A Simplified Method for the Collection and Analysis of a Cosmogenic Radioactive Age Tracer: Sodium-22

The residence time distributions of surface waters give important information on watershed functioning and subsurface storage. Sodium-22 (half-life 2.6 years) can provide residence time information in addition to traditional age tracers (stable isotopes, tritium). A new and straightforward analytical method was created to determine Sodium-22 activities from various water sources. In the field, approximately 400 L of water are passed through a column containing 1.8 kg of cation exchange resin at 4-5 L/min. Columns are returned to the laboratory where they are processed by passing 4L of 3M HCl through the resin columns to elute all cations. The 4L acid is loaded into a Marinelli beaker for gamma ray spectroscopy analysis. The samples are counted for 22Na (1274 keV), determined by using a coaxial germanium detector with a 0.5 keV resolution and less than 3×10-3 counts per second. Samples are counted for seven days in order to acquire sufficient counts above background. The counting efficiency (11.6×10-3 cps/Bq) was obtained by counting a known activity of 22Na standard in the same geometry and matrix (3M HCl in a 4 L Marinelli beaker) as the samples. The net peak area of the samples in a 2.5 keV window is compared to the net peak area of the standard to calculate the 22Na activity. Two entire-season snow samples were collected from the Southern Sierra Critical Zone Observatory in the winter of 2015 and 2016. Approximately 300L of snow melt was collected and processed. The sodium-22 activity was determined to be 141±35 mBq/L and 153±29 mBq/L. Sodium-22 activities in streams, in conjunction with sulfur-35 (87 days) and tritium (12.3), reveal the presence or absence of water with intermediate residence times (0-10 years).