Heavy metals are natural constituents of the Earth’s crust, and the heavy metals in surface soils could originate from the soil parent material (lithogenic source) and various industrial and agricultural activities (anthropogenic sources).  Heavy metals occur in different forms in a soil, such as bound in the matrices of primary and secondary minerals, adsorbed on the surfaces of mineral grains, bound to soil organic matter, and ions and complexes dissolved in soil solutions.  Source identification and apportionment of heavy metal pollution in surface soils are particularly challenging for large-scale regions because of the high spatial variability of heavy metal contents in surface soils caused by both heterogeneous parent materials and widespread human activities.  For the metals that are primarily of lithogenic origins, the contribution of human activities is even more difficult to quantify.  To this end, a novel source apportionment method by combining chemical speciation of heavy metals and stochastic modeling, based on the assumption that heavy metals introduced by human activities are expected to be enriched primarily in the readily and potentially bioavailable and mobile fractions (i.e., non-residual fractions).  This method was applied to investigate the heavy metal pollution in surface soils of the Pearl River Delta (PRD) in southern China.  On the average, 45-86% of Zn, Cu, Pb, and Cd occurred in the non-residual fractions of the surface soils, while only 12-24% of Ni, Cr, and As were partitioned in these fractions.  Combination of sequential extraction, Kriging interpolation, and conditional inference tree reveals that approximately 10, 39, 6.2, 28, 7.1, 15, and 46% of the As, Cd, Cr, Cu, Ni, Pb, and Zn, respectively, in the surface soils of the PRD were contributed by anthropogenic sources.  In the non-residual fractions of the surface soils, the anthropogenic contributions to As, Cd, Cr, Cu, Ni, Pb, and Zn, were 48, 42, 50, 51, 49, 24, and 70%, respectively.  These results demonstrate that a combination of chemical speciation and stochastic modeling is capable of identifying and quantifying the relatively small shares of anthropogenic contributions for the heavy metals in surface soils contributed predominantly by natural sources.<br/>

Source apportionment of heavy metals in surface soils based on their chemical speciation and stochastic modeling

2019 Spring National Meeting

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