Factors affecting the radon (222Rn) emanation from aquifer rock materials: Implications for radiological and groundwater tracer studies

Abstract

Groundwater discharge (GD) is an important component of the water budget in large urban areas with high water demands. Radon is a routinely used groundwater tracer in mass-balances for evaluating GD to surface water bodies. The diffusion of 222Rn from aquifers' sediments parameter is important for GD's assessments. Sediment equilibration experiments were employed with various sediment and rock materials, including sand, granite, gneiss and loess that constitute the Pampeano Aquifer (PA) in the Buenos Aires Province, Argentina. These experiments allowed the 222Rn concentration determination in pore fluids at secular equilibrium and to examine the factors affecting the magnitude of radon 222Rn emanation from the materials under study. We found that radon emanation decreases in a power function (R2 = 0.9, n = 6) with the particle size of the tested PA sediment and rock materials. Based on our results, loess sediments with the smallest particle size and the largest particle surface area have the highest radon emanation. This strongly suggests that these two parameters are the parameters that govern the radon diffusive fluxes' magnitude in the PA. On the other hand, we found that PA basement rocks, primarily granite and gneiss, showed an exhalation rate of radon of 8.1 ± 0.81 Bq∙m−2∙h−1 and 13.2 ± 1.32 Bq∙m−2∙h−1. These rates are two orders of magnitude higher than loess sediments (0.3 ± 0.1 Bq∙m−2∙h−1), owning to the higher natural content of radon's parent isotopes from the 238U natural decay series. These high radon levels are consistent with currently available radon concentrations measured in groundwater in contact with the PA basement rock formations. This study demonstrates the importance of considering site-specific aquifer properties in the radon diffusive fluxes when utilizing radon as a groundwater tracer in hydrological studies. This is the first quantitative study that examines the aquifer characteristics affecting radon emanations in this large hydrogeological system.