Soil heavy metal concentrations and their typical input and output fluxes on the southern Song-nen Plain, Heilongjiang Province, China

China's national “Multi-Purpose Regional Geochemical Survey” (MPRGS) project provided high density data (1 sample/km2) for the assessment of soil heavy metal (e.g., Cd, Hg, Pb, and As) pollution in most agricultural regions in China. As a further study, the fluxes of the soil heavy metal input/output pathways (e.g., atmospheric deposition, irrigation water, fertilization, crop or pasture harvesting, seepage water, and surface water) were observed and evaluated on the southern Song-nen Plain in Heilongjiang Province. It was found that at present, greater than 95% of the total area has soil Cd, Hg, Pb, As, Cu, Zn, Ni, and Cr concentrations lower than the uncontaminated concentration limits given by Environmental Quality Standard of Soil of PR China (GB15618-1995). So, as a whole, the study area is a clean region for agricultural development. Atmospheric deposition, rather than irrigation and fertilization, is the dominant element source of the soil heavy metals (Cd, Hg, As, Cu, Pb, and Zn), accounting for 78-98% of the total input fluxes. Soil seepage water, other than harvesting and surface water runoff, is the dominant soil element output pathway. The observed fluxes from the pathways were evaluated by calculating the changes of the heavy metal concentrations caused by them given that the fluxes keep steady for a certain years, i.e., 5, 10, 15, and 20 years, and there is no other potential pathways. It was shown that the soil in the study area as a whole will remain uncontaminated with little heavy metal hazards in the following decades, i.e., the observed six pathways will not cause severe accumulation or heavy metal hazards in the study area.