Tritium Concentrations in Crops Fertilised with Contaminated Sewage Sludge

The Food Standards Agency (FSA) let a contract to the Radiation Protection Division of the Health Protection Agency (RPD) to exploit the research opportunity afforded by discharges of tritium from a radiopharmaceutical facility to a waste water treatment works (WWTW) in Cardiff. Dried sludge pellets from the WWTW were used as a soil conditioner and the uptake of tritium from the treated soil into crops was investigated.

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The project was divided into two parts. In 2005 a pilot study was performed to determine whether uptake was measurable and decide whether tritium was being lost rapidly from the soil. This pilot study was outside the constraints of the Sludge (Use in Agriculture) Regulations. In 2006 the study was repeated without further addition of sewage sludge which provided conditions that did conform to the regulations. The tritium concentration in soil fell throughout the study.
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The changes in concentrations of tritium in soil over the growing seasons and the low concentrations measured in crops meant that it was not possible to quantify individual soil to crop transfer in terms of conventional concentration ratios. However it was possible to determine an aggregated transfer quotient relating the concentration in the edible part of the crop in Bq kg-1 fresh mass to the original amount of activity applied to the soil in Bq m-2. For 2006 and for all of the crop - soil combinations studied, values of about 2 10-4 were derived. The observed values are likely to depend on factors such as temperature, rainfall and any artificial irrigation. Consequently, those derived in this study are not necessarily applicable in general radiological assessments. However, on the basis of these indicative values it would be reasonable to assume that the proportion of tritium transferred into crops from soil treated with sludge would be small.