Atmospheric transport of radiogenic lead in the vicinity of Ranger uranium mine determined using lead isotope ratios in dust deposited on acacia leaves

2003

Internal Report 451
Bollhöfer A, Honeybun R & Rosman K
Supervising Scientist Division
Department of the Environment, Water, Heritage and the Arts

About the report

To investigate the effect of uranium mines on the airborne dust Pb isotopic composition, a case study was carried out to study the distance that radiogenic dust travels from Ranger uranium mine, the only open cut uranium mine in Australia. Acacia leaves were utilized as passive dust collectors to obtain a spatial snapshot of Pb deposition. In addition, an annual cycle of Pb concentration and Pb isotopic composition of aerosols was obtained at Jabiru East using low volume air samplers to investigate temporal differences due to the distinctively different wet and dry seasons in the Alligator Rivers Region.

The project is part of a larger study, that was instigated at Curtin University of Technlogy, Perth. During 1999 and 2000 baseline aerosol Pb concentrations and isotopic compositions had been obtained at the Cape Grim Baseline Air Pollution Station (Rosman et al, 2000a). Those samples showed extremely low airborne Pb concentrations and occasionally an influence from highly radiogenic Pb, of which uranium mines can be a likely source. The question arose as to what and where the likely source of radiogenic dust might be and a preliminary study was carried out measuring the Pb isotopic composition of dust collected on leaves from 11 sites within Kakadu National Park.

The precision of the measurements of Pb isotopic ratios collected on the leaves during this preliminary stage of the project was quite low, attributed to organic interference occurring during chemistry procedures and analysis using Thermal Ionisation Mass Spectrometry. The interference issue was solved by additional chemistry techniques whereby leaf sample solutions were processed adding 1.5-2 ml 1:5 H2O2/HNO3, combined with microwave digestion and a double column procedure to dissolve and separate organic material from elemental Pb. At least a tenfold increase in precision was observed on selected samples.

However, a consequence of using the new sample preparation technique was the addition of a high blank. The H2O2 used was unable to be cleaned effectively; other reagents used in the column chemistry process (HCl, HBr) were also found to contain high levels of Pb that were deemed uncleanable using standard distilling techniques. A method implementing HNO3 was tested and combined with a double column procedure to obtain blanks of 20-60pg for sample sizes ranging from 200pg-100ng.

A total of 33 leaf samples were collected around the Ranger Uranium Mine during the dry season in June 2002 to measure the dispersion of radiogenic Pb from the mine site. In addition, aerosol samples have been collected between May 2001 and July 2002 and were measured at Curtin University of Technology, Perth, for Pb concentration and isotopic composition in April 2003.

Pb isotope ratios were more radiogenic during the dry season, and showed natural background dust Pb isotope ratios during the wet season. Airborne Pb isotope ratios of up to 4.000 for 206Pb/207Pb and as low as 1.550 for 208Pb/207Pb have been determined in Jabiru East. A contribution of up to 30% radiogenic Pb originating from Ranger U mine to overall Pb levels at Jabiru East during the dry season could be detected.

It was revealed that radiogenic Pb, borne from the decay of 235U and 238U, can be detected up to 40 km away from the Ranger uranium mine site, although the contribution to Pb concentrations is quite small. In addition, Pb isotope ratios are more radiogenic in a northwesterly direction of the mine site, following the main wind direction.

This internal report has been adopted from Honours Dissertation 499-2, Curtin University of Technology, Perth, by R. Honeybun. Temporal Pb isotope data have been measured in April 2003 and were added to the overall results.