Spatial Distribution, Magnetic Monitoring And Pollution Risk Assessment Of Heavy Metals And Phosphorus In Urban Soils Of Lishui City

Urban is the center place for human social,economic,and cultural activities. Urban soil plays an important role for the human living environmental quality and urban ecological functions.Frequent human activities,such as economic and transportation,can make urban soil be strongly interfered,also can cause seriously soil environmental pollution,i.e,heavy metal pollution and excessive accumulation of phosphorus in soil,may cause serious threat to residential and water environment. This study collected 126 urban topsoil samples from Lishui city to analyze their physical and chemical properties,heavy metal(Cr,V,Co,Cu,Mn,Ni,Zn,Cd,Pb,As) contents,soil magnetic parameters and phosphorus contents.By analyzing their relationship and spatial distribution,we try to find out the influence of soil environmental quality in the urbanized process of Lishui city.The results indicated that:(1)The heavy metal contents of Lishui urban topsoil indicated that the average contents of Cu,Zn,Cd,and Pb are 5.75,191.97,0.53 and 63.15 mg kg^-1 respectively, which was about 2.02,2.79,3.17 and 2.47 times higher than the corresponding soil background values in Zhejiang Province.These data showed that the urban topsoil of Lishui city has been polluted by Cu,Zn,Cd and Pb with different levels.(2)Base on geostatistics method,spatial autocorrelation analysis showed that these heavy metals all have great spatial autocorrelation except As.With the geostatistics model of ArcGIS to make the spatial interpolation maps of heavy metal content,they have the similar spatial distribution trend.High value is mainly distributed in the old urban area with great human activities.The pollution risks of heavy metals are evaluated with the integrated pollution index(IPI) and Hankanson's potential ecological risk index(RI).The results showed that the area of IPI≥3 and 150＜RI＜300 mainly distributed in the old urban area.(3) The contents of heavy metals were closely correlated with physical and chemical properties of soils.They had positively significant correlation with each other.The results showed that physical and chemical properties of soil may have a certain relationship with heavy metal accumulation.Principal component analysis (PCA) and cluster analysis(CA) showed that Cr,Ni,Cd,Zn,Cu,and Pb had the same pollution sources in urban soils in Lishui,mainly from traffic and human activity,and Mn,V,Co contents may be from the soil natural forming factors,and Co may have a certain relationship with traffic pollution.(4) Correlation analysis between heavy metal contents and magnetic parameters showed that heavy metal contents were correlated positively with magnetic susceptibility and saturation isothermal remnant magnetization(SIRM).The frequency magnetic susceptibility and heavy metal contents have negative correlation, and Cr,Cd content andχ_fd were significantly negative correlation at p＜0.05 level.The above data showed that a higher magnetic susceptibility and lower frequency magnetic susceptibility indicate a higher heavy metals accumulation in urban topsoils. Additionally,there is a high consistency exist on spatial distribution between magnetic susceptibility and heavy metal contents.All the above showed that the magnetic measurement could be used as a preliminary index for detection of heavy metal pollution.(5) Correlation analysis between soil properties and four kinds of phosphorus contents showed that phosphorus contents was related to soil pH,soil mechanical component,organic matter,total nitrogen,total phosphorus,free Fe₂O₃ contents and so on.Correlation analysis proved significant correlation between total phosphorus contents and four kinds of phosphorus contents.Spatial interpolation analysis on all four kinds of soil phosphorus and total phosphorus content showed an evident regional phosphorus accumulation phenomenon.Anthropogenic area such as industrial,residential areas and the vegetable fields are "hotspot" areas of phosphorus accumulation.(6) The sensitivity of the phosphorus leaching risk was evaluated by using change point method of water-soluble and extractable phosphorus contents in soils. Preliminary results showed that the Olsen-P of 92 mg·kg^-1and degree phosphorus saturation(DPS) of 23.6%can be used as the sensitivity limits of phosphorus leaching risk in Lishui urban soil.The spatial interpolation on the Olsen-P and degree of P saturation indicated the similar spatial distribution pattern.Therefore,both of them can be used to indicate phosphorus leaching persuasively.