Effects Of Dam On The Heavy Metal Distribution In Sediment In A Small Scale River

Natural rivers are considered to have a balanced state in terms of sediments and water.However,when rivers are substantially influenced by construction of dams,this balance can be dramatically changed.The alteration of natural flow regime has led to changes of upstream hydrological conditions.Mainly,dam construction on rivers reduces flow velocity and thus enhances sedimentation.Changes in the amount and composition of sediments,including nutrients and heavy metals,can have an impact on the river ecosystems.Heavy metals are the most persistent pollutant in water ecosystems because of their slow decomposition in natural conditions.Among a variety of contaminants found in aquatic environments,metal species such as copper,cadmium and lead have attracted the attention of scientists around the world in recent decades.Overall,it is necessary to study the effects of dam on heavy metal distribution.In this paper,we used a two-step approach to understand the influence of dam on sediment deposition and heavy metal distribution.The first step is to evaluate the effects of dam construction on sedimentary characteristics based on the changes of hydrological conditions using one-way ANOVA.The second step applies redundancy analysis to examine the relationship between heavy metals(As,Cr,Pb,Cd,Cu,Hg,Zn)and the physical and chemical properties of sediments,including grain size distribution and organic carbon content.The Suo River,a tributary of Huai River,was selected as the study area,which was divided into three parts,including the upper,middle,and lower part,according to the direction of the flow and the position of dam.The hydrological conditions present marked discrepancy:the upper part regularly shows a state of setting off.The middle part have small water flow,whereas,the lower part always contains water.Accordingly,the reservoir-dam construction affected the characteristic and distribution of sediments.In studies investigating grain size distribution,the analyzed physical(grain size)and chemical(organic matter)properties of the deposited sediments showed significant differences between the three parts of the study area.It was determined that silt and clay depositions are higher in middle part,compared to the abundant that is recorded in the upper and lower parts.While the sand deposition is the lowest.The reason might be that the middle part is formed into stable sedimentary environment due to the rapid declines of flow velocity and deposits with finer grain-size are more likely to deposit in the middle part;the silt has the largest proportion of contents in all samples,while the percentage of the clay is smallest.Particularly,reservoir-dam construction led to the priority of tiny grain of material in the middle part.However,researchers have found out that the organic matter can also deposit in the middle part depending on the flow regime and flow rate of the river.One-way ANOVA analysis showed that the majority of heavy metals are higher in middle part except Cr and Cu.Analyze the sediment and heavy metals in both surface layer and subsurface of sediments,the results showed that the contents of clay silt sand and organic matter are almost the same.However,a slight increase in the levels of As,Hg,Cr,Cu,Zn,Pb and Cd in surface layer indicates that heavy metal pollution was exacerbated.Redundancy analysis shows that the particle size plays a significant role in controlling heavy metal concentrations in sediments whereby the concentrations tend to increase with declining particle size.The tiny grain of deposition and the organic carbon are closely and positively related to heavy metals accumulation,such as As,Hg,Cd and Pb.As a consequence,reservoir-dam construction can increase the risk level of heavy metal pollution by changing the hydrological conditions.