Stable (Pb-206, Pb-207, Pb-208) and radioactive (Pb-210) lead isotopes in 1 year of growth of Sphagnum moss from four ombrotrophic bogs in southern Germany: Geochemical significance and environmental implications

摘要

The surfaces of Sphagnum carpets were marked with plastic mesh and 1 year later the production of plant matter was harvested in four ombrotrophic bogs from two regions of southern Germany: Upper Bavaria (Oberbayern, OB) and the Northern Black Forest (Nordschwarzwald, NBF). Radioactive, Pb-210 was determined in solid samples using ultralow background gamma spectrometry while total Pb concentrations and stable isotopes (Pb-206, Pb-207, Pb-208) were determined in acid digests using ICP-SMS. Up to 12 samples (40 x 40 cm) were collected per site, and 6-10 sites investigated per bog. The greatest variations within a given sampling site were in the range 212-532 Bq kg(-1) for Pb-210 activity, whereas Pb-206/Pb-207 and Pb-208/Pb-206 varied less than 1%. The median values of all parameters for the sites (6-10 per bog) were not significantly different. The median activities of Pb-210 (Bq kg(-1)) in the mosses collected from the bogs in NBF (HO = 372 +/- 56, n = 55; WI = 342 +/- 58, n = 93) were slightly less from those in OB (GS = 394 +/- 50, n = 55; KL = 425 +/- 58, n = 24). However, the mosses in the NBF bogs exhibited much greater productivity (187-202 g m(-2) a(-1)) compared to those of OB (71-91 g m(-2) a(-1)), and this has a profound impact on the accumulation rates of Pb-210 (Bq m(-2) a(-1)), with the bogs in the NBF yielding fluxes (HO = 73 +/- 30; WI = 65 +/- 20) which are twice those of OB (GS = 29 +/- 11; KL = 40 +/- 13). Using the air concentrations of Pb-210 measured at Schauinsland (SIL) in the southern Black Forest and average annual precipitation, the atmospheric fluxes of Pb-210 at SIL (340 Bq m(-2) a(-1)) exceeds the corresponding values obtained from the mosses by a factor of five, providing the first quantitative estimate of the net retention efficiency of Pb-210 by Sphagnum. When the Pb-210 activities of all moss samples are combined (n = 227), a significant decrease with increasing plant production rate is observed; in contrast, total Pb concentrations show the opposite trend. The contrasting behaviour of Pb-210 and total Pb in the mosses may reflect differences in the particle size distribution of the corresponding aerosols, their physical and chemical properties, the extent of their interaction with plant surfaces, or some combination of these factors. The Pb-206/Pb-207 ratios from NBF (HO = 1.159 +/- 0.002, n = 19; WI = 1.157 +/- 0.003, n = 48) and OB (GS = 1.157 +/- 0.003, n = 28; KL = 1.159 +/- 0.003, n = 15) are uniform and indicate that both regions are impacted by Pb from predominately anthropogenic sources. Although Sphagnum moss represents an established receptor in monitoring atmospheric Pb deposition, the physical characteristics (size, morphology, composition) of the three predominant kinds of Pb-bearing aerosols considered here, namely Pb-210 (adsorbed onto aerosol surfaces, following decay of Rn-222), anthropogenic Pb (sub-micron aerosols from high temperature combustion processes) and lithogenic Pb soil-derived mineral dusts ( tens of microns, from chemical weathering of crustal rocks), are fundamentally different and these have consequences for the retention efficiency of the three kinds of particles. (C) 2015 Elsevier Ltd. All rights reserved.