Migration Characteristics And Controlling Factors Of Heavy Metal Elements In Polje Soil

The high geological background level for heavy metals together with the influence of human activities are the main causes for the high heavy metal content in soils observed within the karst areas of Southwest China.Allogenic water is one of the main driving forces of karstification,which affects the development of karst landforms and the sediment spatial distribution.The polje formed under the local geological structure and prevailing hydraulic conditions impact the main occupational and residential locales for residents in the karst area.The soil quality affects regional ecological security and human health in the polje.The formation of soil in polje is closely related to factors such as fluviatile transport and soil erosion.The spatial heterogeneity of karst environment has complex effects on the heavy metal content and availability in soil.However,research regarding the effect of allogenic water on the migration,distribution,and activation of soil heavy metals in polje soil is still scarce.In this study,the surface samples and profile samples of different parent materials were examined in a typical karst polje in Guangxi,China.In addition,the content of heavy metal elements and major oxides was analyzed.Geochemical fingerprints were characterized for soil source analysis in a small watershed of the border polje.The fingerprints indicated the influence of allogenic water on the formation of polje soil and element geochemical behavior.Rice and rhizosphere soil samples were collected at the polje to analyze water-rock constraints on the bioaccumulation and control factors of heavy metals in the soil-rice system.The findings provide a scientific basis for solving the migration and transformation of heavy metals in soil in karst areas and the management and control of heavy metals in high geological background areas.The main results are as follows:（1）The main oxide composition of soil in the study area is mainly Si O₂,Al₂O₃,TFe₂O₃;while Na₂O,K₂O,Ca O,Mg O are relatively deficient.The average content and regional background values are lower than the national average content.The chemical weathering index（CIA）of soil and a A-CN-K triangle diagram were analyzed.The soil in the study area soil showed a high level of chemical weathering.In terms of spatial distribution,the content of soil Ca O was low in the non-karst areas in the south,east,and west.The p H showed strong acidity,while the Si O₂content of soil in the karst area in the central and northern parts was low while the p H value was high,showing the opposite trend.The high heavy metal elements concentrations for As,Cd,Cr,Cu,Hg,Ni,Pb,and Zn were also distributed in the carbonate rock area.The geochemical characteristics of heavy metal elements in soil were controlled by the parent material,which was significantly correlated with soil p H.There were significant differences in element content between karst and non-karst areas.River transport affected the spatial distribution of regional soil geochemistry in polje.（2）The content of soil organic carbon（Corg）in the karst weathered soil profile（SPk）was significantly higher than that in the border polje（SPbp）in the non-karst area（SPnk）.Corg gradually decreased with increasing soil depth.The SPk p H value is high for SPbp and SPnk,while there is a slight increase with profile depth.The Na₂O and K₂O contents also increased slightly with the depth of the profile,but the contents of different profiles varied significantly.Ti O₂decreased with the depth of the profile.The other major elements only varied slightly in the soil surface and in underlying layers.The correlation analysis of the elements showed that the elements were products of weathering of the underlying bedrock.During the weathering process,Ca O,Mg O,Na₂O,and K₂O were leached to varying degrees,while Al₂O₃and TFe₂O₃were enriched,indicating an aluminum-rich and iron-rich weathering stage.However,the content of major element oxides in SPbp change intricately with depth.This demonstrates that the soil may originate from different ratios of mixing of parent materials in the surrounding karst and non-karst areas.（3）The hydrochemical type of ground water in the karst area was of a HCO₃-Ca type.The allogenic water recharged to an underground river（Wag）and promoted karstification.The calcite saturation index changed from unsaturated to saturated.Moreover,the elements in the dissolution products or soil leaching liquid were transferred to the water body for migration.Allogenic water entered the karst area in the form of surface water or underground rivers.This resulted in an increase in the content of most heavy metals.Heavy metal elements in water mainly originated from rock weathering.In addition,some elements were affected by redox conditions and changes in Fe and Mn contents.（4）Based on geological background,land use,and cluster analysis,the soil sources of border polje were divided into five groups.Three composite fingerprints were generated based on the conservation of elements and the fingerprints’ability to discriminate among the five source groups.The bi-plot discriminant function demonstrated the ability to distinguish more than 85.6%of soil sources in border polje.The goodness of fit of the non-mixed model exceeded 0.75,indicating the accuracy of source analysis for BP soils by geochemical fingerprints.（5）The parent material in the non-karst area contributed more than 90%to the soil in the border polje.The border polje was mainly derived from the water level fluctuation zone of the non-karst reservoir.The slope land in the non-karst area only contributed to the soil near the toe of the slope in the polje.This indicated that the soil may be affected by residual slope accumulation and siltation during floods.Karst depressions contributed more than half to the soil of dry land near the limestone foothills in the border polje rim,followed by water level fluctuation zone of the non-karst reservoir.Therefore,the geological background concentrations and the influence of human activities must be considered when analyzing the soil sources of border polje by geochemical fingerprinting.（6）The element content in the polje soil is generally between that of soils comprised of carbonate rock and non-carbonate rock.The soil properties and elemental composition of border polje is similar to that of non-karst areas;however,the elemental contents are different in different spatial locations,which are affected by fluvial transportation.The average heavy metal content in soil may be expressed in ascending order starting with an underground river recharged by allogenic water polje<narrow and long border polje<polje recharged by karst water<narrow and long dry polje with seasonal rivers.The allogenic water transports the parent material from the non-karst area into the karst area.This transport dilutes the total quantity of heavy metals in the soil.In addition,the karst underground river that receives allogenic water recharge transports a significant quantity of base ions.This results in the p H of the soil in the irrigation area being greater than that of the non-karst area and the border polje.（7）Differences of the contribution ratio of karst and non-karst to the border polje soil caused a large spatial difference in soil elemental content.The soil heavy metal content near the karst area is high,whereas that near non-karst area or exogenous water channel is low.The availability of heavy metal elements in soil is greater between carbonate rocks and sand shale parent materials.The Cd and Pb of some rice samples exceed the maximum allowable values for rice prescribed in the national food safety standards.This violation may cause health risks,resulting in the fact that the polje soil with high geological background of heavy metals has certain ecological risks when soil acidification conditions prevail.（8）When exogenous water enters the karst area,it causes leaching of salt-based ions,the acidification of soil,and activation of heavy metals.These processes result in excessive heavy metal elements in rice.The karst underground river water supplied by exogenous water is rich in salt-based ions such as Ca.Ca maintains a relatively high soil acidity and alkalinity and effectively reduces the risk of heavy metal activation.In addition,it reduces the proportion of heavy metals exceeding the standard.The bioaccumulation factor of heavy metals in the soil-rice system contained within the polje is related to the contribution ratio of carbonate parent materials to soil and its spatial location.For cultivated land intermediate between karst and non-karst,and intermediate between exogenous river and karst,it is necessary to consider changing land use patterns and planting crops with low enrichment of heavy metals.In addition,for such cultivated land,the monitoring of p H,salt-based ions,and available heavy metals should be improved to achieve the safe utilization of soil in border polje.At present,the high geological background of karst environment is an active research topic.The results of this study improve the understanding of the migration law of soil heavy metal elements in karst areas subject to the constraints of hydrological conditions and karstification.The results not only facilitate a better understanding of the source,transport,and fate of heavy metals in the polje soil,but also improve understanding regarding the interaction and mechanism of heavy metals and base ions in the polje soil-rice system under hydrological constraints.In addition,this study also provides a significant volume of basic data and scientific and technological support for the control and monitoring of heavy metal soil content in arable land within the national karst area.Further in-depth research is needed in the future on the effect of chemical weathering on element migration during soil erosion and deposition,the effect of allogenic water on karst groundwater quality,and the biogeochemistry cycle of heavy metal elements in soil-crop systems.