Pb-210 and fly ash particles in ombrotrophic peat bogs as indicators of industrial emissions

Peat cores were collected from a Sphagnum-dominated Selisoo bog, which is located about 40 km from the large oil shale-fired power plants (PPs) in Estonia. These PPs have been operational from the 1960's and had the largest negative impact on the surrounding environment during the 1970's and 1980's. Nearby ombrotrophic peatlands are good indicators of atmospheric pollution due to their properties of effectively adsorbing mineral matter and pollutants. Collected peat cores (S1 and S2) from Selisoo peat bog were sliced into 1 cm thick layers and measured gamma spectrometrically. In addition, spherical fly ash particles (SFAP) originating from the combustion of the PPs were counted. The maximum concentrations (particles per cm³) of the SFAP remained between 7 and 12 cm for core S1 and between 11 and 17 cm for core S2. The concentration profiles of the SFAP reflect the combustion and emission history of the PPs. Pb-210 activity concentrations have the maximum values up to 500 Bq kg^-1 and 413 Bq m^-2 for S1 and for the S2 the values are 441 Bq kg^-1 and 535 Bq m^-2 (dry weight). The unsupported ²¹⁰Pb inventory is around 4250 Bq m^-2. This represents a ²¹⁰Pb deposition flux of 133 Bq m^-2 y^-1. The estimated ²¹⁰Pb deposition via fly ash from the PPs at Selisoo area remains between 0.2 and 2.2 Bq m^-2 y^-1. Considering the annual ²¹⁰Pb deposition from the atmosphere (with a precipitation rate of 600 mm y^-1) between 92 and 133 Bq m^-2, which is regarded as the natural background value, we show that the radiological burden due to the power plants at these distances is negligible. As the peat cores exhibit noticeable differences from each other (in terms of radionuclide concentration distribution), the SFAP can provide a good additional parameter to improve the validity of results obtained only from radiometric methods in the chronological studies. SFAP can also act as a possible tool to estimate the radionuclide deposition rate via fly ash in the vicinity of the PPs.