The Eco-geochemical Assessment Of Chagan Lake In West Of Jilin Province

1. Natural geography and regional geologyThe research area is located on the Songnen plain hinterland,suited at the transition zones of semi-dry and semi-humid climates, the temperature differences is big,and the frost-free period is short.Surface water resources in the area is less,and most of it is hard to using.The soils of the area are mainly chernozem, saline-sodic soil, and meadow soil.Crops were harvested chiefly in chernozem,salinized pasture were distributed in saline-sodic soil and meadow soil.The plant resources are mainly swamp marsh vegetation and salinized meadow vegetation.Stratigrahpy includes Cretaceous system of Mesozoic,Tertiary and Quaternary systems of Cenozoic.The area as a part of the Songliao fault-depression basin continues to subside on the base of Cenozoic tectonic movement.The faults were developed with the area,and mostly striking is NW faulting.2 . Samplings and analysesOnly 6 water sampling sites were set within Chagan lake,and to sample water samples by two seasons(summer and winter).Each water sample were analyzed for about 54 chemical index.Only 2 suspended material sampling sites were arranged within Chagan lake,and and to sample suspended samples by two seasons(summer and winter) too.Each suspended sample were analyzed for about 50 chemical index.The 32 superficial sediment samplings were collected at the bottom of the lake.The analyzing index for the samples were As, Hg, Se, Co, Cl, Cr, Cu, Ni, Mn, N, P, Mo, Pb, Th, U, Al2O3, Fe2O3 MgO, CaO, Na2O, K2O, Cd and organic carbon.Three sediment cores were collected in the lake.The elements for analyzing were As,Hg,Fe2O3,Cu,Mn,P,Pb,Zn,Co,Ni,Rb,Sr,Cr,Ba,Nb,Ta,Th,U,REE and 210Pb.Fish samples were carried out at the same time with water sampling.In summer,only 8 fish samples were collected,and in winter 12 fish samples were sampled.The elements for fish testing were As,Se,Zn,Hg,Ca,Cd,P,Fe,Cr,Co,K,Mg,Mn, Pb,B,Cu,Mo,Ni,I,Na. 3. Water geochemical characteristics of Chagan lake and the water quality evaluationThe content sequences for major cations and anions in different seasons were nearly the same,which implied that season variations can not affect cation and anion contents and their proportions too much.Dissolved oxygen(DO),CODMn,TN,TP in summer and winter seasons were all up to the national third surface water quality standards.The water chemical type for Chagan lake was HCO3–Na.In the lake,the trace element contents in summer season were generally higher those in winter season except for Fe and Hg.The REE normalized patterns of wasters from the summer and winter seasons were nearly parallel,but only with some differences in HREE.The general characteristics of elements in the suspended in the lake were that the contents of Ca,Mg,Al,Fe,P,K,Na,Mn,Ti,Sr,Ba were relatively higher, and others were relatively lower.REE and transition elements were mainly derived from soils and forcks within the lake drainage areas.The water quality of Chagan lake was evaluated with mono-factor assessment methods and modified Nemero index methods.It was concluded that the water quality of the lake was quite good,and the water quality in summer season was superior to that in winter season.4. The temporal and spatial variations of elements in bottom sediment from Chagan lake and the potential ecological risks assessment of heavy metalsThe assemblages of the quartz-plagioclase-potash feldspar-illite-minerals with the mixed layer of illite and montmorillonite occurring in the lake bottom sediments indicated that the weathering in the area is weak,and calcite and dolomite appearing in the sediment showed that Ca and Mg contents were high in water,soils and rocks within the area.Ca,Mg,Cl,Hg,Cd in superficial sediments of Chagan lake were enriched,and Co, Na2O and organic carbon were somewhat delepted.The contents of elements in the superficial sediments varied less,which implied that Hg,Cd etc were not disturbed by human activcity.C/N ratio was suggested that the organic material was come from the phytoplankton within the lake.Using Pearson's analysis methods to calculate the coefficients of the elements in lake sediments.Pb and Th had no relation to others.Na2O,K2O had the negative correlation with others.But for these,there were some positive correlation among the the rest elements.Adopting potential ecological risk assessment and geo-index nethods to evaluate the potential ecological risks of heavy metals in the superficial sediments of the lake. The potential ecological risks of Hg,Cd,As,Cr,Cu,Zn in surface sediments were less.To date the three sediment cores by 210Pb dating methods.The 210Pb activities of the three cores were changed quite different with the depth increasing,which can be subdivided into narrowly oscillation,exponential decay, the mixed types.Using the constant flux supply model (CRS) to calculate the sediment ages and sedimentation rate of the three cores,with sedimentation rates 1.33cm/a,1.2cm/a and 0.99cm/a,respectively,and the overall sedimentation rate for lake sediments was 1.17cm/a in average.The heavy metal contents in cores were decreased from past to present.In the three cores,Fe2O3,Cu,Zn,Co,Ni,Cr,Cd had the same variation trends.The sediment may derive the same sources.The of heavy metal in sediment profiles were also less,which implied that the amounts of heavy metals vertically charged into the water of Chagan lake were little.5. The relationships among water-lake sediment-fishTo test the heavy metal in the fish of Chagan lake,and to calculate the enrichment coefficients of As,Hg,Se,Cu,Ni,Co,Pb,Zn,Mo between fish-water and fish-lake sediments.The linear relationships among fish,water and lake sediments were not clear.