Understanding Argon-39 groundwater age dating: Importance in water resources protection and to identify potential alternative sources of low-radioactivity argon

The ability to detect the level of depletion of 39Ar in groundwater relative to the modern atmospheric abundance provides valuable opportunities to determine groundwater age dis- tributions and increase our understanding of groundwater systems. As an intermediate age tracer, 39Ar provides better constraints on groundwater age distributions than those determined from young and old age tracers alone (3H, 14C, etc.). Typically, groundwater used for drinking and irrigation in many communities is from shallow, freshwater aquifer systems. Mixing of young and old water occurs in these systems and can introduce contaminants from the surface to precious groundwater resources. Knowing the age of the groundwater can also indicate areas where water resources are being depleted at a much faster rate than it is being replenished. Therefore, being able to determine groundwater age distributions is critical to protect our most valuable resource. Groundwater increases in salinity with depth and becomes unsuitable for drinking without costly treatment. However, deep groundwater is typically older in age and very old water will be depleted in 39Ar, as long as there is no in situ production. In situ production of 39Ar typically takes place in groundwater areas where uranium, potassium, and thorium are abundant. Therefore, to find subsurface sources of argon that are depleted in 39Ar, we must look to identify deep hydrologic environments with very old water and with rocks that have a relatively low abundance of uranium, potassium and thorium. Knowledge gained in better understanding groundwater age distributions will in turn support identification of these areas. This talk will discuss the importance of understanding groundwater age to support both environmental applications and the search for low-radioactivity argon. We will provide an overview of PNNL’s application of 39Ar age dating of groundwater to support water resources studies, and briefly describe the types of groundwater systems that may be suitable sources of argon to support fundamental physics experiments. This presentation was used for the Low-Radioactivity Underground Argon Workshop held at Pacific Northwest National Laboratory in Richland, Washington on March 19 - 20, 2018