Contribution Of Atmospheric Deposition And Rock Weathering To Soil Cadmium Accumulatio

Cadmium（Cd）is known to be both extremely toxic and ubiquitous in natural environments.It is also the heavy metal with the largest pollution area with the most serious environmental impact in China.The main sources of soil Cd are firstly,the natural sources of weathering residue of rock minerals;secondly,the anthropogenic sources including waste gas,waste water and waste residue produced by industry and mining;third,agricultural sources based on fertilizer input and others.Northwest Guizhou is a typical geological high background area of heavy metals.The region has a history of lead-zinc ore smelting with serious exogenous pollution of heavy metals in the regional soil such as cadmium（Cd）,lead（Pb）,and zinc（Zn）.In this study,rocks and soils（slope soils developed under natural vegetation in the uncontaminated areas）from 12 strata of three geological ages in the carbonate rock,clastic rock,and basalt area in northwestern Guizhou were selected as the research objects.Under natural weathering,the geochemical behaviors of heavy metals such as Cd,Pb,Zn,As,Cr,Cu,Ni,and major elements such as Si,Al,Fe,and Ti in rock-soil systems,including spatial distribution,enrichment and inheritance relationships,and their impact on contribution of the accumulation of heavy metals such as soil Cd explored.The polluted small watershed in MS with zinc-powder production plant and unpolluted small watershed as the control（CK）were selected to systematically study the distribution law of atmospheric deposition and the characteristics of heavy metal content.PCA,UNMIX and PMF models were used in the experiment,for source analysis,while Pb-Cd stable isotope combined tracer technology was used to investigate and identify the superposition process of atmospheric pollution of soil cadmium in the geological high background area and its influence mechanism.This provides theoretical basis and data support for the treatment of heavy metal pollution in regional soil.The results of the study are as follows:（1）The heavy metal elements such as Cd,As,and Pb,and major elements including Si,Al,and Fe in rocks from different lithologic formations have high variation coefficients,large dispersion,and uneven distribution.The Cd content in the rocks were in order of carbonate rock area（0.49 mg/kg）>the siliclastic rock area（0.37 mg/mg）>basalt（0.32 mg/km）,exceeding the average abundance of Cd in Chinese rocks.The topsoils Cd from different formations demonstrate an order of carbonate rock area（1.90mg/kg）>basalt area（1.72mg/kg）>siliclastic rock area（1.61mg/kg）,while the bottom soil Cd displays carbonate rock area（0.87mg/kg）>basalt area（0.39mg/kg）>clastic rock area（0.27mg/kg）.The soil Cd content of different lithological formation areas has strong surface enrichment phenomenon,and the bottom layer was consistent with the regional background value,indicating that the region is affected by serious exogenous pollution superposition.Due to the characteristics of"secondary enrichment"in the process of rock weathering and soil formation in the carbonate rock area,the surface and top soil Cd are higher than those in the basalt and siliclastic rock areas.The conserved element Ti O₂/Al₂O₃has a significant linear correlation between carbonate rock（R²=0.82）and siliclastic rock area（R²=0.85）,and it has good in-situ weathering characteristics.The average p H value of soil in carbonate rock area is 6.43,and there is a secondary weathering process of soil minerals under weak acid conditions.This phenomenon of"secondary enrichment"may be one of the reasons for the higher Cd content of soil in carbonate rock area than in other areas.Soil Cd in the study area has no significant correlation with inert elements Si and Al,but has significant or extremely significant correlation with Zn,As,Pb and Ni,and has homology.The average ratio of ²⁰⁸Pb/²⁰⁴in rocks,soils and atmospheric deposition in the study area is rock（39.4205）>soil（39.1578）>atmospheric deposition（38.2347）,which indicates that soil Pb in the study area has the characteristics of accumulation of geological sources and atmospheric pollution sources,and indirectly indicates that there is multi-source coupling of regional soil Cd.（2）The topsoil Cd in different Formation was Longtan Formation>Huanglong Formation>Xuanwei Formation>Feixianguan Formation>Emeishan Basalt Formation>Maping Formation>Suining Formation>Erqiao Formation>Maokou Formation>Jialingjiang Formation>Guanling Formation>Ziliujing Formation,with contents of 5.37,5.21,2.54,2.42,2.09,1.95,1.74,1.59,1.15,0.81,0.65 and0.49 mg/kg,respectively.The corresponding rock content ranged from 0.18 to 0.156mg/kg.The correlation between rock Cd content and soil cotent is minimal.The Cd content of the Permian Longtan Formation is the highest,and the corresponding rock content is 0.12 mg/kg.The Longtan Formation is widely distributed in coal-measure strata and contains a large amount of carbonaceous shale rich in organic matter,which results in strong soil Cd enrichment.Carboniferous Huanglong Formation presents a high Cd content owing to the widely Zn-Pb mineralization in the Formation.The Cd content in the Emeishan Basalt Formation soil is 2.09 mg/kg.The soil Cd in the Jialingjiang,Guanling and Ziliujing Formations is relatively low,but all exceed the background value of 0.47 mg/kg in northwestern Guizhou.（3）The total amounts of heavy metals Cd,Pb,and Zn deposited in the atmosphere from the MS watershed of the Zinc-Powder Factory were 108.33,4935.77,and 8940.19 g/ha/y,respectively.These values are much larger than the control values20.05,783.93,and 1480.77 g/ha/y for Cd,Pb,and Zn,respectively.In terms of spatial distribution,the input of heavy metals to the soil by atmospheric deposition in MS showed an initial increasing trend and then decrease with the increase of the distance from the pollution source,and the maximum deposition was at the main wind direction of 300 meters.The Cd content in MS atmospheric TSP is also high reaching a value of 20.95ng/m³,the mean Cd of soil in small watershed of zinc powder factory is 6.92 mg/kg.The Cd content in TSP in MS atmosphere is also as high as 20.95ng/m³.The atmospheric deposition and long-distance transport of TSP in the polluted area of zinc smelting may lead to large-scale soil Cd pollution.Based on the estimation of20 million tons of waste residue from indigenous zinc smelting in northwest Guizhou Province,the emission of heavy metal Cd in waste gas from indigenous process zinc smelting is up to 22,100 tons.Based on the minimum control value of 1.5 mg/kg（Cd）of the agricultural land soil quality standard（GB15618-2018）,the 20 cm of topsoil polluted area by atmospheric deposition is estimated to reach 120.31×104 ha²,accounting for 88.27%of the area of agricultural land contaminated by heavy metals in northwest Guizhou,and the pollution superposition is serious.（4）UNMIX model source apportionment results revealed that the geological sources of heavy metals such as Cd,Pb,and Zn in the study area account for 75.60%,and the atmospheric deposition sources account for 24.40%.Pb isotopic analysis shows that the geological sources of carbonate and siliclastic rocks account for78.91%and 58.27%.The analytical geological sources of Cd isotopes in carbonate and siliclastic areas are 80.15%and 87.13%.The results of the two methods were basically consistent.Rock weathering soil are the main source of the regional high geological background of heavy metals,and about 20%of the heavy metals originated from atmospheric deposition.PMF model results showed that the Cd source apportionment in the small watershed MS were 25.6%,24.7%,40.6%and 9.1%of geological source,industrial source（atmospheric deposition）of zinc powder factory,the historical source of indigenous zinc smelting,and agricultural source,respectively.The results of Pb isotope ratio analysis were 34.2%,14.6%,30.8%,20.4%,and Cd isotope ratios were 19.9%,42.8%,37.3%,0%,respectively.The contribution of atmospheric deposition to soil Cd accumulation in small watersheds in the 16 years of zinc powder factory production ranges from 24.7%to 42.8%,while the contribution of historical zinc smelting to soil Cd accumulation ranged from 30.8%to 40.6%.The soil cadmium content of different lithological strata is significantly different,and the enrichment characteristics of carbonate rock area are more obvious than that of siliclastic rock and basalt area.Under the influence of coal measures strata and metal minerals,Cd in the soils of Longtan and Huanglong Formations was abnormally enriched.The amount of atmospheric deposition in zinc powder plant is large,and the Cd content in atmospheric deposition and TSP is high.The maximum contribution to soil Cd accumulation in small watershed was 42.8%,while soil Zn accumulation in the MS small watershed is about 30-40%.In conclusion,the contribution of rock weathering to soil Cd accumulation in naturally developed soils in the northwestern Guizhou is about 60-80%,and the contribution of atmospheric deposition to soil Cd accumulation ranged from20 to 40%.The rock weathering is the main source of the high geological background of heavy metals in northwestern Guizhou.The atmospheric deposition of zinc smelting and the long-distance transport of TSP are also important causes of large-scale soil Cd pollution in the region.The heavy metal cadmium in the soil of the high geological background and pollution superimposed area in Northwest Guizhou mainly comes from the weathering of rocks and the atmospheric deposition of historical artisanal zinc smelting.At present,the contribution of industrial sources and agricultural sources to soil cadmium accumulation is relatively small,so we should pay attention to the safe utilization of polluted farmland in this area,ensure the safe production of agricultural products,and reduce the risk of food chain to human health.