Radiochemical Analysis of Low-Level Naturally Occurring Radionuclides for Environmental Applications

Naturally Occurring Radioactive Materials (NORM) covers all naturally existing radioactive elements in the environment. The environmental presence of natural radiation stems from two primary sources: cosmic radiation and the decay of naturally occurring radionuclides. Some of these naturally occurring radionuclides, inherent in the Earth’s crust, are part of natural decay chains, resulting in the production of various radionuclides that disperse extensively throughout the environment. The concentration of these radionuclides can be altered by human activities such as extraction of oil and gas. The interest in measuring these radionuclides increased after the discovery of elevated concentrations in waste
originating from the oil and gas industry.

Certain radionuclides commonly found in NORM waste include polonium210 (²¹⁰Po), radium226 (²²⁶Ra), and lead210 (²¹⁰Pb). The objective of this PhD was to develop methods to determine these radionuclides across various environmental samples. This would enable the determination of their presence and concentration within the environment.

A rapid method was developed for measuring ²¹⁰Po in soil and sediment samples. Measuring ²¹⁰Po in solid samples can pose a challenge owing to its volatility, which eliminates the use of dry ashing. The developed method used alkaline fusion to decompose soil samples without using hydrofluoric acid. The average recovery of ²¹⁰Po for the proposed method was 81.2 ± 6.0% with a detection limit of 0.1 mBq/g.

A simple and rapid method was developed for measuring ²²⁶Ra in drinking water samples by directly loading a 300 mL acidified drinking water sample (0.01 M HNO₃) onto TK101 resin. The elution of ²²⁶Ra was performed using 4 M HNO₃, which was then evaporated and redissolved in 2% HNO₃ for measurement on ICPMS. The average yield of the proposed method was 94.7 ± 2.8% with a turnaround time of less than 5 hours. With a preconcentration factor of 150, the limit of detection was 3.73 × 10⁵ ppt (0.001 Bq/L), which is less than the guideline level from the European Union of 0.5 Bq/L for ²²⁶Ra in drinking water samples.

A similar method was developed for measuring ²¹⁰Pb in drinking water samples on ICPMS. A 1 L acidified drinking water sample (0.01 M HNO₃) was loaded directly onto the TK101 resin, and ²¹⁰Pb was eluted in 8 M HCl. The average yield was 61.4% ± 7.9%. The detection limit of the method was 0.007 ppt (63 mBq/L), which is below the guideline levels from WHO and EU.

The proposed methods were used for investigating the concentrations of ²¹⁰Po, ²²⁶Ra, and ²¹⁰Pb in environmental samples acquired in the surrounding environment of a storage site for NORM waste from the Danish oil and gas industry. The measured concentrations did not exceed the levels observed in the natural background, and the concentration of ²²⁶Ra and ²¹⁰Pb in the drinking water samples was below the detection limit. Based on these measurements, it was concluded that the release of NORM radionuclides from the storage site did not contribute to an increase in background concentration.