HIGH URANIUM CONCENTRATIONS IN THE CENTRAL OKLAHOMA AQUIFER NEAR GUTHRIE, LOGAN COUNTY, OKLAHOMA

The Central Oklahoma Aquifer (COA) is a major source of drinking water in central Oklahoma, and underlies an area of about 3,000 square miles in Cleveland, Lincoln, Logan, Oklahoma, Payne and Pottawatomie counties. COA bedrock strata include the Permian Garber Sandstone, Wellington Formation and the Chase, Council Grove and Admire groups. Strata dip to the west at about 50 feet per mile, but most of the aquifer flow is from west to east. The COA consists of an upper fresh water (Ca-Mg bicarbonate) zone (total dissolved solids (TDS) less than 5,000 mg/L) and a lower saline water(Na-Cl brine) zone (TDS greater than 5,000 mg/L). The base of the fresh water ranges from 100 feet below the surface to over 1000 feet in the Garber Sandstone. The COA is bounded on the north by the Cimarron River and on the south by the Canadian River, the western border of Oklahoma County to the west and the Pennsylvanian Vanoss Formation to the east. Most recharge is from precipitation. In Logan County (the focus of this study), shallow, intermediate and deep wells were tested as well as a spring fed lake. Shallow wells are less than 90 feet in depth, intermediate wells 90-295 feet and deep wells are considered to be greater than 295 feet in depth. In December 2003, the Environmental Protection Agency (EPA) set a Maximum Contaminant Level (MCL) of 30 μg/L, for the level of uranium in drinking water. The uranium content in two of the three shallow wells tested was less than 1 μg/L, the other well was 136 μg/L. Levels of uranium in 14 of 34 intermediate depth wells exceeded EPA MCLs with the highest level at 194 μg/L. Uranium levels, in four of five deep wells tested exceed the EPA MCLs with the other four between 120 and 185 μg/L. The uranium is naturally occurring and is probably derived from deep ground water interacting with micas and clays in the Wellington Formation, Chase, Council Grove, and Admire Groups. Data suggests that the deeper older groundwater has a higher uranium content then does younger shallower groundwater.