Mode of occurrence and environmental mobility of oil-field radioactive material at US Geological Survey research site B, Osage-Skiatook Project, northeastern Oklahoma

Two samples of produced-water collected from a storage tank at US Geological Survey research site B, near Skiatook Lake in northeastern Oklahoma, have activity concentrations of dissolved ²²⁶Ra and ²²⁸Ra that are about 1500 disintegrations/min/L (dpm/L). Produced-water also contains minor amounts of small (5–50 μm) suspended grains of Ra-bearing BaSO₄ (barite). Precipitation of radioactive barite scale in the storage tank is probably hindered by low concentrations of dissolved SO₄ (2.5 mg/L) in the produced-water. Sediments in a storage pit used to temporarily collect releases of produced-water have marginally elevated concentrations of “excess” Ra (several dpm/g), that are 15–65% above natural background values. Tank and pit waters are chemically oversaturated with barite, and some small (2–20 μm) barite grains observed in the pit sediments could be transferred from the tank or formed in place. Measurements of the concentrations of Ba and excess Ra isotopes in the pit sediments show variations with depth that are consistent with relatively uniform deposition and progressive burial of an insoluble Ra-bearing host (barite?). The short-lived ²²⁸Ra isotope (half-life = 5.76 a) shows greater reductions with depth than ²²⁶Ra (half-life = 1600 a), that are likely explained by radioactive decay. The ²²⁸Ra/²²⁶Ra activity ratio of excess Ra in uppermost pit sediments (1.13–1.17) is close to the ratio measured in the samples of produced-water (0.97, 1.14). Declines in Ra activity ratio (excess) with sediment depth can be used to estimate an average rate of burial of 4 cm/a for the Ra-bearing contaminant. Local shallow ground waters contaminated with NaCl from produced-water have low dissolved Ra (<20 dpm/L) and also are oversaturated with barite. Barite is a highly insoluble Ra host that probably limits the environmental mobility of Ra at site B.