Radon (222Rn) in Ground Water of Fractured Rocks: A Diffusion/Ion Exchange Model

Abstract

Ground waters from fractured igneous and high-grade sialic metamorphic rocks frequently have elevated activity of dissolved radon (²²²Rn). A chemically based model is proposed whereby radium (²²⁶Ra) from the decay of uranium (²³⁸U) diffuses through the primary porosity of the rock to the water-transmitting fracture where it is sorbed on weathering products. Sorption of ²²⁶Ra on the fracture surface maintains an activity gradient in the rock matrix, ensuring a continuous supply of ²²⁶Ra to fracture surfaces. As a result of the relatively long half-life of ²²⁶Ra (1601 years), significant activity can accumulate on fracture surfaces. The proximity of this sorbed ²²⁶Ra to the active ground water flow system allows its decay progeny ²²²Rn to enter directly into the water. Laboratory analyses of primary porosity and diffusion coefficients of the rock matrix, radon emanation, and ion exchange at fracture surfaces are consistent with the requirements of a diffusion/ion-exchange model. A dipole-brine injection/withdrawal experiment conducted between bedrock boreholes in the high-grade metamorphic and granite rocks at the Hubbard Brook Experimental Forest, Grafton County, New Hampshire, United States (42°56'N, 71°43'W) shows a large activity of ²²⁶Ra exchanged from fracture surfaces by a magnesium brine. The ²²⁶Ra activity removed by the exchange process is 34 times greater than that of ²³⁸U activity. These observations are consistent with the diffusion/ion-exchange model. Elutriate isotopic ratios of ²²³Ra/²²⁶Ra and ²³⁸U/²²⁶Ra are also consistent with the proposed chemically based diffusion/ion-exchange model.