gas tracers along with case studies that describe their applications to ground water flow problems.