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

Abstract

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.