Lithological control of groundwater 222Rn concentrations in fractured rock media

The parameters controlling and/or influencing the relative source strength, release mechanisms and accumulation of 222Rn in the pore fluids of fractured rock lithologies are discussed. The results of approximately 150 radon analyses of groundwater wells sampled from fractured lithologies in eastern Connecticut (United States of America) are examined to determine the intralithological and interlithological variability. Although there is primary control of 222Rn activity in pore water by uranium concentration in the host lithology, the intralithological and interlithological variabilities are dominated by site specific parameters. The primary geological parameters controlling the activity of radon in the pore fluids of fractured lithologies are evaluated. These are (with their contribution to the variability): (1) the specific fracture surface area (>103), which is functionally dependent on the crack width; (2) the emission efficiency, which is functionally related to the fracture spacing (>102x); and (3) the surficial enrichment of uranium on the fracture surface (∼ 103x). Permeability and porosity do not provide additional information nor do they constrain the controls on radon activity in pore fluids. Thus, the activity of radon in fractured lithologies is difficult to predict, and the measurement of radon activity does not uniquely constrain the geometry of the rock or the chemical inhomogeneity. (author). 19 refs, 7 figs, 2 tab