Mercury accumulation in upland acid forest ecosystems nearby a coal-fired power-plant in Southwest Europe (Galicia, NW Spain)

This study was carried out to determine total Hg concentrations (HgT) in acid soils and main plant species in forest ecosystems located in the river Sor catchment, which is located 20 km to the NE of the biggest coal-fired power-plant in southwestern Europe (Galicia, NW Spain). Mercury enrichment factors and Hg inventories were also determined in the soils, which were regularly sampled between 1992 and 2001. The presence of elemental Hg was estimated by simple thermal desorption at 105 °C. The highest HgT concentrations occurred in upper soil layers (O and A horizons) with values up to 300 ng g− 1. HgT decreased with depth, achieving the lowest values in the bottommost horizons (i.e. the soil parent material, < 6 ng g− 1), except in podzolic soils. A similar trend occurred for Hg enrichment factors (HgEF) which showed values from 40 to 76 in topsoils. Upper soil mineral horizons (A or AB) made the largest contribution (> 50%) to the HgT inventory despite showing lower concentrations than the organic horizons. The role of vegetation in capturing atmospheric Hg and subsequent deposition to soil agrees with the sequence of HgT in plant material: wood < living leaves/needles < litterfall. The thermal desorption test revealed that almost all Hg is matrix-bound. Multiple regression analysis indicated that total organic carbon and Fe extracted in Na-pyrophosphate explains 87% of the variance of HgT concentrations, suggesting that organic matter (total amount and the presence of metal–humus compounds) plays a key role in Hg accumulation in the studied acid soils.