Development of Novel Method for Analyzing Cosmogenic Radioactive Age Tracers Sodium-22 and Sulfur-35 and Applications in Hydrology

Abstract:

Residence time distributions of surface waters give valuable information on catchment hydrology and subsurface storage. Cosmogenic produced sodium-22 (half-life 2.6 years) and sulfur-35 (half-life 87 days) can provide valuable information on catchment hydrology to complement more traditional age tracers like stable isotopes and tritium. Newly developed in-situ field collection techniques and laboratory analytical methods were developed at LLNL to determine sodium-22 and sulfur-35 activities from groundwater and surface water samples. The relatively short half-lives of these isotopes make them valuable hydrologic tracers for determining residence times and flow paths on short times scales and can be used to investigate impacts on groundwater dependent ecosystems, evaluate aquifer vulnerability to contamination and climate change, as well as determining recharge and transport at managed aquifer recharge (MAR) sites.

We have successfully applied our novel methods in multiple catchment areas including the Southern Sierra Critical Zone Observatory where sodium-22 activities in precipitation ranged from 0.130 to 0.459 mBq/L. Stream values were lower due to mixing and ranged from 0.037 to 0.092 mBq/L. Sulfur-35 activities in precipitation ranged from 3.0 ± 0.4 mBq/L to 103 ± 1 mBq/L with stream values ranging from below detection limit to 6.7 mBq/L.