CHLORINE-36 SYSTEMATICS IN THE MERCED RIVER BASIN, CALIFORNIA

Chlorine is generally assumed to behave conservatively in natural waters. To the extent this is true, bomb-pulse ³⁶Cl may be used as a groundwater age indicator in shallow aquifers or alpine catchments, much the same way tritium has been used. Chlorine-36/chloride ratios in shallow groundwater (<100 m) in the Wawona catchment of Yosemite National Park indicate a bomb-pulse source of ³⁶Cl. Precipitation collected in the area has ³⁶Cl/Cl ratios near 100x10^-15, where groundwater measured in the top one hundred meters from ground surface have ³⁶Cl/Cl ratios ranging between 1000-10,000x10^-15. Chlorine-36/Chloride ratios measured in the Merced River in Yosemite National Park also had values similar to those measured in shallow groundwater. This would suggest that the mean residence time of water flowing in the Merced River was on the order of ~40 yrs. Helium-3, Tritium, CFCs, and SF6 samples collected in springs in the Sierra Nevada indicate that groundwater is on the order of a few years to four decades old. Water balance considerations suggest that only a fraction of the water discharging in the Merced River may be four or five decades old, but much of the water may be significantly younger. It is possible that chloride may not behave conservatively in the Merced River Basin, and using ³⁶Cl as an age dating tool may over estimate the mean groundwater travel time. Recent findings suggest that chloride is taken up in heavily forested ecosystems, which would then recycle the chloride to the saturated or unsaturated zones. Other findings also show that ³⁶Cl has an affinity to low molecular weight humic substances in soils. Comparison of ³⁶Cl “ages” with water ages determined by radon, tritium, and other means provides evidence for the degree of attenuation of chloride in the Merced River basin, and raises questions concerning its utility as a conservative tracer in hydrologic systems.