Geohydrologic, geochemical, and geologic controls on the occurrence of radon in ground water near Conifer, Colorado, USA

Abstract

Integrated studies of geohydrology, geochemistry, and geology of crystalline rocks in the vicinity of Conifer, Colorado, reveal that radon concentrations do not correlate with variations in concentrations of other dissolved species. Concentrations of major ions show systematic variations along selected groundwater flowpaths, whereas radon concentrations are dependent on local geochemical and geologic phenomena (such as localized uranium concentration in the rock or the presence of faults or folds). When radon enters the flow system, concentrations do not increase along flowpaths because its decay rate is fast relative to groundwater flow rates. Radon-222 is not in secular equilibrium with 238U and 226Ra in the water. Therefore, most of the 238U and 226Ra necessary to support the waterborne 222Rn must be present locally in the rock. High concentrations of dissolved radon are not found in zones of high transmissivity, and transmissivity is not correlated with rock type in the study area. A higher transmissivity can be indicative of higher water-volume to rock-surface-area ratios, which could effectively dilute 222Rn entering the water and/or may indicate that emanated radon is carried away more rapidly. Water samples collected from individual wells over periods of several months showed significant fluctuations in the dissolved 222Rn content. This fluctuation may be controlled by changes in the contributions of water-producing zones within the well resulting from seasonal fluctuations of the water table and/or pumping stresses.