Abstract
The transport and radioecology of the therapeutical radioisotope 131 I has been studied in Canberra, Australia. The isotope has been detected in water samples and its activity quantified via characteristic J-ray photo peaks. A comparison of measurements on samples from upstream and downstream of the Canberra waste water treatment plant shows that 131 I is discharged from the plant outflow into the local Molonglo river. This is consistent with observations in other urban environments. A time-correlation between the measured activities in the outflow and the therapeutical treatment cycle at the hospital identifies the medical treatment as the source of the isotope. Enhanced activity levels of 131 I have been measured for fish samples. This may permit conclusions on 131 I uptake by the biosphere. Due to the well-defined and intermittent input of 131 I into the sewage, the Canberra situation is ideally suited for radioecological studies. Furthermore, the 131 I activity may be applied in tracer studies of sewage transport to and through the treatment plant and as an indicator of outflow dilution following discharge to the environment.
Highlights
The radioisotope iodine-131 (131I) with a radioactive halflife of t1/2_rad = zärt d is routinely and successfully applied in the medical treatment of thyroid cancers
Sewage is treated at the Lower Molonglo Water Quality Control Centre (LMWQCC) that is located on the western fringe of the city and it releases treated water into the Molonglo River, which is a subsidiary of the Murrumbidgee River
The radioecology of the therapeutic isotope 131I is important, since it is applied in nuclear medicine at very high activities
Summary
Activities of several GBq are given orally to patients. Data on the pathways of 131I into the environment and possible uptake by the biosphere are scarce, despite it being readily detectable via a characteristic J-ray line It bio-accumulates in algae [4,5] and the uptake by other flora and fauna may be expected. The frequency of discharge into sewage and the attrition of the isotope due to radioactive decay are well matched. This may permit the tracing of individual discharges of 131I through the sewage system and assist with the interpretation of measured 131I activities in the outflow from the wastewater treatment plant. The well-calibrated, but sporadic nature of the input activities may permit 131I to be used as a tracer of wastewater flow and dilution
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