Distribution Characteristics Of Phosphorus Speciation In Sediments In The Riparian Zone Of The Lanzhou Section Of The Yellow River And Its Ecological Risk Assessment

Riparian surface sediments are an important component of terrestrial and aquatic ecosystems and reservoirs of nutrients for the overlying water environment.Part of the phosphorus imported into the river is adsorbed in the sediment through a series of migration and transformation processes,and under the condition of controlling exogenous pollution,the phosphorus concentration in the water will be reduced,and the phosphorus in the sediment will be re-released into the overlying water,becoming the endogenous pollution source of river eutrophication.The occurrence and release risk of phosphorus in sediments have important implications for the stability and balance of aquatic-riparian ecosystems.This paper takes the surface sediment and overlying water of the Lanzhou section of the Yellow River as the research object,studies the physical and chemical characteristics of the overlying water near each sampling point,and analyzes the morphological distribution characteristics and temporal heterogeneity of phosphorus in the surface sediment of the riparian zone.The total phosphorus pollution index,phosphorus adsorption equilibrium constant（EPC₀）,phosphorus adsorption index（PSI）,phosphorus adsorption saturation（DPS）,and phosphorus release risk index（ERI）were used to evaluate the potential release risk of phosphorus from the surface sediment of the riparian zone of the Lanzhou section of the Yellow River.The conclusions of the study are as follows:（1）The p H of the water in the Lanzhou section of the Yellow River was 7.32～8.44,which belongs to the p H range of the natural water and shows weak alkalineity;The water conductivity,turbidity value,dissolved oxygen（DO）,ammonia nitrogen（NH₃-N）,permanganate index(COD_Mn),and total phosphorus（TP）concentration were all within the organic matter content range of natural water,which met the environmental quality standards of Class II surface water,and the water quality at each point was normal,and there was no special effect on the sediment at the sampling point.（2）The TP content of surface sediment in the surface sediment of the Lanzhou section of the Yellow River in the dry season was 1140.78～1345.06 mg/kg,and the TP content in the wet season was 1189.32～1453.16 mg/kg,and there was no significant difference between the monitoring points,and the spatial distribution of TP content was not much different.Inorganic phosphorus（IP）in sediments in dry season accounted for 84.57%～97.35%of TP,IP accounted for 79.20%～97.76%of TP in wet season,and phosphorus in surface sediments mainly existed in the form of IP.The content of organophosphorus（OP）in the sediments was relatively low,and there were some spatial differences in distribution.The overall distribution trends of phosphorus in various forms were calcium-bound phosphorus（Ca-P）>residual phosphorus（Res-P）>reduced phosphorus（Fe-P）,metal oxide-bound phosphorus（Al-P）>Weakly adsorbed phosphorus（NH₄Cl-P）.The content of NH₄Cl-P was low,and it did not show the regularity of change with the water period,and the NH₄Cl-P content in the upper and lower reaches of the Lanzhou section of the Yellow River in the dry period was slightly higher than that in the flood period,and the overall fluctuation was small.The Fe-P content was generally low,and the Al-P content in sediments in the water-rich period was higher than that in the dry period.The northern part belongs to calcareous soil,Ca-P is the main component of TP in sediment,and the Ca-P content of each monitoring point in the two water periods does not show the regularity of changes with the water period.The content of Res-P fluctuated between different sampling points,and there were certain spatial distribution differences.（3）The contents of NH₄Cl-P,Fe-P and Al-P at the back-end sampling point（4#）of Qilihe sewage treatment plant were higher than those at the front-end sampling point（3#）and other points of the sewage treatment plant during the two water periods,and the 4#sampling point was located in the tail water receiving section of the Qilihe Sewage Treatment Plant,and the tailwater discharge had a certain impact on the morphology and content of phosphorus in the sediment,which increased the content of easily released phosphorus and increased the risk of phosphorus release in the sediment.In the correlation analysis of dry season,TP was significantly correlated with Ca-P（p=0.05）,indicating that the TP content in the sediment was mainly derived from Ca-P.NH₄Cl-P was significantly correlated with Al-P（p=0.05）,and Fe-P and Res-P（p=0.01）.Al-P and Res-P（p=0.01）were significantly correlated,indicating that there was a certain mutual transformation relationship between these morphologies.In the correlation analysis of the wet season,Fe-P and Al-P（p=0.05）were significantly correlated,indicating that Fe-P and Al-P could be converted into each other under specific physicochemical conditions.（4）The TP pollution index of the surface sediment in the riparian zone of the Lanzhou section of the Yellow River was greater than 1 in the two water periods,indicating that TP in the sediment had the lowest ecological effect,that is,there was a low ecological risk.The proportion of bioavailable phosphorus（BAP）in TP content in the dry season was less than20%,and the risk of bioavailability of phosphorus released from sediments was small.The ratio of BAP to total phosphorus at some points in the wet season increased,the risk of phosphorus released from sediments for bioavailability increased,and the BAP content at each point was affected by human activities,with spatiotemporal differences,and there was no obvious spatiotemporal law.（5）The release risk of phosphorus in sediments in the study area was explored by different evaluation methods,and the results showed that phosphorus in the surface sediment of the Lanzhou section of the Yellow River had a certain release risk.The EPC₀of phosphorus in the surface sediment of the riparian zone was significantly higher than the average phosphorus concentration value of 0.02 mg/L,and much higher than the TP concentration of the water near the sampling point measured by the experiment,indicating that the sediment played the role of"phosphorus source"and had the risk of release.By phosphorus adsorption index（PSI）experiment,the PSI of sediments in the study area was less than 20（mg·P/100g）/（μmol/L）,indicating that the surface sediment of the riparian zone has poor buffering capacity for phosphorus,and the risk of phosphorus release into the water is high.Through experiments,the variation range of DPS in the dry season sediment was10.72%～17.95%,the average value was 14.78%,and the change range of phosphorus adsorption saturation（DPS）in the sediment in the water-rich period was 11.85%～16.82%,and the average value was 13.59%.The phosphorus release risk index（ERI）in sediment was obtained from the ratio of DPS to PSI,and the ERI of phosphorus in the surface sediment of the riparian zone of the Lanzhou section of the Yellow River was greater than 25,indicating that the phosphorus in the surface sediment of the riparian zone in the study area had a high release risk,and the W(P_Ex/Fe/Al)/W(P_Ca)of the surface sediment was less than 0.5,and the release intensity of phosphorus in the sediment in the acidification environment was large.