New Mexico Geological Society Annual Spring Meeting — Abstracts

Chlorine-36 can be a powerful tool when examining residence times and sources for waters in a river system. In this case, it is known that the 36Cl/Cl ratios in the headwaters of the Rio Grande are much higher than the natural deposition resulting from cosmic-ray activation of Ar in the atmosphere. This input is due to the pulse of chlorine-36 produced by nuclear testing during the 1950's. This impulse is being recycled in the biological environment, by trees, based on our recent chlorine-36 analysis of two Rocky Mountain Douglas Fir trees that revealed an average chlorine-36 ratio of 14,516 (36Cl/10¹⁵ Cl) in the vegetation. Recycling by the biota provides a constant input of chlorine-36 to the headwaters of the Rio Grande. This function will enable future work on tracing the bomb pulse through the subsurface.

Understanding the source of solutes in the Rio Grande has been an increasingly important problem due to the increase in population living in the Rio Grande valley. Up to this point, several studies have been done on the causes of this salinity increase. Recent investigations by our research group indicate that brines are contributing chloride to the Rio Grande at distinct locations which coincide with faults that cross the river. Such brines are characterized by high Cl/Br and low 36Cl/Cl ratios. Chlorine-36 results from a synoptic sampling of the Rio Grande performed in 2001 show that the 36Cl/Cl ratios decrease while the Cl/Br ratio and chloride concentration increase in the downstream direction. These results support the hypothesis that saline brines are contributing chloride to the Rio Grande.