Assessment of the radiation field from radioactive elements in a wood-ash-treated coniferous forest in Southwest Sweden

Abstract

The distribution of natural and antrophogenic radioactive elements in soil and wood was investigated in a 30-year-old forest stand of Norway spruce. Forest plots treated with a single dose of granulated wood ash in 1989 were compared with untreated control plots. It was observed that the retention of radiocesium and radiostrontium by the forest soil is rather strong in spite of the high annual precipitation (1100 mm a −1) and the relatively acidic conditions of the soil. Most of the deposited nuclear weapon fall-out of radiocesium and radiostrontium is still residing in the forest soil. Radiostrontium, but not radiocesium, was found in the intrasoil water collected with lysimeters at soil depths of 20 and 50 cm. Wood xylem radial distributions of radiostrontium indicated a decreased bioavailability with time after deposition of nuclear weapons fall-out, and no major differences could be observed on comparison of wood from ash-treated plots with wood from untreated plots. The activity concentration of radiocesium in tree rings formed prior to 1986 and grown at the ash-treated plot was about two to three times that found in wood from the untreated control plot. In order to make a dosimetric assessment, the spectroscopic change in the radiation field was investigated at the forest plots and at a deposit for industrial waste (southeast Sweden). A portable spectroscopic system employing a semiconductor-type detector (HPGe) and a scintillation detector (3″ × 3″ NaI(Tl)) was used in order to compare the radiation field from deposited ash of various dates of disposal and ash-treated forest plots to controls, respectively. From depth distribution data obtained by activity determination of cores from the ash deposit, theoretical calculations of the primary radiation field showed a reasonable agreement (±2–8%) with results obtained from the field-gamma measurements. The main contributions to absorbed dose in air (105–130 nSv h −1) on the ash and sludge deposit are 214B, 40K, 208Tl, 137Cs, 214Pb, 228Ac, 134Cs and 212Pb. In the ash-treated forest and the corresponding control plots 137Cs and 40K were the main contributors of absorbed dose rate in air. A set of biomass effective filtration factors and biomass effective linear attenuation factors was calculated in order to depict how the forest stand modifies the intensity of the primary radiation field from the ash application.