喀斯特地区小尺度农业土壤砷的空间分布及污染评价

贵州岩溶地貌分布广泛,部分地区的土壤砷(As)污染问题比较突出.为了解高As背景地区农业土壤As含量分布特征,选取贵州省兴义市西南部具有代表性的As污染区(典型喀斯特区、亚喀斯特区)为研究对象,并以非喀斯特区作为对照组,采用地统计分析与GIS相结合的方法研究农业土壤As的空间变异和污染状况,并采用Moran's I系数分析在小尺度下土壤As的空间自相关性及其方向性特征.结果表明,不同地貌区土壤中As含量从高到低依次为:典型喀斯特区亚喀斯特区非喀斯特区,其中典型喀斯特区农业土壤As的算术均值与几何均值分别为47. 9 mg·kg~(-1)和43. 3 mg·kg~(-1),亚喀斯特区其值分别为36. 8 mg·kg~(-1)和30. 1 mg·kg~(-1),两个区域农业土壤As含量明显高于贵州土壤As背景值,As的超标率分别为98. 5%和96. 7%,表现出明显的As累积,而非喀斯特区As超标率仅为6. 7%;独立样本T检验结果表明,在3个不同地貌类型中,农业土壤与自然土壤As含量均无显著性差异(P 0. 05).农业土壤As的Moran's I系数为0. 45,Z值为11. 61,在本采样尺度下具有显著的正空间自相关(P 0. 05),存在空间聚集,尤其是东北-西南方向空间自相关性较好,以结构性变异为主.农业土壤样本As总体上处于轻微污染至轻度污染之间,分别占27. 10%、29. 02%,部分地区处于中度污染级别,而处于无污染状态的样本占41. 94%.

Spatial Distribution and Pollution Assessment of As at a Small Scale in Agricultural Soils of the Karst Region

The karst landforms in Guizhou are widely distributed, and the problem of soil arsenic (As) pollution is prominent in these areas because of the high environmental background levels. In order to study the distribution characteristics of As contents in agricultural soils with high background values of As, representative As polluted regions (typical karst region, semi-karst region) in southwestern Xingyi City of Guizhou Province were selected as the research objects, and the non-karst region served as the control group. Geostatistical analyses were then combined with GIS data to study the spatial variability and pollution conditions of As in agricultural soils. Furthermore, Moran''s I statistic was used to analyze the spatial autocorrelation and directional characteristics of As at a small scale in the soil. The results showed that As contents in soils from different geomorphological regions were ranked as follows:typical karst region > semi-karst region > non-karst region. The arithmetic mean value and geometric mean value of As in agricultural soils in the typical karst region were 47.9 mg·kg^-1 and 43.3 mg·kg^-1, respectively. Meanwhile, the arithmetic mean value and geometric mean value of As in agricultural soils in the semi-karst region were 36.8 mg·kg^-1 and 30.1 mg·kg^-1, respectively. The As content in agricultural soils from these two regions was significantly higher than the background values of As in Guizhou. In addition, the standard exceedance rates of As in those two regions were 98.5% and 96.7%, respectively, thus demonstrating a high degree of As accumulation. In contrast, the standard exceedance rate of As in the non-karst region was only 6.7%. Among these three afore-mentioned landform types, the results of independent sample T tests showed that there were no significant differences in the content of As between agricultural soil and (natural) soil (P>0.05). The Moran''s I coefficient of the As content in agricultural soil was 0.45, and the Z value was 11.61, thus suggesting that there was a significant positive spatial autocorrelation at the small scale (P<0.05), especially in the northeast-southwest direction, and the structural variation was dominant. The As polluted agricultural soils were generally at the slight pollution and mild pollution levels, which accounted for 27.10% and 29.02% of the samples, respectively. However, some regions were at the level of moderate pollution. The non-polluted samples accounted for 41.94% of the samples.