| Soil heavy metal pollution is particularly prominent among the many soil environmental problems at present because of its difficult degradation,easy enrichment and easy entry into human body through the food chain.As a typical karst region in southwest China,Guizhou Province presents high geological background values of soil heavy metals Cd,which poses a serious threat to ecological environment quality and human health.At the same time,the special geomorphological type and geological background of karst region make the pollution,risk and influencing factors of soil heavy metals have more complex relationships,therefore,understanding the influencing factors of soil heavy metals distribution and clarifying the risk areas in karst region is important for soil pollution prevention and control and ensuring the safety of agricultural products.In addition,the spatial distribution of soil heavy metals has complex spatial differentiation characteristics,which leads to the need to deploy high-density points to ensure the accuracy of the survey results when monitoring the soil environment.In order to meet the demand for long-term monitoring of soil heavy metal sites,it is necessary to optimize the sites in the monitoring area to ensure the accuracy while characterizing the soil quality with a more appropriate number of sites.In this study,taking Guiyang city(except part of the main urban area),a typical karst landscape,as the study area,the soil heavy metal Cd element was selected as the study object,based on mathematical and statistical methods,spatial autocorrelation analysis,and geostatistical methods,the spatial partitioning characteristics of Cd element were analyzed,and the risk situation was initially studied;the geographic detectors were used to identify the main control influence factors quantifying the spatial partitioning of soil heavy metal Cd,and to define The risk target areas of Cd elements were defined by using geodetectors to identify the main controlling influence factors to quantify the spatial variation of soil heavy metals,and to define the risk target areas of Cd elements.The main research results are as follows:(1)Soil heavy metal Cd in the study area has strong spatial heterogeneity,dominated by structural heterogeneity,and the spatial distribution is related to natural factors such as stratigraphy,topography,and soil type.The results of spatial autocorrelation showed that soil heavy metals had strong spatial autocorrelation globally and showed clustering characteristics locally as well.The analysis of the spatial pattern characteristics of soil heavy metals in the study area using geostatistical methods found that the exponential model was the best-fit semi-variance function model,the spatial distribution of soil heavy metals Cd in the study area was dominated by the medium value,the high value area was concentrated in Kaiyang County,Qingzhen City and Huaxi District in the study area,and the low value area’s mainly spread from the north to the south,and the comparison between the spatial distribution of Cd elements and environmental factors found that The distribution of high value area is similar to the distribution of yellow loam in soil type,sphagnum system in stratum,higher soil p H and lower soil organic matter area,meanwhile the richer the mineral resources area,the more enriched the Cd content,and the distribution of heavy metal Cd in the study soil is not obvious in the construction land,which indicates that the enrichment of soil heavy metal Cd in the study area has some connection with the spatial distribution of different natural and anthropogenic factors.(2)According to the results of geo-ethical sounding,it can be seen that the stratum has the greatest explanatory power for the soil heavy metal Cd in the study area,and DEM,p H,and mineral resources also have some influence on the distribution of Cd.The ecological sounding results show that there is no significant difference between these influencing factors.The explanatory power among the interacting factors was greater than the sum of the explanatory power of the single factors.The interacting factor pairs with the highest influence were stratum ∩ organic matter and stratum ∩ development area,indicating that stratum was the main factor influencing the spatial distribution of soil heavy metals in the study area.Although the independent explanatory power of soil-forming parent rocks on soil heavy metals under a single factor was the smallest,the different interactions between soil-forming parent rocks and other factors also enhanced their influence on the spatial distribution of soil heavy metal Cd content.It indicates that the pollution sources of soil heavy metals in the study area are complex and not completely controlled by a single factor,and the interaction between natural and anthropogenic factors causes the enrichment of heavy metals.(3)By analyzing the relationship between different types of influence factors on the spatial distribution of soil heavy metals,it was found that the distribution of Cd is closely related to stratigraphy,with enrichment in the Carboniferous and Permian systems;the special geological conditions of karst lead to higher concentrations of Cd in carbonate areas;the concentration of Cd in the study area is high in grasslands and woodlands with high vegetation cover;the heavy metal Cd is prone to migration in areas with high altitude.Element Cd is widely distributed in the study area in such zonal soil types as yellow loam and yellow-brown loam;the concentration of soil heavy metal Cd in the study area decreases with increasing distance from development areas,roads and mineral resources;the migration of heavy metals is affected in areas with small soil p H and high physical and chemical organic matter in the soil,leading to the differentiation of heavy metal enrichment.(4)The risk target areas in the study area are mainly distributed in Kaiyang County,Xiuwen County,Qingzhen City and Huaxi District,so priority should be given to focus on these areas when carrying out soil environmental control in the study area.The spatial distribution of risk areas in Kaiyang County and Xiuwen County is similar to the distribution of the Carboniferous and Permian systems with high soil heavy metal Cd content.The identification of soil heavy metal risk areas through the dominant types of main control factors provides a certain guiding basis for the key regulatory risk areas of soil heavy metals and the development of control measures.(5)To spatially optimize the soil sample points in the study area,the study area was divided into five partitions to ensure that the variance within the partitions was small and the variance between the partitions was large,and the partitioning results showed that the 15 partitioned areas were homogeneous and reasonably partitioned.The layout of the soil sample points was optimized using the MSN method on the basis of homogeneous partitioning.The results showed that a total of 4,554 sample points were optimized by univariate and 6,139 sample points were optimized by integrated optimization based on auxiliary variables,and the integrated optimization was more consistent with the original data characteristics.It shows that the MSN method is applicable to the optimization of soil sampling points and can provide some reference for the reasonable spatial placement of soil monitoring points. |
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