Tritium age dating groundwater
On a regional scale, the distribution of isotopic compositions are controlled by several factors: H values of precipitation remain fairly constant from year to year.
This happens because the annual range and sequence of climatic conditions (temperatures, vapor source, direction of air mass movement, etc.) remain fairly constant from year to year.
The simulated groundwater travel times increase with depth in the aquifer, ranging from about 1.5 to 6.5 years for the shallow wells (screen bottoms 3–4 m below the water table), from about 10 to 25 years for the medium-depth wells (screen bottoms 8–19 m below the water table), and from about 30 to more than 40 years for the deep wells (screen bottoms 24–26 m below the water table).
Apparent groundwater ages based on CFC- and He-dating techniques and model-based travel times could not be statistically differentiated, and all were strongly correlated with depth.
Shallow ground-water H values of the precipitation before the water reaches the saturated zone (Gat and Tzur, 1967).
Low temperature diagenetic reactions involving silicate hydrolysis can sometimes cause increases in the D values of waters.
In the case of snow, various post-depositional processes, such as melting and subsequent infiltration of surface layers and evaporation, may alter the isotopic content of the snowpack, often leading to meltwater values that approach uniformity in time and space, and approximate a damped reflection of the precipitation over a period of years.
It is important to remember that although an individual storm may be large and isotopically very different from the "old" water in the catchment, the amount of precipitation that infiltrates will likely be small compared to the amount of "old" water in storage.