Effect of leachability on environmental risk assessment for naturally occurring radioactive materials (NORM) in petroleum oil fields

Elevated concentrations of NORM often occur in petroleum oil fields. The NORM generated by oil field operations comes from 238U and 232Th contained in geologic materials. The predominant NORM radionuclide brought to the surface by produced water is radium, which co-precipitates with barium in the form of complex compounds of sulfates, carbonates, and silicates in sludge and scale. These NORM deposits are highly stable and very insoluble under ambient conditions at the surface. However, the co-precipitated radium matrix is not thermodynamically stable at reducing conditions, and all of the radium trapped in it could be released to the environment. The leachability of radium from NORM deposits due to acid-rain and other aging processes are generally unknown. Experiments have been performed on soil samples collected from eastern Kentucky oil fields to find the leachability of radium due to change in pH, temperature, sulfate ion concentration, and accelerated aging of the soil. The average concentration of radium in the soil is in the order of 5.92 Bq g-1. The risk assessment for different scenarios using the RESRAD code show that the annual total dose to a person living or working on this NORM contaminated soil will be in the range of 6.4-9.6 mSv for the initial 100 years. However, as the soil ages from exposure to acid rain, ultraviolet light from the sun, irradiation from the decay of 226Ra and its progeny and other kinds of reducing conditions, the solubility of radium in the NORM also changes. Results of accelerated aging and other leaching studies with these samples of soil show that the available radium for uptake increase over time with a corresponding increase in dose to the exposed individuals