Phosphorus Forms In River Sediments And Phosphorus Loss Risk From Typical River Watersheds Of Chao Lake Basin

The contents and forms of phosphorus in river sediments can provide important information for source apportionment of phosphorus at the river watershed scale and assessment of its impact on downstream receiving waters.The phosphorus index(PI)model can be used to identify the critical source areas and assess the risk of phosphorus loss.Two typical tributaries of Chao Lake Basin,Fengle River and Zhegao River,were selected.Totally 27 sampling sites were set,with 14 at Fengle River and 13 at Zhegao River,where river sediments and river water were sampled in different seasons and under different hydrological conditions.Sediment samples were analyzed by SMT(Standards Measurements Testing)for different phosphorus forms,including total phosphorus(TP),inorganic phosphorus(IP),organic phosphorus(OP),iron-aluminum phosphorus(Fe/Al-P),calcium-phosphorus(Ca-P).Water samples were analyzed for TP and dissolved phosphorus(DP).Based on watershed division,a new PI model was constructed to calculate the phosphorus loss risk index,with the sediment phosphorus content,soil total phosphorus content,phosphorus fertilizer application rate and point source as source factors,and soil erosion,surface runoff,and shape coefficient of sub-watershed as transfer factors.The model was also applied to predict the phosphorus loss risk under an extreme rainfall scenario.The results showed that the average contents of TP,IP,OP,Fe/Al-P,and Ca-P in Fengle River sediments were 502.6,264.6,111.9,101.6,122.2 mg/kg and that in Zhegao River sediments were 1060.70,755.74,125.31,250.36,307.89 mg/kg,respectively.For both rivers,the average content of IP in sediment was higher than that of OP and Ca-P was higher than that of Fe/Al-P.The total contents of TP,IP,OP,Fe/Al-P,and Ca-P in Zhegao River sediments were higher than that in Fengle River.There was a significant spatial difference for different forms of phosphorus content in sediments for both rivers.Average TP concentrations of water in Fengle and Zhegao were 0.11 and 0.12 mg/L,respectively,which were mainly composed of DP and reached the level of eutrophication.No obvious seasonal variation of the contents of different forms of phosphorus in the sediments was observed for both rivers,except for IP,which was slightly higher in spring for both rivers as well.The content of TP in the sediments of Fengle River decreased slightly in the summer flood period compared with that of the non-flood period in summer,but a slight increase was found for the Zhegao River.No obvious variation was found for other forms of phosphorus in sediments sampled during the summer non-flood period and flood period.PI model simulation results showed that the PI values were lower in the upper reaches and higher in the middle and lower reaches of the Fengle River watershed,with 29% of the subwatersheds having PI values greater than or equal to 6,which was regarded as the critical source area of phosphorus loss.But 78% of the sub-watersheds of the Zhegao River watershed were counted as critical source areas.The high-risk area of phosphorus loss was related to many factors,including the content of soil phosphorus in source factors,the amount of phosphorus fertilizer applied,the shape coefficient of point source pollution factors,and transfer factors in sub-watersheds,and the amount of soil erosion.The validation results of the new PI model were consistent with the running results of the model.The PI values increased or decreased slightly in some sub-watersheds,still belonging to the same grade.The new PI model was proved reliable and practical in estimating and predicting the risk of phosphorus loss at the watershed scale.The simulation on the extreme rainfall scenario with 50 mm rainfall during 24 hours demonstrated that PI values of half of the sub-watersheds of Fengle River were upgraded by one grade,and more than half of the sub-watersheds of Zhegao River were upgraded by one grade indicating the obvious effect of climate change on the loss of non-point source phosphorus.Both Fengle and Zhegao were typical agricultural watersheds.Agricultural nonpoint sources inflow and the input of town sewage were the main contributions of phosphorus in river sediments and water.The crop and vegetable fields,towns,or villages of the Zhegao watershed are mostly located along or near the riverbanks,where the phosphorus from agricultural non-point sources,and that from domestic sewage in greater intensity,could be more easily transported to river channels.Zhegao had a much higher proportion of critical source areas than Fengle.Besides,the Zhegao watershed was closer to Chao Lake and might pose a greater risk to the eutrophication of Chao Lake.