Influence Of Suspended Sediment On Salinity Measurements And Its Correction

Salinity is a basic parameter of oceanography as well as coastal dynamics,which is mainly measured by electrical conductivity.However,suspended sediment affects conductivity and the conductivity-based salinity measurements.It is of great significance to study the influence of suspended sediment on the salinity measurements,which can improve the accuracy of salinity measurement and enrich the theory of coastal dynamics,etc.Therefore,the effects of suspended sediment on the salinity measurements are explored through laboratory experiments in many aspects,including suspended sediment concentration,sediment particle size and attachment of organic matter.Importantly,the mechanisms of these effects on salinity measurements are analysed.The main results are as follows:Firstly,the influence of suspended sediment concentration on conductivity and salinity measurements is studied by laboratory experiment.The results show that the suspended sediment affects the conductivity of saltwater,thereby affecting the conductivity-based salinity measurements.The measured salinities in turbid saltwater under different suspended sediment concentration(SSC)are all lower than those of clear saltwater.For a given reference salinity,the greater the suspended sediment concentration is,the larger the absolute values of the difference in salinity between turbid water and clear water are.For example,with the salinity concentration of 30 g/kg,the absolute values of the difference were 0.076 and 1.698 PSU under SSC of 5 and 70 g/L respectively.Furthermore,the measured salinity increases with increasing SSC and there is linear relationship between them.Interestingly,for a given SSC,the relative salinity differences in terms of conductivity were up to the maximum value of 6.31%with the initial salinity of 10 g/kg.Secondly,the influence of sediment grain size and adhesive organic matter on salinity measuements is surveyed.It can be concluded that for a given reference salinity and SSC,the finer the sediment particle is,the greater its influence on salinity measures is.The absolute values of the difference are 1.285,1.654 and 1.882 PSU for meadian particle sizes of 50 ?m,37 ?m and 23 ?m,respectively.The organic matter adhereing to particle surface can affect the physicochemical property of sediment particles.Therefore,a comparative experiment was carried out to present the influence of organic matter on salinity measurements:one includs organic matter and the other is free of organic matter.The results show that for a given SSC,suspended sediment with organic matter will result in slightly lower measured salinities than those without organic matter.Thirdly,the influence of SSC,sediment grain size and adhesive organic matter on adsorption is also investigated.The absolute values of salinity difference increase w:ith the increase of SSC.In addition,the adsorption effect appears to be enhanced for finer sediment particles,thus causing larger measured salinity deviation.The adsorption effect is enhanced due to organic matter.Above all,the salinity deviation caused by adsorption effect accounts for about 2.5%?10%in the total deviation value.Fourthly,based on the experiment mentioned above,the effects of suspended sediment particles on conductivity-based salinity measurements are analyzed from four aspects:blocking,occupying,adsorption and flocculation effect.Sediment particle blocks the electric ion movement,thus the blocking effect decreases the directional movement speed of electric ions as well as conductivity and measured salinirty.Occupying effect is caused by sediment particle which occupies the saltwater volume,thereby the ion number per unit volume decreases,electrical conductivity and measured salinity.What s more,sediment particles can adsorb the electric ions in the saltwater,which causes a decrease in active electric ion number in the measuring cell.Thus,adsorption effect decrease conductivity and measured salinity.Salt ions from the water are absorbed during flocculation.As the flocs are formed,the electric ions in closed water are unable to participate in directional movement.Therefore,flocculation effect can reduce conductivity and measured salinity.In a word,the directional movement speed and per unit number of electric ions which caused by the effects mentioned above are the main reasons of the suspended sediment particles reducing the conductivity-based salinity.Finally,based on the experimental data,a correction formula is developed using linear fitting method twice for correcting measured salinity affected by suspended sediment concentration.This formula has the advantages of high calculation precision,simple form and harmonious dimension,etc.In order to consider the sediment gradation or other influence factors,adjustable parameters are introduced to expand the application range of the formula.Then,the correction system of the suspended sediment effect on salinity measurement is developed with Python.