Purification and Detection of 39Ar in Groundwater Samples via Low-Level Counting

Abstract:

Argon-39 can be used as a radiotracer to age-date groundwater aquifers to study recharge rates and to better understand the mean residence time, or age distributions, of groundwater. Argon-39 (with a half-life of 269 years) is created in the atmosphere by cosmic rays interacting with argon in the air (primarily ⁴⁰Ar). The use of ³⁹Ar as a radiotracer fills a gap in the age dating range which is currently covered by ³H/³He or ⁸⁵Kr (< 50 years) and ¹⁴C (>1000 years); ³⁹Ar fills the intermediate time scale range from 50-1000 years where the previously established radiotracers are not adequate.

We will introduce the process for purifying and detecting ³⁹Ar in ground water using ultra-low-background proportional counters (ULBPCs) at the shallow underground laboratory at Pacific Northwest National Laboratory. Argon-39 is detected through direct beta counting using ULBPCs loaded with a mixture of geologic argon (extracted from a carbon dioxide well with no measureable ³⁹Ar activity) and methane, which enhances the sensitivity for ³⁹Ar measurements. The ULBPCs have been shown to have a background count rate of 148 counts per day (cpd) in the energy range 3-400 keV when filled with 10 atm of P-10 counting gas (90% geologic Ar, 10% CH₄).

Initial demonstration samples were collected from groundwater aquifers in Fresno, California supported by the United States Geological Survey (USGS). A discussion of the sampling technique to degas the water from these wells and to then purify it for counting will be presented. In order to quantify the ³⁹Ar contribution in the groundwater samples, the ULBPCs were characterized to determine two components: 1) the detector efficiency to modern levels of ³⁹Ar, and 2) the remaining detector background (using geologic sourced argon which is free from ³⁹Ar – no measureable ³⁹Ar activity). These characterization results will be presented along with a discussion of the quantification of the ³⁹Ar age of the demonstration measurements.