Spatial Characterization Of Heavy Metals In Topsoils And Loadings In Surface Water And Their Responses To Urbanization

Anthropogenic activities increase heavy metal levels in catchment soils while changing land use ina watershed and further impair the receiving water. This study attempted to understand how land usepatterns in an urbanizing watershed contributes instream heavy metal loadings via a long-termmonitoring in Xiamen, china. The study integrated satellite image interpretation, field survey of soilheavy metal level, and GIS geo-statistics to explore relationships of soil heavy metal loads and theirpotential ecological risk to land use pattern in the urbanizing watershed. The urbanizing watershed wasequipped with timing-and rainfall-triggered autosamplers at six points in five subwatersheds andinstream loadings of anthropogenic heavy metals (Cu, Zn, Pb, Cr, Cd, Ni, and Mn) were monitored fromMarch2012to December2013. In total150soil sampling points were assigned in the watershed basedon area of each land use patch with minimal3samples to satisfy statistical requirement. A compositesample of5top-10cm soil samples was collected in a10x10m square at each sampling point. Sixmetals, Cu, Zn, Pb, Cr, Ni, and Cd, were analyzed in these topsoil samples. The main results are asfollows:In comparison with the reference site (the head water point), anthropogenic activities on land usechanges increased heavy metal loadings in stream water metal-dependently. Farm land dominatedsubwatersheds had higher metal loadings at monitoring points than others. Assessed by Chinese surfacewater quality standard (GB3830-2002), instream loadings of Cu and Zn occasionally exceeded the ClassI thresholds at monitoring points within farmland dominated subwatersheds while Mn loadings werefully greater than the limit for drinking water source at all monitoring points. Farm land use highly andpositively contributed to R2of linear models for instream loadings of Cu, Zn, Cd, and Mn while urbanland use was positively higher than the farm land use for Pb and Cd loadings. Rainfall played a crucialrole in heavy metal loadings in stream water not only as a source (Cu and Zn in rain water exceeding theâ… class level of chinese surface water quality standard) but also as a driver (improving the linear modelfitness for heavy metal loadings against land-use pattern in subwatersheds). In short, urbanization isleading to server water quality degradation in the urbanizing watershed.Urban land use and agricultural land use significantly increased heavy metal loads in topsoils incomparison with the forest land use and green land use in the watershed. The enrichment factor ofheavy metals calculated on the basis of the regional background values showed that urban land use had higher values than agricultural land use, followed by green land use and forest land use. Accordingly,the potential ecological risk factor of each heavy metal showed the same pattern. Urban land useincreased 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 fromatmospheric deposition, in relation to local air pollution. Regardless of land use types in the watershed,total Cr and Ni contents in topsoil were below the regional background values but significant increaseswere observed within the urban land in comparison with the forest land.