基于小流域尺度的土壤重金属分布与土地利用相关性研究——以厦门市坂头水库流域为例

Distribution of heavy metals in topsoils affected by land use patterns at a small watershed scale: a case study in the Bantou Reservoir watershed in Xiamen,China

人类活动改变流域集水区土地利用方式同时提高了土壤重金属水平,并影响地表水重金属负荷。以具有明确地理边界的流域作为研究单元可能实现计量区别不同人类活动或因其引起的土地利用方式对土壤重金属空间分布及其环境风险的贡献。但不同人类活动及其改变的土地利用方式对土壤和地表水重金属负荷的相对贡献研究缺乏可操作性方法论。以我国典型的山塘水库小流域—福建省厦门市坂头水库流域为例,结合遥感影像解译、野外土壤重金属水平调查和GIS地统计方法,针对小流域尺度土壤重金属空间分布及其环境风险与土地利用方式相关性研究进行方法论的探索。坂头水库流域覆盖205km2,土地利用类型有林地、农业用地、城镇用地、绿地和水面等5大类。按照不同土地利用类型覆盖面积为权重,以统计最小样本数为准则,在该流域内共采集150个表层土壤样品,针对6种具有人为源特征重金属(Cu、Zn、Pb、Cr、Ni、Cd)土壤总量及富集水平进行分析,以潜在生态风险指数评价流域土壤重金属风险,并结合土地利用类型分析了其空间特征。结果表明:与林地和绿地相比,城镇用地和农业用地表层土壤重重金属水平显著较高;土壤重金属富集水平及其潜在生态风险依次为:城镇用地>农业用地>绿地>林地;城镇化加重了流域土壤重金属Cu、Zn和Cd的污染风险;土壤Pb富集水平与土地利用方式无关,可能为大气沉降来源;所有土地利用方式土壤Cr和Ni总量低于区域土壤背景值,但城镇用地的富集指数显著高于林地。基于土地利用方式的反距离加权空间插值(LU-IDW)也清晰地揭示了土壤重金属富集及其潜在生态风险与人类活动密切相关,尤其是城镇化。以野外调查结合遥感卫星图像解译和基于土地利用方式的空间插值的研究方法有效地揭示了研究流域土壤重金属的空间分布特征及其潜在风险评价空间模式,为小流域尺度的环境质量演变研究提供了方法论和案例。

Anthropogenic activities increase levels of heavy metals in catchment softs while changing land use in a watershed and further impairing the receiving water. Using a watershed as a study unit might help to determine the contributions of various anthropogenic activities, or their driven land use changes to spatial loadings and environmental risks of heavy metals in catchment soils because of its close geographic boundary. Few studies have been performed at a watershed scale to understand spatial relationships between anthropogenic land use changes and heavy metal enrichment in catchment soils, and have combined this with an overall potential ecological risk of heavy metal enrichment. This study integrated satellite image interpretation, field surveys of soil heavy metal level, and geo-statistics to explore relationships of soil heavy metal loads and their potential ecological risk in an urbanizing watershed in Xiamen, China. The Bantou Reservoir watershed covers 205 km： and consists of forested land, agricultural land, urban land, green land, and water surface. Interpreted by the digital elevation map （ DEM）, the studied watershed is comprised of five subwatersheds. In total, 150 soil-sampling points were assigned in the watershed based on coverage of each land use type with a minimum of three samples in each subwatershed to satisfy statistical requirements. A composite sample of two top- 10 cm soil samples was collected in a 10 x 10 m square at each sampling point. Six heavy metals, Cu, Zn, Pb, Cr, Ni, and Cd were analyzed in these topsoil samples. The enrichment factor and potential ecological risk factor of each heavy metal for each land use type, and total risk index of six heavy metals for each land use type were calculated. Results indicated that urban and agricultural land use significantly increased heavy metal loads in topsoils in comparison with forest and green land use in the watershed. The enrichment factor of heavy metals calculated on the basis of the regional background values showed that urban land use has higher values than agricultural land use, followed by green and forest land use. Accordingly, the potential ecological risk factor of each heavy metal followed the same pattern. Urban land use increased the enrichment of Cu, Zn, and Cd in topsoil of the watershed in comparison with other land uses. Lead enrichment factor was not related to land use type, suggesting that Pb was derived from atmospheric deposition. Regardless of land use types in the watershed, total Cr and Ni contents in topsoil were below the regional background values, although significant increases were observed within urban land in comparison with forest land. The land use based-inverse distance weighting （LU-IDW） spatial interpolation revealed that heavy metal loads and potential ecological risks are clearly related to anthropogenic activities, especially to urbanization. This study demonstrated that the methodology we applied to understand relationships between land use change with urbanization and heavy metal enrichment and even their ecological risks at a watershed scale is feasible. The integration of traditional field survey and satellite image interpretation techniques can serve as a powerful methodology for watershed-scale studies on environmental changes.