[Selenium Geochemical Characteristics and Influencing Factors of Paddy Fields in Du'an County, Guangxi].

This study used 21 rock samples and 193 sets of paddy soil and rice grain samples collected form Baiwang Town, an area of clastic sedimentary rocks in Guangxi, China, to evaluate the potential for selenium-rich agriculture. We analyzed the concentrations of selenium and heavy metals in the soil and rice samples, and discuss the influencing factors by means of statistics and correlation analysis. The results showed that the selenium-rich rate of paddy soil and rice grain in Baiwang Town were 67.4% and 64.8%, respectively, but the content of cadmium in the selenium-rich soil samples was generally higher than the baseline value for China and the background value for Guangxi. In comparison to the screening value of soil pollution risk for agricultural land(GB 15618-2018), the over-standard rate of cadmium was 46.1%, while the over-standard rates of other heavy metals were negligible. The comparison shows that the selenium-rich rate of rice was 61.1% in the southern area of Baiwang Town with a negligible heavy metal content, and the advantages of green selenium-rich rice planting were clear. The correlation analysis showed that secondary enrichment was the main form of selenium enrichment in the study area. The soil pH and texture of the root-zone soil also affected the selenium content of the soil. The selenium content of rice seeds was mainly affected by the selenium content, active selenium content, pH, and aluminum oxide content of the root-zone soil. The risk assessment showed that the level of dietary exposure to cadmium is generally within safe limits, but it is still necessary to strengthen the monitoring of cadmium in rice and control cadmium pollution to reduce the levels of dietary exposure, especially in the central region of Baiwang Town.

[Characteristics and Origins of Heavy Metals in Soil and Crops in Mountain Area of Southern Sichuan].

Excessive heavy metals in soils and crops will affect the human living environment and health. In order to study the enrichment characteristics and causes of heavy metals in soils and crops in a mountainous area of southern Sichuan, 1699 cultivated soil samples, 30 rice seed samples, 31 corn seed samples, 15 tea samples, and 76 crop root soil samples were collected. The enrichment characteristics and origins of heavy metals in the soil were studied using mathematical statistics, principal component analysis, and correlation analysis. The enrichment characteristics of heavy metals in crops and their correlations with root soil were discussed. The results showed that the content of heavy metals in cultivated soil was higher than the national soil background value, and many sampling points exceeded the risk screening value. Among them, 770 points of Cd, 176 points of Cu, 116 points of Cr, and 106 points of Ni exceeded the standard. Combining the factor analysis results, it was found that Cd, Cr, Cu, Ni, and Zn in cultivated soil mainly originated from the geological background, and Cd was also affected by soil organic matter and pH. Meanwhile, Hg and Pb were mainly related to human activities. The results showed that the contents of heavy metals in the crops were lower than the national standards; the enrichment coefficients of heavy metals in crops were generally small, and were mainly in the following order: tea > rice > corn. The correlation analysis of the heavy metal contents in the root soil and crops showed that there was no significant correlation among them. Thus, the heavy metals in the soil generally had a high content and low activity, which did not affect the quality and safety of the three crops. However, Cu in corn and CaO in root soil, As in corn seed and Fe2O3 in root soil, and Ni in corn seed and Fe2O3 in root soil showed significant correlations, whereas As in rice seed and CaO in root soil and Hg in rice seed and MgO in root soil showed significant correlations, and no significant correlations were found in tea.

[Ecological Risk Assessment of Heavy Metals at Township Scale in the High Background of Heavy Metals, Southwestern, China].

Heavy metals (HMs) are naturally occurring elements that have high natural background levels in the environment. Therefore, it is important to conduct ecological risk assessment and identify potential sources of HMs. In the past, studies were conducted at the regional scale. The accuracy of those studies could not meet the needs of spatial planning and natural resource management. Therefore, it is necessary to conduct ecological risk assessment at the township scale. In this study, 1092 soil samples (from 0-20 cm depth) were collected in the town of Reshui, an area with high background levels of soil HMs with the parent material of carbonatite, which is commonly found in Southwest China. The town of Reshui is a multi-ecological risk superimposed area where the ecological risk is high. In this study, concentrations of HMs (Cd, Cr, As, Hg, Pb, Cu, Zn, and Ni) in the topsoil were analyzed, and statistical analysis (SA), geographic information system (GIS) modeling, and positive matrix factorization (PMF) analysis were performed. The geoaccumulation index (Igeo) and potential ecological risk index (PERI) were applied for the ecological risk assessment and quantification of the sources of the soil HMs. The mean values of HM concentrations in the topsoil were 18.1, 1.18, 174.1, 202.2, 0.09, 71.1, 34.9, and 167.2 mg ·kg-1for As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn, respectively, which were considerably higher than the average background value (ABV) in soils in Yunnan Province except for As and Pb. The average concentrations of Cd, Cr, Cu, and Ni exceeded the screening values specified in the soil contamination risk in agricultural land (GB 15618-2018) by 5.82, 1.16, 4.04, and 1.02 times, respectively. The Igeo value shows that the major pollutant is Cu in the surface soil of the study area, followed by Cr, and Cd. Speciation analysis of HMs indicates that HMs (Cr, As, Pb, Cu, Zn, and Ni) mainly exist in the residual form, mostly from the geological background with low bioavailability. The potential effective components of Hg have higher levels, but the total amount of Hg and its pollution risk are lower. Cd has a high bioavailability ratio, is easy to enter the soil solution and be absorbed by crops, and is the HM with the highest pollution risk in the study area. The PERI shows that the proportions of low ecological risk, moderate risk, and high risk soil samples are 44.23%, 54.40%, and 1.37% of the total number of samples, respectively. Hg and Cd were the major sources of risk because of their high toxicity coefficient. The PMF analysis indicates that there are four major sources of HMs in the study area: human activity, natural sources, coal mining and traffic emissions, and agricultural sources with the risk contribution ratios of 9.29%, 53.67%, 11.23%, and 25.81%, respectively. The PMF analysis effectively quantified the ecological risk from these sources, providing a reference for further pollution control and prevention measures.