Spatio-temporal Distribution Of Nitrogen And Phosphorus And Its Geochemical Modeling In Poyang Lake Catchments

Lake pollution, induced by human activities with the development of society andeconomy, has been a serious environmental problem, of which the eutrophication isone of the most important and harmful environmental problem. Eutrophication isdefined as the enrichment of surface waters with nutrient. Of these nutrients nitrogen(N) and phosphorus (P) are important influence factors for primary productivity inaquatic ecosystems. In order to prevent and control the eutrophication effectively, thesources and spatio-temporal distribution of the nitrogen and phosphorus in lakeshould be understood comprehensively.Poyang Lake catchments situated at the south of Yangtze River midstream consistof five rivers (including Ganjiang River, Fuhe River, Xiushui River, Raohe River andXinjiang River) and Poyang Lake. The water chemistry and spatio-temporaldistribution of the nitrogen (N) and phosphorus (P) were studied in Poyang Lake andthe five rivers, and determined the N and P concentrations in lake water, the mainriver water, and some agricultural drainage, groundwater and urban sewage at thecircumference of Poyang Lake. And the sources of the N and P in the Poyang Lakewere discussed primarily. The hydrogen and oxygen isotope were measured in thelake water, river water, groundwater and rainwater. The chemical species and thesaturation indices for minerals were calculated by PHREEQC. The main conclusionshave been attained as follows.The type of water chemistry in Poyang Lake, Ganjiang River, Fuhe River, XiushuiRiver and Changjiang River is [C]Na,CaI at high water period, but in Lean River andXinjing River is [C]NaI. At average water period, the type of water chemistry inPoyang Lake and other rivers is [C]NaI, except that of Xinjiang is [C]Na,Cal. Andthat of Poyang Lake and fiver rivers is [C]NaI at low water period. According to theaverage concentration of the ion at one year, the type of water chemistry in PoyangLake is [C]NaI, different from the result that the type of water chemistry in PoyangLake is [C]CaI at year 1988. This indicates that the Poyang Lake water wasinfluenced by the human action increasingly. The water is influenced by rock weathering mainly. The relationship betweenδD andδ¹⁸O for the lake water and riverwater indicates riverwater is the dominant water source of Poyang Lake.The slope oftheδD-δ¹⁸O relation is less than that of the Craig Line. That is the result of localevaporation. The heavier hydrogen isotopes occurring in groundwater may be theresult of mixing with other types of water (e.g. soil water).The concentration of various form N and P is the lowest in Xiushui River,following is in the Fuhe River. The concentration of NO₃^--N is the highest inGanjiang River. The concentration of NH₄⁺-N is the highest in Changjiang River andLean River. The concentration of TN and TP is the highest in Xinjiang River. Theconcentration of NO₃^--N, NH₄⁺-N, TN and TP in Poyang Lake is higher than theresult of that in 1988. The concentration of TN and TP is higher than theconcentration, at which it is occurred eutrophication, and this indicaes that the PoyangLake is polluted by the N and P aggravatly and the eutrophication will appear whenthe condition is meeted. Poyang Lake is at middle trophic state at whole year, and thestate is different at different time. It is at middle and eutrophic state in high waterperiod, while at eutrophic state at average and low water period.According to the turn from the low water period to the average water period to thehigh water period, the concentration of the various forms of N and P is declining inPoyang Lake, but the concentration of NO₃^--N and NH₄⁺-N is ascending in GanjiangRiver. The concentration of NO₃^--N in the other rivers is the lowest at average waterperiod, and is the highest at low water period. The concentration of NH₄⁺-N is thelowest at average water period, and is the highest at the low water period in FuheRiver, but it doesn't present significant temporal characteristic in the other river. Ataverage period, the concentration of TN is the highest in Changjiang River andXinjiang River, and is the lowest in Ganjiang and Fuhe River. The concentration ofTP presents no significant temporal characteristic in Fuhe River, Xinjiang Rive andLean River. The concentration of TP of Xiushui River is the lowest at average waterperiod, and that of Changjiang River is the lowest at low water period, but that ofGanjiang River is the highest at high water period.At low water period, the concentration of the various forms of N of other rivers isthe highest at downstream, but that is the highest at upstream in Xiushui River. The concentration of TP in five rivers water is the highest at downstream, and is thelowest at midstream in Ganjiang River, Fuhe River and Xinjiang River. At averagewater period, the concentration of the various forms of N and P is increasing with thewater flow in Changjiang River and Lean River. The concentration of NO₃^--N islowest at midstream in Fuhe River, Xiushui River and Xinjiang River, but increasingwith the water flow in Ganjiang River. The concentration of NH₄⁺-N is increasingfrom upstream to downstream in Changjiang River and Lean River, but it presents nosignificant spatial characteristic in other rivers. The concentration of TN is the highestat downstream in other rivers, except at upstream in Xiushui River. The concentrationof TP is the highest at downstream in other rivers but Ganjiang River. At high waterperiod, it presents no significant spatial characteristic in all rivers in the N and P'sconcentrations. The concentrations of N and P are higher in surface water than inbottom water due to the degradation of sedimentary organic matters and disturbance.NO₃^--N dominated in the forms of N in Poyang Lake, most of those came fromGanjiang River water. The agricultural drainage, groundwater and urban sewagewhich contain high concentration of N and P is the main sources of the N and P inPoyang Lake and the five rivers.With PHREEQC, chemical species were calculated for surface water in PoyangLake catchments. The results shows the contents of major complexes are influencedby TDS. The contents of Ca²⁺, Mg²⁺ decrease but the contents of CaSO₄⁰,MgSO₄⁰increase with the increasing of the contents of SO₄^2-. The major species of P is HPO₄^2-and H₂PO₄^-; their contents are correlated with pH significantly. The major species ofN is NO₃^-,NH₄⁺,NH₃,NO₂^-, the result of calculation is similar with that ofexperiment.The calculation result of the mineral saturation indices indiating thatgoethite and hematite is saturated in most of sampling sites, they will deposit. But thecalcite,dolomite, gypsum and halite etc are non-saturated, they will soluble.