Phosphorus Load In Rich-phosphorus Sediments In Cascade Reservoirs Of The Huangbai River

The Huangbai River,with a large phosphorite deposit in the basin upstream and a very high level of phosphorus in sediments,is key drinking-water source of Yichang city,China.Since the water bloom breaking out in 2013,the water quality has aroused the concern of the community.Studies have shown that the release of internal phosphorus is considered to be the main factor causing poor quality of the water in rivers and lakes.Therefore,to maintain the stability of the water ecosystem and improve water quality in the basin,it is imperative that make a study of internal phosphorus load.In this paper,the sediments which is from the three cascade reservoirs in the upper reaches of the East Branch of the Huangbai River are used as the research object.The reservoirs are Xuanmiaoguan（XMG）,Tianfumiao（TFM）and Xibeikou（XBK）.Through the in-situ samplings and indoor analysis experiments,the basic properties of sediment like porosity,moisture and particle size,total organic carbon（TOC）and total nitrogen（TN）were measured to investigate the relationship between phosphorus characteristics and each factor.The contribution from phosphorus characteristics to water,and the phosphorus flux across the sediment-water surface were preliminarily discussed at 3 reservoirs,which provides reference for controling of internal phosphorus in the basin.The main understanding is as follows:（1）The sediments of the three reservoirs are mainly composed of silt,accounting for51.2%,followed by sand and clay,accounting for 42.6%and 6.2%respectively.There was a strong correlation between moisture（39.8～62.4%）,porosity（64.6～78.5%）and volume weight（0.569～0.938 g/cm³）.There is also a significant positive correlation between the median size and the average size,and the both sizes were dominated by D25 and D90.The overlying waters of the three reservoirs in the Basin are neutral or weak alkaline.In summer and autumn,the overlying waters of XMG and TFM is at an anaerobic state,which increases the release of phosphorus.（2）The results showed that the concentration of total phosphorus（TP）decreased from the upstream to downstream reservoirs of the basin and the mean of the TP were 8070.0,2681.2 and 2656.6 mg·kg^-1 respectively.It’s at a high-polluted level.The order of the phosphorus fractions was HCl-P>OP>Na OH-P for the most sites.The average proportion of HCl-P is 76%,the average proportion of OP is 15%,and the proportion of Na OH-P is only 9%.HCl-P is the main component of IP and TP,and there is a significant positive correlation between the three phosphorus fractions.The source of OP may be mainly derived from D90-sized pullutants.The correlation between TOC and TN,TOC and OP were also positive.（3）In the shorter time scale,the middle of TFM and the head of XMG are mainly based on the releasing state,while the rest of the sampling sites are retained.In the longer time scale,the heads of reservoirs and the rail of XMG are at the releasing state.In the case of fast release and slow release,the contribution of phosphorus fractions in sediments was dominated by HCl-P,followed by OP and Na OH-P.（4）The total phosphorus concentration in the interstitial water ranged from 0.09 to9.54 mg·L^-1,with the more than 10 times ratio of overlying water.To estimate the phosphorus release flux from the reservoirs,according to the Fick’s Law,it was found that all three reservoirs showed a“source”of phosphorus.The annual average daily phosphorus release fluxes of the XBK,TFM and XMG ranged from 0.2930 to 1.2617,0.1850 to1.6360 and 0.1133～1.1674 mg·m^-2d^-1 respectively.The average contribution of internal phosphorus from the three reservoirs to water was 20.8%,10%,and 6.4%.