Environmental Engineering Geological Characteristics And Pollution Assessment In Shuimuchong Tailings Of Tongling

Sulfide-rich tailings produced by mineral processing are prone to oxidation and cause many pollution problems in the surrounding environment;this issue has become a focus of attention.The Tongling area of Anhui Province is an important rich area of copper,iron,gold and sulfur deposits in China.In the process of mining and smelting metals,it is inevitable to produce a large amount of waste rock and tailings,resulting in a large number of Tongling tailings pond problems.In this paper,the Tongling Shuimuchong tailings and surrounding soil were the main research object.The accumulation and oxidation characteristics,granularity mineralogical structure,element distribution and surrounding soil pollution in copper tailings were studied in depth through various research methods such as consulting relevant information,survey analysis,field observation,sample collection and testing,data analysis,and that were analyzed by using multi-disciplinary cross-theories such as geology,environmental science,statistics.These research results can provide theoretical support for soil remediation around the tailings pond.The main results were as follows:（1）Both geophysical detection methods showed that the surface layer of tailings was seriously oxidized by rainwater leaching,forming a plate layer of about 1m.This is because the tailings contact with water and atmosphere during the accumulation process to promote sulfide oxidation,and the iron element reacts with it to form iron salt precipitation.The thick oxidized hard layer results in a steady environment for tailings accumulation,and it can be initially determined that the tailings dam is relatively steady.（2）The particle size of the tailings oxide layer was significantly larger than that of the non-oxidized layer.As the tailings oxidation degree decreases,the content of sand and fine sand in the tailings gradually increases,and the tailings in the non-oxidized layer mainly existed in the form of fine sand and silt.The data indicated that the size of tailings will larger after weathering and oxidation.The main primary minerals in tailings were gypsum,quartz and calcite,and the secondary minerals were jarosite,pyrrhotite,goethite,siderite and so on.The XRD test results showed that the pyrite and goethite were higher in the tailings,accompanied by some pyrite.A large amount of iron-bearing minerals in the tailings undergo chemical reactions under natural weathering conditions to form colloids,caused the tailings surface to harden.（3）The primary compounds of the tailings were Fe₂O₃,SiO₂,CaO,Al₂O₃ and so on.The content of Fe₂O₃ and S in the oxidized hard layer of tailings surface was much higher than that of loose layer.The sulfate aggravated the oxidation of Fe and formed an oxidized hard layer of tailings rich in Fe₂O₃.At the same time,the acidic wastewater generated by the sulfate accelerated the leaching of heavy metals in the tailings and local enrichment in the surface layer.The average contents of Cu,Cd,Zn,and As in the tailings were 1865,1.46,774.4,and 78.34 mg/kg respectively,which had reached 37.3 times,4.87 times,3.87 times,and 1.96 times of the risk screening values in GB15618-2018,and greatly exceed the soil background value in Tongling City,causing the pollution of surrounding farmland soil environment.The content of Hg and As in the oxidized lamellar layer was higher than that of the tailings loose layer.This is because the exposure of sulfide to air and water will lead to the release of high concentration of Hg,As from acid leachate.The Hg and As were easily adsorbed and precipitated by iron oxide,and then enriched in the lamellar layer.（4）The surrounding soil of Shuimuchong tailings reservoir showed a weak acid environment.The Hg,As and Cd in soil have exceeded or approached the risk control value in GB 15618-2018,the average content of Cu was 224.7 mg/kg,and reached4.49 times of the risk screening value in GB15618-2018.The heavy metal content of the soil in the study area decreased as the distance from the tailings pond increased.The content of As and Cd in the sampling point 500m from the tailings pond still exceeded the risk screening value,and the content of As in 80%of the sampling points has exceeded the risk control value in GB 15618-2018.The data indicated that due to the migration and diffusion of heavy metals in the tailings pond,the heavy metals in the surrounding farmland soil exceeded the standard,and the heavy metals in the crops in the soil around the study area should be monitored.The results of BCR four-step extraction method showed that the heavy metals in the surrounding soil of tailings reservoir mainly composed with residual fractions,the proportion of non-residue states of Cd was the highest,and the proportion of As was the lowest,indicated that the mobility and bioavailability of Cd were the highest,and does the greatest harm to the surrounding environment.（5）The single-factor pollution index indicated that there was no accumulation of heavy metals in the surrounding soils except Cr and Ni,Cd,As,Cu,Zn,Hg and Pb were all accumulated to a certain extent.There were many points above the slight pollution in the surrounding soil,and Cd,Cu,As had a serious extremely pollution,and the attention and prevention of heavy metals Cd,Cu,As should be strengthened.The Nemerow comprehensive pollution index method indicated that the soil around the study area showed heavy pollution.The comprehensive pollution index method indicated that the soil at the sampling points within 100m from the tailings pond was all heavily polluted,and the two sampling points from the tailings pond at 500m were medium degree of pollution.Pollution monitoring and remediation of contaminated cultivated land need to be strengthened.