{sup 36}Cl in groundwater: Is it a conservative tracer?

Fallout from nuclear weapons` testing substantially increased the levels of several cosmogenic nuclides present in precipitation between the years 1951 and 1976. The record of these heightened inputs has been preserved in groundwater profiles in the regions of near vertical recharge. {sup 36}Cl measurements in one such depth profile correlate well with earlier measurements of tritium at the same site. When a 2-D flow model was used to simulate the tritium data, the depth to the bomb peak was best reproduced by a recharge rate between 135 and 170 mm a{sup -1}. However, a storage effect in the near surface must also be included in the simulation to reproduce the shape of the observed {sup 36}Cl profile. Processes hypothesized to explain this effect include uptake of chloride by vegetation (and subsequent delayed leaching from litter), and slow release of chloride bound to clay particles in the upper portions of the soil profile. These hypotheses are currently being tested.