Geochemistry And Risk Assessment Of Redox-Sensitive Elements At The Water-Sediment Interface In Caohai Wetland

Wetlands are one of the most important productive ecosystems on the earth,which play an important role in maintaining biodiversity and regulating climate.With the rapid socio-economic development and urban expansion,many wetland ecosystems are threatened by large uncontrolled inputs of pollutants.Pollution prevention and control of wetland ecosystems are a hot topic of current research.Sediment has an important impact on wetland diversity and function.It serves as a reservoir of nutrients and pollutants but releases residual pollutants into the environment when the environment changes.Therefore,research on wetland sediment pollution is necessary.In this study,the sediment and pore water of the Caohai were used as the research objects.Based on the field sampling and analysis,the multivariate statistical analysis and GIS were applied to study the mineralogical characteristics,the content of redoxsensitive elements,and their enrichment factors of sediment and typical coal mines in the Late Permian Longtan formation of western Guizhou,and revealed the spatiotemporal variation characteristics and pollution sources of redox-sensitive elements in the surface sediment of Caohai.The main findings of this study are as follows.(1)The surface sediment contains mineral components such as quartz,calcite,pyrite,kaolinite,illite,chlorite,and gypsum.The mineral composition is mainly influenced by the intensity of regional weathering.The transformation between illite,chlorite and kaolinite reflects the process of climate change from dry to wet during the period of mineral formation.The source of gypsum may be related to the large amount of harmful gas and dust emissions from the clay-based zinc refinery around Caohai.(2)The organic matter,total nitrogen,total phosphorus and p H of the surface sediment showed significant spatial variability over time.The high-value areas are all distributed over the southwest lake area,and the content is low in the northwest and east lake areas.Compared with other inland type lakes,organic matter,total nitrogen and total phosphorus in Caohai wetland are at a higher level,with obvious accumulation,and endogenous loads cannot be neglected.Organic matter and total nitrogen reached several times of the lowest level of ecotoxic effect(1% and 0.055%),respectively,indicating a high possibility of triggering a more serious ecotoxic effect and thus causing harm to benthic organisms in the wetland.The content of different redoxsensitive elements in the surface sediments of the Caohai varied greatly.The RSE content was influenced by p H.In the surrounding shallow water area,which is more polluted,Cd and Pb accumulate gradually in the sediment as the p H increases.The content of each element in the pore water did not exceed the CCC and CMC standards proposed by USEPA.The high partition coefficients of V and Cr indicated that they were not easily migrated from the sediments.The relatively low partition coefficients of Pb and Zn indicated that they were easily migrated.(3)Seasonal variations in the content of different redox-sensitive elements in sediment were evident.During the dry period,the high contents of Pb,Zn and Cd were mainly distributed in the center and southwest of the lake,decreasing to a tongue-like manner to the northeast.The high contents of V,Cr,Ni,Cu and Mo were distributed in the western and southern regions,decreasing to the east.During the abundant water period,the high contents areas of Pb,Zn and Cd were shifted from the southwest to the northwest;the contents of V,Cr,Ni,Cu and Mo were greater in the lakeshore zone than in the lake center.The elements in the sediments such as V,Cr,Ni,Zn,Cd and Pb may influenced by mining and agricultural activities,Cu and Mo may originate from endogenous release,and Fe and Mn may be mainly influenced by minerals.(4)The mean value of the integrated modified ecological risk index(MRI)in the study area was 344,which was at a severe ecological risk level.Zinc and Pb reached a more moderate pollution level.Cadmium was a heavy ecological risk level.Zinc,Pb and Cd have a tendency of continuous enrichment.Combining the results of the sediment quality benchmark evaluation in three countries(China,the United States and Canada).Zinc and Cd often have negative biological effects.RSE within the sediment has a non-negligible non-carcinogenic risk and a higher carcinogenic risk(oral ingestion predominant)for local residents,and children facing a greater carcinogenic risk than adults.