Distribution Characteristics And Source Analysis Of Heavy Metals In Mangrove Wetland Surface Sediments In Jiulong River Estuary

The surface sediments of the mangrove wetland in Jiulong River Estuary were studied in this paper.The physic-chemical parameters,the distribution characteristics of heavy metal contents and chemical fractions of surface sediments were analyzed.The Geo-accumulation index（Igeo）,enrichment factor（EF）,potential ecological risk index（PERI）,sediment quality guidelines（SQGs）,the ratio of secondary phase to primary phase（RSP）and risk assessment code（RAC）were applied to assess the degree of heavy metals pollution in the region.The sources of the heavy metals in surface sediments were studied using multivariate statistics and isotopic ratios of lead,strontium and neodymium.The physic-chemical parameters of wetland surface sediments were relatively stable overall.The pH range（average）of surface sediments was 6.12～8.05（6.90）,mostly neutral or slightly acidic.The redox potential（Eh）range（average）was-34mV～67 mV（24.22 mV）,mainly reducing.The soluble salt contents range（average）was 1.99‰～7.04‰（3.81‰）.The total organic carbon contents range（average）was10.82 g?kg^-1～61.93 g?kg^-1(22.11 g?kg^-1).The grain size characteristics were mainly silt（47.15%～70.72%）,followed by clay（19.65%～37.58%）and sand（1.09%～29.76%）.The order of heavy metals contents(mg·kg^-1)of surface sediments was:Fe（38928.38±1653.36）>Mn（1077.32±289.81）>Zn（146.57±10.31）>V（86.81±2.74）>Pb（73.54±6.88）>Cr（63.98±3.76）>Cu（34.25±7.29）>Ni（28.62±2.09）>Co（13.88±0.57）>As（12.99±1.44）>Cd（0.229±0.11）>Hg（0.150±0.060）.Except that Fe and V contents were close to the soil environmental background values in Fujian Province,the other heavy metals contents exceed the background values to varying degrees.In terms of the spatial distribution of heavy metals contents,the contents of V,Cr,Mn,Fe and Zn were lower in the north and south edges of the wetland,and the contents in middle region were higher which was related to the characteristics of regional physical-chemical parameters（pH,redox potential and grain size characteristics）.There was no correlation between As,Hg and Cd contents and physic-chemical parameters,while the contents of other heavy metals had correlation with physic-chemical parameters.The contents of Mn and Pb in all heavy metals were more closely related to physic-chemical parameters.The contents of Mn and Co were significantly positively correlated with pH and significantly negatively correlated with Eh.The contents of Cu and Pb were significantly negatively correlated with pH and significantly positively correlated with Eh.There was a significant negative correlation between the contents of Zn,Pb and soluble salt contents.There was a significant positive correlation between total organic carbon contents and Pb contents.Clay composition content was positively correlated with the contents of V,Fe,Zn and Pb,while significantly negatively correlated with Mn content.Silt composition content was significantly positively correlated with the contents of Mn and Ni,while significantly negatively correlated with Pb content.The sand composition content was significantly negatively correlated with the contents of V and Cr.The results of modified Community Bureau of Reference（BCR）sequential extraction procedure showed that V,Cr,Fe,Ni,Zn,As,and Hg mainly existed in the residual fraction.Mn and Cd showed the highest portion of acid extractable fraction.Pb mainly existed in the reducible fraction,and the oxidizable fraction of Hg had a certain proportion.The reducible,oxidizable,and residue fractions of Cu all occupied a certain proportion.Overall,the bioavailable fraction proportion of Cd and Mn was relatively high（65.76%～97.84%、60.70%～80.33%）,and the bioavailable fraction proportion of Pb,Cu and Co was also quite significant（55.59%～78.56%、48.83～81.29%、44.77%～61.24%）.The proportion of bioavailable fraction of V,Cr,Mn,Fe,Co,Ni,Zn,and As in the wetland marginal area was lower than those in the central area,and gradually decreased from the highest proportion area toward the north and south.The proportions of bioavailable fraction of Co,Ni and Zn in the area near the tidal creeks were significantly higher than those in the area farther away from the tidal creeks.There was no significant difference in the spatial distribution of Hg bioavailable fraction.The results of all heavy metals pollution degree assessment methods showed that,the potential ecological risk of heavy metals in surface sediments in the region was mainly higher.Among the 12 heavy metals,the surface sediments of the mangrove wetland in the Jiulong River Estuary were polluted by other heavy metals to varying degrees except V and Fe.The pollution degree of Cd,Mn,As,Ni,Hg and Pb was relatively high,especially Cd and Mn,which reach a degree of moderate pollution and moderate enrichment.This should be taken seriously.The multivariate statistical analysis results showed that Zn,V,and Fe were mainly derived from natural sources,and Co,Ni,Cr,Pb,and Mn pollution was mainly from coal combustion and industrial activities.Cu and Hg pollution was mainly from aquaculture.Cd and As pollution mainly came from agricultural activities.The contribution rates of the above potential sources to heavy metals in wetland sediments were 54.20%,19.38%,24.76%,and 1.66%,respectively.The contents,geochemical characteristics and distribution patterns of rare earth elements（REEs）suggested that the wetland surface sediments were mainly from terrestrial sources and also affected to some extent by other sources.The results of Pb isotopic ratios in surface sediment samples and potential sources indicated that the Pb accumulated in the mangrove wetland was mainly from parent material,Fujian Pb-Zn deposit and industrial sources,with the contribution rates of 32.73%,52.12%and 15.15%,respectively.Compared with the other three fractions,values of ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb of the residue fraction was farthest from the anthropogenic sources and the least affected.The bioavailable fraction of Pb was easily affected by the anthropogenic sources.According to the strontium isotope composition,the contribution rate of natural sources to strontium in wetland sediments was 85.53%,and the contribution rate of anthropogenic sources to it was 14.47%.The combined tracer of total rare earth contents（REEs）and neodymium isotope indicated that the REEs in the surface sediments mainly came from the parent material and the Fujian lead-zinc deposit,and had also been affected by a certain degree of anthropogenic sources,but the impact was relatively small.The combined plots of the Sr–Nd–Pb isotopic ratios demonstrated that natural sources were still the main sources that had a greater impact on wetland sediments.Comprehensive analysis results of different methods for source identification of heavy metals showed that natural sources were the main source of heavy metals in the wetland sediments,but anthropogenic sources also contributed to it and should not be ignored.