Spatial and seasonal variations in soil and river water mercury in a boreal forest, Changbai Mountain, Northeastern China

Abstract

The distributions of total mercury (THg) and methyl mercury (MeHg) in soils and river water on the north slope of the Changbai Mountain were investigated in April and September, 2009. Concentrations of THg in soils ranged from 0.08 to 0.73mgkg−1 (dry weight), with an average of 0.24mgkg−1. This average was much higher than the background value on the north slope of the Changbai Mountain (0.055mgkg−1). MeHg in soils ranged from 0.05 to 0.56μgkg−1, averaging 0.25μgkg−1 and accounting for 0.1% of THg. The storage of THg and MeHg calculated for soils of 35cm was estimated at 304–946gha−1 and 419–875mgha−1, respectively. Seasonal and altitudinal changes in THg and MeHg were observed in forest floor soils and river water. In the O horizon of low altitude sites, THg accumulated significantly during the cold season. It was expected that Hg releases from local anthropogenic activities would be an important Hg source in boreal forest. THg concentration in river water collected in April (8.3±4.8ngL−1) was almost 2 times higher than that collected in September (3.2±1.1ngL−1). From these values, and the statistically significant relationships found between particulate Hg and total Hg in river water, we deduce that surface runoff is the dominant form of Hg transportation from the upland forest floor to downstream regions. Concentrations of MeHg in soils were higher in September than in April. The highest concentration was observed in the O horizon; this may be due to the impacts of temperature and organic matter on Hg methylation. The dynamics of THg and MeHg in the O horizon and river water may represent a process of seasonal accumulation and degradation of Hg in the boreal forest floor. Soils in the boreal forest of Changbai Mountain were demonstrated to be an important source of THg and MeHg in the Songhuajiang River.