Spatial Distribution Characteristics And Migration Simulation Of Heavy Metals In Non-Ferrous Smelting Site And The Surrounding Soils

Heavy metal（HM）contamination is prominent in the site soil due to smelting activities.Complex pollution sources and high spatial heterogeneity in soil HM lead to unclear mechanisms of HM contamination in soils around smelting sites.In this study,we used meta-analysis with literature data collection,combined with investigations of typical Cu-smelting and Zn-smelting sites to reveal the spatial distribution characteristics and influencing factors of HM contamination in soils around smelting sites.The adsorption and transport mechanisms of cadmium（Cd）in miscellaneous fill and weathered slate were revealed through laboratory experiments.Based on the mass balance theory and ArcGIS,the stochastic model for HMs（SSG-TMs）and the source-soil-groundwater environmental risk prediction model for HMs（RSSG-TMs）were developed to simulate long-term accumulation and transport of Cd,predict the spatiotemporal variation of Cd leaching risk from soil to groundwater.The main research contents and conclusions are as follows:（1）Based on literature data collection and meta-analysis,arsenic（As）,Cd,copper（Cu）,lead（Pb）,and zinc（Zn）contamination were prevalent in the surface soil（0-20 cm）surrounding non-ferrous smelting sites,with Cd contamination being the most serious.The smelting type mainly affected the HM contamination in soils.The mean contents of As,Cd,chromium（Cr）,Cu,nickel（Ni）,Pb,Zn,and mercury（Hg）in surface soils around smelting sites were 278,20.7,68.9,1203,119,1991,1620,and 2.21 mg/kg,respectively,exceeding the background values of the corresponding elements in the earth’s crust by 48.7,345,0.94,44.6,3.49,79.6,21.6,and36.8 times.The accumulation of HMs in the surface soil was closely related to the distance from the smelting site（p<0.05）,and the HM contamination was mainly concentrated in the 2 km range around the smelting site.The mean value of the HM pollution index in surface soil was in the order of smelting site>agricultural land>forest land.The main contamination elements in soils around Cu-smelting sites were Cd,Cu,and As,and those around Zn-smelting sites and Pb-smelting sites were Cd and Pb.Therefore,considerable attention should be paid to the contamination of Cd and other HMs in soils within 2 km of the surrounding smelting sites.（2）The vertical distribution of As,Cd,Cu,Pb,and Zn pollution in soil profiles around the Cu-smelting and Zn-smelting sites was highly spatially heterogeneous and mainly influenced by the land use type and soil physicochemical properties.The HM content in soil profiles around the smelting site decreased rapidly with increasing soil depth,mainly accumulating within 2 m.The HM accumulation in the soil profiles was smelting site>agricultural land（abandoned land）>forest land.Arsenic,Cd,Cu,Pb,and Zn were mainly accumulated in miscellaneous fill layers（0–2 m）at smelting sites.Meanwhile,the five HMs were primarily distributed in the 0–0.6 and 0–0.2 m soil layers in agricultural land（abandoned land）and forest land,respectively.Heavy metal contamination in soil profiles was significantly positively（p<0.05）correlated with SOM,Fe,Al,S,and HM contents and sand percentage but negatively（p<0.05）correlated with soil depth and silt and clay percentages.Soil Al,Fe,and SOM contents,soil texture,and HM content in surface soils around smelting sites were the key factors affecting the vertical distribution depth of HM contamination.（3）Soil water conditions and salt ion types could affect the adsorption and transport process of Cd in soils at smelting sites.Overall,the adsorption capacity of miscellaneous fill and weathered slate for Cd decreased as the particle size increased.The Cd in soils at smelting sites mainly exchanged with Fe and occupied weak adsorption sites in the form of（Fe OH）₂Cd OH⁰.The reduction of Fe³⁺to Fe²⁺and SO₄^2-to S^2-was the main reason for the decreased percentage of acid-extractable Cd under the stagnant water condition.The Na⁺in reclaimed water reduced the adsorption capacity of profile soil for Cd,and Cl^-could form Cd Cl⁺with Cd²⁺to inhibit soil Cd adsorption.The morphological characterization results showed that salt ions promoted the erosion and fragmentation of coarse particles to form fine soil particles.The above results indicated that salt ions in reclaimed water for flushing would promote Cd transport to deep soil and groundwater through ion exchange,complexation,and erosion fragmentation.（4）The SSG-TMs model based on the finite difference method and Monte Carlo simulation was developed to simulate the transport and probabilistic risks of Cd in heterogeneous soil–groundwater systems under slag stacking and reclaimed water flushing scenarios.The model results were validated by field measurements and laboratory leaching experiments,and the simulation error was less than 20%,suggesting that it can successfully predict the probability of leaching risk occurrence for TMs in a heterogeneous slag-soil-groundwater system.Under the scenario of slag stacking amount increasing yearly,the Cd content and leaching flux were 1.43 and 39.4 times higher than that under fixed stacking amount in the100^th year,respectively.The probabilistic risk of Cd leaching from the soil to the groundwater exceeded 99.9%under the scenario of slag stacking amount increasing yearly.The pollution risk of Cd leaching from soil to groundwater under long-term（>20 years）flushing of Na Cl-type reclaimed water increased by 1.38 times compared to the rainwater leaching scenario.A plateau in the slag yard caused a sharp increase in the risk of Cd leaching during the late stage of the simulation,which was influenced by the input flux of Cd and the adsorption capacities of miscellaneous fill and weathered slate for Cd.The results of the sensitivity analysis showed that the surface runoff interception rate（IRCR）,Cd input flux（I）,and stacking time（ST）were key factors affecting the risk of Cd contamination in groundwater.The IRCR needed to be increased to at least 80%through the intervention measures to obtain ideal risk prevention effects.（5）The spatiotemporal trends of Cd pollution risk for regional soil and groundwater were predicted by the RSSG-TMs model.The model results were validated by field measurements（R²=0.629–0.879）,suggesting that it could effectively predict the distribution of HM pollution in soils.Under the smelting in-process（scenario A）simulation for 30 years,the area of Cd pollution(I_geo>3)in soils at the smelting site and surrounding agricultural land（abandoned land）and forest land was up to 100%,and the area of Cd leaching from soil to groundwater was 2.45%.Compared to scenario（A）,the area of soil Cd pollution under smelting shutdown（scenario B）and slag removal（scenario C）was reduced by 0.15%and 9.41%,respectively,and the area of Cd leaching from soil to groundwater was decreased by 0.55%and 1.13%,respectively.The results of ArcGIS spatial analysis showed that the leaching risk of Cd from soil to groundwater was high within 0.86 km from the smelting production area,wastewater runoff area,and slag runoff area.Atmospheric deposition（I_a）and surface runoff input flux（I_w）were the key factors affecting the pollution risk of Cd leaching from soil to groundwater.In conclusion,the RSSG-TMs model can be used to predict the trend of Cd pollution risk in soils around smelting sites and identify potential high-risk areas and key influencing factors.This study（1）revealed the spatial distribution characteristics of HM contamination in soils around non-ferrous smelting sites and the key influencing factors;（2）proposed the main pollution scope of HM contamination in soils around smelting sites;（3）developed the risk prediction model for HM contamination in heterogeneous soils around the smelting-affected area.These results could provide the scientific basis for preventing,controlling and remedying HM pollution at smelting sites and surrounding soil and groundwater.66 Figures,22 Tables,469 References...