Hydrogeologic and geochemical processes affecting the distribution of 222Rn and its parent radionuclides in ground water, Conifer, Colorado

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Authors:Lawrence, Errol P.
Source:Institution: Colorado School of Mines, Golden, CO, United States
Publication Date:1990
Note:In English. Master's thesis
Summary:A 30 square mile area near Conifer, Colorado, underlain by igneous and metamorphic rocks, was studied to determine the processes controlling the distribution of radon (222Rn) and its parent radionuclides in ground water. Ground water samples collected from forty-six water supply wells were analyzed for major and minor elements, 222Rn, radium-226 (226Ra), and total uranium (U). Specific-capacity pump tests, static water-level (SWL) measurements, well records, and field observations were used to characterize the ground-water flow regime. A potentiometric surface map was used to delineate potential ground-water flowpaths along which the chemical evolution of the ground water was examined. Radon-222 values ranged from 1340 pCi/1 to 166,000 pCi/1, with a median value of 6670 pCi/1. Radium-226 concentrations were below 4 pCi/1 in 40 of 42 samples. Concentrations of dissolved U ranged from 0.14 ppb to 1200 ppb, with a median of 3.1 ppb. All sampled ground water was sufficiently oxidizing that reduced U solids were not supersaturated. Major dissolved constituents exhibit systematic variations along inferred ground-water flowpaths. Radionuclide concentrations along the flowpaths are erratic and do not mimic the changes occurring in the major ions. Radon-222 does not accumulate in the ground water along the flowpath because its decay rate is fast relative to estimated ground-water flow rates. Concentrations of dissolved U and 226Ra in the ground water do not support the concentrations of dissolved 222Rn. This lack of secular equilibrium in the water and the undersaturation with respect to U solids indicate that 222Rn is derived from U and 226Ra adsorbed to solid surfaces. Ground water flows under unconfined conditions through the fractured crystalline aquifers within the area. Calculated transmissivities range from 3 to 9300 gallons day-1 foot-1. There is a nonlinear correlation between calculated transmissivities and dissolved 222Rn. High 222Rn concentrations are not present in wells that had high transmissivity. Aquifers of high transmissivity may effectively dilute or disperse dissolved 222Rn as a result of greater water-volume to rock-surface area ratios or greater ground-water flow rates. Significant fluctuations in the dissolved 222Rn content of water samples collected from individual wells over periods of several months are probably controlled to a large extent by hydrologic processes.
Subjects:Alkaline earth metals; Geochemistry; Ground water; Hydrochemistry; Hydrogeology; Isotopes; Levels; Metals; Movement; Noble gases; Oxidation; Ra-226; Radioactive isotopes; Radium; Radon; Rn-222; Solutes; Transmissivity; Colorado; United States; Conifer Colorado
Record ID:1992062095
Copyright Information:GeoRef, Copyright 2018 American Geosciences Institute.
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