Environmental Geochemistry Of Heavy Metals In Sediments Of The Sheyang River Basin

The coastal area of Jiangsu Province composes an especially important part of the Yangtze River Delta for rapid economic development and intensive anthropogenic activities. During previous decades, enormous industrial effluxes and domestic sewage had been discharged into rivers, resulting in severe environmental pollution events and degradation problems in this area along with the advancement of coastal exploitation and development. Heavy metal in sediments causes extensive concern due to the vast sources, toxicity, persistence, nonbiodegradability and bioaccumulation properties, thus becomes one of the major limiting factors to a healthy sedimentary environment and a hot topic in research worldwide. Riverine sediment, as a prominent sink of heavy metal in aquatic ecosystems, accumulates and documents metal elements from various origins.Therefore, the identification of heavy metals sources, the reconstruction of sedimentary records, and the analysis of distribution and migration of heavy metals, are of primary importance for the demonstration of pollution characteristics, for the assessment of ecological risks and the evaluation of anthropogenic impact for the studied area.Sheyang River, the largest natural river in Yancheng City that has been seriously polluted by heavy metal, was specified for the present study. According to a constant rate of supply (CRS) model, dating of the sediment cores and the velocity of deposition were counted from the profiles of 210Pb activities. The concentrations of severn heavy metals (Cu,Zn,Pb, Ni,Cr, Fe,and Mn) and their fractions were determined to reveal the spatial distribution and speciation characteristics. The natural and anthropogenic sources of heavy metals were idnetified based on multivariate statistical analysis. Geographically weighted regression (GWR) was applied to verify and detail the influences of different land uses to sedimentary heavy metal pollution. The results would be a helpful and general auxiliary method to contamination control and environmental protection. The main results are as follows:(1) Spatial distribution of heavy metals in surface sedimentsThe average concentration of studied metals in surface sediments followed the decreasing order as Fe > Mn > Zn > Cu > Ni > Pb > Cr. Except for Fe and Mn,concentrations of all the other heavy metals (Zn, Cu, Ni, Pb and Cr) greatly exceeded the corresponding background values. In the studied area, Cu and Zn were significantly enriched,indicating a severe pollution condition. While, the distribution of Pb and Cr presented a significant spatial diversity and were mostly moderately enriched, which means the anthropogenic activities had different effects on the enrichment of these heavy metals in surface sediments. In all sites, only Mn and Fe were relatively low with nearly constant values, underlining that Mn and Fe were mainly from a lithogenic origin and no significant anthropogenic source was detectable. Altogheter,the spatial distribution of the determined heavy metals showed similar characteristics that the higher concentrations of heavy metals presented in upstream and Xiaoyang River compared to that in downstream and Sheyang River. The upstream subareas characterized by higher heavy metal concentrations were mainly the regions with higher percentage of urban lands and industrial lands, where the activities of human beings were more intensive. High discharge of industrial wastewater and domestic sewage,directly or indirectly fed into the river, enlarged the load of metals in sediments sourcing from human activities. However, the sites characterized by lower heavy metal concentrations especially near the estuary of Sheyang River, which were dominated by agricultural lands with low population density, showed lower anthropogenic loads of heavy metals.(2) Bioactivity of different heavy metals in surface sedimentsThe heavy metal fractions showed that different species of metals might had distinct spatial pattern of total concentration and different fraction speciation. Five metal species (Cu, Zn, Pb, Cr and Fe) had the greatest proportion in residual fraction,while Ni and Mn had higher proportions in non-residual fractions. The evaluation of the bioavailability and potential risks for the environment based on the fractions had slight difference compared with that based on the total concentrations. The results showed that Cu, Zn, Pb and Ni had higher bioactivity and mobility, signifying high bioavailability and potential ecological risk to the environment. The proportion of bioavailable and potential bioavailable form were much higher at the sites locating in upstream of Sheyang River and Xiaoyang River, suggesting a high ecological risk in these subareas.(3) Metal accumulation preserved in the sedimentary records for the last century and its response to human activtiesThe sediment ages and sedimentation rate for three tested cores (S9, S10 and S16)were estimated based on the 210Pbex activities. According to the CRS model, S9 covered the time period of 1901-2014, S10 covered the time period of 1855?2013, and S16 covered the time period of 1864?2013. An average sedimentation rate of 0.32 g/cm2 a for S9, 0.10 g/cm2·a for S10 and 0.09 g/cm2·a for S16 were obtained through CRS model. Owing to the filtration by the river, the suspended particles in water became very few and only a small proportion of particles were transported to the estuary, thus the sedimentary rates and content of fine ingredients were much higher in upstream than in downstream. The century-long sedimentary history could be divided into three stages. First, from 1980 to 1995, the industralization developed rapidly in costal Jiangsu with poor awareness of environmental protection from the governments and corporations. Under the influences of industrial effluents, coal combustion, leaded gasoline, ore, and smelting, a drastically enrichment of metals in the sediment had been created. Since this period, anthropogenic inputs had become prevailing over nature sources. Second, during 1996 to 1999, two national natural reserves protecting red crowned crane and elk were established, a number of manufacturing enterprises in Sheyang County were forced to relocated. An obvious decrease in metal enrichment were found in this specific duration. However, this decrease is not as durable or reliable when confronted with the continuously rapid economic development, constantly advancing of industrial modernization and urbanization, which soon made the enrichment of meals increased again. Last, entering the 21st century, the industries still developed rapidly but the EFs of metals did not increase unlike that in the 1990s. This might be related to the rationalization of industrial structure and improvement of pollutant treatment.(4) Origins of heavy metals in sediments of Sheyang River watershedMultivariate statistical methods including correlation analysis and principal component analysis were applied to identify the contributing sources of heavy metals in sediments. It has been well documented that Cu, Zn and Pb had high affinity to anthropogenic activities, such as industrial effluents, coal combustion and leaded gasoline, etc. Fe and Mn, however, were mainly from natural sources, lithogenic origin instead of anthropogenic input. While the origin of Cr and Ni was more complex and influenced by both anthropogenic activities and natural settlement. According to comparison of stable lead isotope ratio in sediments (206Pb/207Pb and 208Pb/207Pb) and the possible anthropogenic sources, the results suggested that the possible sources of lead in sediments of Sheyang River were mainly from coal fly ash and metal smelting,among which the anthropogenic contribution rate was about 19.23%.(5) The influence of land use type on heavy metal pollution in the sediment of Sheyang RiverSpatial autocorrelation analysis of heavy metals in sediments of the study area demonstrated a concentration area of multiple metals in the region of Xiaoyang River with low spatial variations, which required attention in management. Most of the heavy metals in the lower reaches of the Sheyang River, especially near the estuary, had low concentration areas, indicating that the location of the estuary was less polluted by heavy metal which should be further protected.According to the analysis based on GWR model, a positive correlationship could be found between the proportion of urban land and the accumulation of heavy metals,including Cu, Zn, Pb, Fe and Mn. A more substantial positive correlationship could be found in highly-urbanized watersheds than less-urbanized watersheds that were dominated by agricultural or natural land. It could be explained that anthropogenic inputs dominate within urbanized watershed sediments. While, the percentage of agricultural land had a relatively weaker association with sediment heavy metal pollution in these watersheds, compared with urban land. This might be explained that in the agriculture-dominated watersheds agricultural land might contribute to heavy metal pollution, but in highly-urbanized watersheds the agricultural contribution was much negligible, comparing with urban sources. This might be explained that in the agriculture-dominated watersheds, agricultural land contributed to heavy metal pollution, but in highly-urbanized watersheds this contribution was much negligible comparing with urban sources. This relationship helped to predict metal pollution in sediment in less urbanized areas. Based on GWR modelling, the percentage of forest and grass land had negative correlationships with most of heavy metals in sediments(Cu, Zn, Pb, Fe and Mn) at all sampling sites, indicating that forest and grass land coverage might be a fine negative representative indicator of metal pollution. Intensive anthropogenic acitivities, including industrial manufacture, urban traffic and mining normally occurred in lands less covered with forest and grass land,and on contrast,this type of land cover could significantly reduce heavy metal pollution within the subwatershed thus lower the impact of human activities. Subsequently, this negative correlationship between the cover of forest and grass land and heavy metal pollution could be much stronger in less-urbanized subwatersheds, while in highly-urbanized subwatersheds the R2 values were commonly lower. The percentage of forest and grass land could explain most of the spatial variation of heavy metal pollution in less-urbanized areas, but little in highly-urbanized watersheds.