Soil and ambient air mercury as an indicator of coal-fired power plant emissions: a case study in North China

Coal-fired power plants (CFPPs) are an important anthropogenic mercury (Hg) source in China, and it is crucial to understand the environmental impacts of this detrimental element emitted from this source. In the present study, field experiments were conducted for measuring Hg in ambient atmosphere and upland agricultural soils within a radius of 10 km surrounding a large scale coal-fired power plant (1550 MW) in Tangshan, Hebei province. Short-term (20 min) average of gaseous elemental mercury (GEM or Hg⁰) in ambient air varying from 1.5 to 9.0 ng/m³ and total Hg (THg) in surface agricultural soil (0-20 cm) varying from 9.2 to 43.5 μg/kg at different sites were observed. THg in two soil cores decreased with depth, with concentrations being 2-2.5 times higher in the surface layer than that in the deep layer (50-60 cm), indicating the possibility of the atmospheric input of Hg. Based on the information of the total atmospheric Hg emission since this CFPP's operation in 1970s and the increased THg in nearby soils, it was estimated that about 3.9% discharged Hg has accumulated in the nearby agricultural soils. The low retention rate of the total emitted Hg by soils is a result of high proportion of Hg⁰ (79.5%) in stack gas emission and potential loss of Hg from soil surface reemission. The positive shifting (~ 0.5‰) of Hg isotopic signature (δ²⁰²Hg) from deep soil to surface soil reflected Hg deposition from nearby CFPP emissions that are featured with much heavier Hg isotopic signatures inherited from feed coal (δ²⁰²Hg: -0.50‰) and different combustion products (δ²⁰²Hg: -0.95 to 3.71‰) compared with that in deep soil layer (δ²⁰²Hg: ca -1.50‰). Overall, this study demonstrated that this CFPP has a slight but distinguishable effect on the elevation of ambient GEM and agricultural soil THg in the local environment.