TRITIUM AND HE-3 ISOTOPE RATIOS FOR DIRECT ESTIMATION OF SPATIAL VARIATIONS IN GROUNDWATER RECHARGE

In the absence of strong dispersion the ratio of atmospherically derived tritium (H-3) and its stable daughter, helium 3 (He-3), can be used to accurately date shallow groundwater with ages ranging from 0 to about 50 years. The effects of dispersion of H-3/He-3 ages are examined by numerical simulation in simple one- and two-dimensional flow systems. For a H-3 input function that is relatively constant in time the difference between H-3/He-3 ages and purely advective groundwater travel times is controlled by the relative magnitude of the groundwater velocity, the H-3 decay constant, and the dispersivity of the porous media. In many shallow sandy aquifers the groundwater velocity is sufficiently large and the dispersivity sufficiently small that the difference between the true groundwater travel time and the H-3/He-3 age near the water table can be neglected (< 10% difference). This suggests that measurements of the H-3/He-3 age gradient near the water table can be used to estimate recharge rates and vertical velocity values with a high degree of spatial resolution. In contrast to H-3 activities, modeled H-3/He-3 ages are relatively insensitive to uncertainties with respect to the H-3 input function. Thus combined measurements of H-3 and He-3 offer considerable advantage over measurements of H-3 alone. The ability to obtain reliable estimates of spatial variations in recharge rates combined with accurate groundwater ages in shallow unconfined aquifers will undoubtedly provide valuable constraints when calibrating models of groundwater flow and solute transport in such systems.