Radon-222 in the groundwater surrounding the Anton Lake Basin, Hockley County, Texas
Bartolino, James R
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Because lake basins on the Southern High Plains have been determined to be areas of groundwater recharge, a study of an area including the Anton lake basin was initiated to investigate the deposition of uranium in the Ogallala aquifer by water percolating downward from the Anton lake basin to the aquifer. The Anton basin, which once contained a 3-square-mi1e lake in Quaternary time, is located in northeastern Hockley County. The lacustrine sediments underlying the basin (primarily light-gray to white clay and silt) are more than 100 feet thick in some parts of the basin. Uranium mineralization in the upper part of these sediments is shown as a marked peak on borehole gamma-ray logs. The uranium daughter product radon-222 was chosen to trace the uranium distribution in the aquifer. The liquid scintillation method for analyzing radon-222 in water was chosen for its accuracy, inexpensive cost, and speed. Water samples from 50 wells in the study area contained radon-222 activities that ranged from 100 to 2,500 picocuries per liter (pCi/L). The average radon activity in the samples was 760 pCi/L, and the median activity was 450 pCi/L. The line of equal activity of 600 pCi/L has a plume shape that appears to originate in the Anton lake basin and a playa-lake basin in the northwestern corner of the study area. The radon-222 belt corresponds to a paleovalley at the base of the Ogallala aquifer and a depression in the potentiometric surface. Analyses for total uranium and radium-226 in groundwater determined no clear relation between the presence of these elements in solution and radon-222, indicating that uranium and radium-226 have precipitated onto the rock matrix away from the recharge zone, where more reducing conditions are present in the groundwater. The apparent source of the radon in groundwater in the study area is uranium mineralization in the lacustrine sediments of the Anton lake basin (and presumably the other playa-lake basin.) The source of the uranium in this zone probably is a weathered volcanic ash.