Polluted Characteristics Of Sediments And Phosphate Adsorption And Release Characteristics At The Sediment-water Interface In Nansi Lake, China, And Its Main Estuaries

Nansi Lake is the largest lake in Shandong province and the northern area of China,which is an important buffer lake of the South-to-North Water Diversion Project (east route).Eutrophication has become the central problem of water environment in Nansi Lake where the eutrophic state is from mesotrophic state to hypereutrophic state. Afer the water diversion project,the route of the water will be changed that is from the lower lake to the upper lake.The main inflow rivers that are heavily polluted all locate in the upper lake and the water of the inflow rivers will be a tremendous hidden trouble to the water coming from Jiangsu province.It is important to carry through the relative investigations on the heavy metals and organic compounds contained in the surface sediments of the inflow rivers' estuaries for making managements of Nansi Lake. Under the control of external load,some sedimentary phosphorus can be released at certain periods of the year and sedimentary phosphorus will determine the quality of water.So it is crucial to investigate the phosphate adsorption and release characteristics of lake sediments.To investigate various P-binding forms and biologically available phosphorus(BAP)will do good to find out the rule of phosphorus' transference and transformation at the sediment-water interface.It is a potential method to alleviate the degree of eutrophication by controlling and cutting BAP.Relative researches of the surface sediments that were sampled from the lake and inflow rivers were carried on and the main results are as follow.The overlay of earth's surface is analyzed according to the spectral data of spot-5. The analytical results show that the overlay of earth's surface is complicated which is obviously different from the other shallow lakes in China.There are five leading types of earth's surface in Nansi Lake,including open water area,pound,hydrophytic vegetation,farmland and terraneous vegetation.The proportion of open water area to whole lake area is only 41.71%。The quality of water which is heavily polluted could not meet the Grade V of the 'China surface water quality standard(GB3838-2002)'.The water quality of mostly water area exceed the Gradeâ…¢of GB3838-2002 and the concentrations of total phosphorus are degressive from the north to the south.The concentrations of Cr,Cu,Mn,Pb,Zn could meet the Gradeâ… of GB3838-2002.The concentrations of heavy metals(Al,Ca,Fe,Mn,Zn,Cr,Cu,As,Pb,Ni and Cd)in the sediments of Nansi Lake and its inflow rivers show that Xizhi River,Si River, Wanfu River and Liangji Canal are heavily contaminated by heavy metals.The concentration of heavy metals in the surface sediment is relatively low and contamination of heavy metal is not the chief problem of Guangfu River.The concentrations of every kind of heavy metals are litter in the sediment of Si River. Combined with spot investigation and analysis of concentrations of heavy metals in the surface water,Si River is badly contaminated.It is necessary to combine chemical components,concentrations of heavy metals in the surface water,hydrological condition with the concentrations of heavy metals in the sediments to evaluate the degree of contamination.The intercorrelations between Cu,Pb,and Zn contents in the sediments are significant,especially Cu and Zn.Mn and Al,As,Cd,Cr,Ca have significant relationships,and Al,As,Cd,Cr,Ca always compound with Mn in the sediments.Fe only has correlation with Pb.Corrasion and efflorescence of sediment and soil affect the concentrations of Ca and Al,As,Cd,Mn in the sediments together,and Ca has prominent relativity with Al,As,Cd and Mn.The evaluative results of I_geoand RI both shows that the sediments of Nansi Lake are badly contaminated by Cd,and contamination caused by As takes second place. Other heavy metals just contaminate the sediments lightly.As an extremely dangerous element,Cd ranges from 2.65 to 5.04 mg·kg^-1in the Nansi Lake sediments.The concentrations of Cd are all higher than the lakes' background concentration cognizanced by Environment Monitoring Station in China,so governmental department should pay more attention to Cd in the Nansi Lake sediments to ensure the water quality.The investigative results of total phosphorus and phosphorus fractions in the sediments show that the average content of total phosphorus(TP)is 0.08%,and the sediments of Nanyang Lake,Dushan Lake,and the rivers in the west of Nanyang Lake have higher TP contents compared with Zhaoyang Lake,Weishan Lake and the rivers in the east of Nanyang Lake.Different phosphorus fractions contents varies greatly,and the phosphorus fractions of the sediments are IP＞Ca-P＞OP＞Ads-P＞Fe-P＞Al-P. Inorganic phosphorus(IP)contents are higher than organic phosphorus(OP),and calcium bound phosphorus(Ca-P)is the major phosphorus fraction.The concentrations of IP and Ca-P in the sediment of Sea-route are the largest,and Ads-P contents of Sea-route and Liangji Canal where are heavily affected by human activities are higher than other sampling sites.Iron bound phosphorus(Fe-P)contents are less than 17.72 mg·kg^-1except for Guangfu River.OP content of Liangji Canal that is an important part of Jinghang Canal is the highest of all sediments because Liangji Canal is a main channel of wastewater coming from Jining City and polluted badly.As a rule,OP content of river sediment is higher than it of lake sediment,but in the research OP contents of lake sediments are relative high.It is the main reason causing high concentrations of OP that a great deal of hydrophytes' debris enriched in the sediment of lake,after decomposition,leads to high content of organic matter.The relationships between TP content and Ca-P,Ads-P and OP contents are significant,and Ca-P and Ads-P have the most significant relationship.Among all inorganic phosphorus only Ads-P has a correlation with OP.In the course of phosphorus release,acid extractable phosphorus could be converted to inorganic phosphorus by microbe that is prone to be used by aquatic organism,and the conversion probably is a crucial part of eutrophication course.Besides the positive correlationships,Fe-P has negative correlationship with Ca-P,so there is mutual transformation between Fe-P and Ca-p in the sediment.The sediment of Guangfu River has the highest content of BAP,and the other sediments with high concentrations of BAP were obtained from the Hui River,Wanfu River,Sea-route,and Liangji Canal that were polluted heavily by municipal sewage, chemical fertilizer overusage,and wastewater of ships.The BAP of the sediments in the four lakes are Dushan Lake＞Weishan Lake＞Zhaoyang Lake＞Nanyang Lake.The high ratio of BAP to TP could bring more concerns with the sediments of Dushan Lake, Zhaoyang Lake and Weishan Lake.BAP could be considered as an important index to indicate contaminated degree of Nansi Lake sediment and to reflect the human influences on the lake.BAP and OP have significant relationship merely,so OP content could be cursorily considered as potential phosphorus that could be used by aquatic organism in the sediment.TP has an obvious relationship with Alox,MnO and has no evident relationship with the other physical chemistry parameters.The positive relationships between OP content and organic matter,CaO contents are found and OP content to a great extent lies on organic matter content.Fe-P greatly influences eutrophication degree of Nansi Lake and Fe-P has close relationship with PO₄^3-content in water.The low Fe-P content partly due to few amount of active iron in Nansi Lake sediment and most of iron are integrated with silicon.The majority of phosphate adsorptions on the samples have similar trends that are almost completed in 10 h and the sorption process reaches equilibrium after 48 h. Distribution of maximal amount of phosphorus adsorbed(Q_max)has a visible trend,that is to say Q_maxgradually decreases from Nanyang Lake to Weishan Lake.In a condition of the experiment,zero equilibrium phosphorus concentration(c_EPC)ranges from 0.010 to 0.157 mg·L^-1,and the sediments of Hui River,Fuxin River,Guangfu River,Dasha River,the four lakes will not desorb phosphorus and will not threaten the water at present.Ads-P has positive relationships with all characteristic parameters of phosphorus adsorption.Ads-P could be considered as an important index to indicate contaminated degree of Nansi Lake sediment and water,and characteristic parameters of phosphorus adsorption close associate with contaminated conditions,c_EPChas positive correlation with Ads-P,TP,IP and Ca-P.In the same condition,the sediment with higher trophic level desorbs phosphorus easily leading to aggravate trophic degree.Adsorption efficiency(m)has remarkable positive relationship with total maximal amount of phosphorus adsorbed(TQ_max).The m is the sum of adsorption efficiency of the sediments adsorbing the phosphorus coming from external and the phosphorus desorbed from the sediments.Native adsorbed phosphorus(NAP)has positive correlation with c_EPCand the trend is that the sediment with higher NAP easily desorbs phosphorus to the water,contrarily the sediment with lower NAP easily adsorbs phosphorus.Phosphorus release quantity of sediment at different trophic level in a acid condition or a alkali condition is larger than in a neutral condition.Fe-P/Ca-P could be considered as a index to indicate phosphorus release arousing by acidification or basification.The index of Nanyang Lake and Weishan Lake is 1.05 and 0.24.Therefore basification is prone to accelerate phosphorus release of Nanyang Lake sediment and Weishan Lake sediment is sensitive to acidification to release more phosphorus. Transformations of Fe-P,Al-P and Ca-P are different in dissimilar pH,as a result the effect of pH on phosphorus release is discrepant.