Simulation Study On The Effects Of Pools And Flow Rate On Solute Transport In Karst Conduits

Quantitative tracer tests can be a powerful tool for characterizing solute transport in karst conduits.They provide direct information about hydraulic connections inside a complex karstic network and give breakthrough curves(BTCs)form which solute transport parameters can be easily obtained.The correct interpretation of BTCs is the basis for understanding and predicting solute transport process in karst conduits.The classical advection-dispersion equation can not explain the appreciable tailing generally observed in field BTCs.Thus,study on the effect of various factors on BTCs is of great significance to correctly interpreting the BTCs.A pool may lead to the heavy tails of BTC,and the flow rate has a very complex impact on the transient storage and the BTC in karst conduits.Study on the effect of a pool and flow rate on solute transport in a karst conduit plays an important role in interpreting the field BTCs and predicting the solute transport process in the karst conduit.The Fluent software was used to simulate the physical process in karst conduits,preliminarily discussing the influence of a pool on the flow field and solute transport in the karst conduits.Three conduit structures were designed:a conduit,the conduit with a symmetrical pool and the conduit with an asymmetrical pool.The asymmetrical pool had a diagonally opposite entry/exit,whereas the symmetrical pool had a parallel opposite entry/exit located in the middle of the pool.The flow field simulations show that when the water flows through the pools,a part of water is trapped in the both sides of symmetrical pool,forming closed recirculation zone,and a part of water is trapped in the unclosed large eddies formed in the whole asymmetrical pool.The flow velocity in the asymmetrical pool is larger than within both sides of the symmetrical pool.The solute transport simulations show that the transient storage in the pool is translated to solute retention(BTC tailing).When the tracer plume passes through the pool,more solute is temporarily trapped in the asymmetrical pool than in the symmetrical pool,causing lower peak concentration for the asymmetrical pool.However,the solutes detained in the asymmetrical pool would be released into the main channel more rapidly,yielding shorter tails for the asymmetrical pool.After the main tracer plume passes through the conduit system,the concentration in immobile zone(pool)is much larger than in the mobile zone(main central conduit)for the symmetrical pool.Honever,the opposite conclusion is obtained for the asymmetrical pool.This may be related to that the flow field in the downstream reach of the asymmetrical pool is more complex.To further study the effect of the pool on solute transport in karst conduits,a pipe,the pipe with the symmetrical pool and the pipe with the asymmetrical pool were designed and a series of tracer experiments were conducted.The geometries for the two pools are the same as in Fluent simulation.The breakthrough curves(BTCs)are generated after instantaneous injections of NaCl tracer solution.In order to test the feasibility of reproducing the BTCs and study solute transport processes,three modeling approaches have been applied:the equilibrium model(EM),the linear graphical method(LGM)and the two-region nonequilibrium model(TRM).The investigation results show that the pools induce the appreciable tails of BTCs.The tailing of the BTC increases,and the peak decreases in magnitude,as the number of pools increases from zero to three.With the increase of pools,the peak time remains almost constant for the symmetrical pool while the peak arrives later obviously for the asymmetrical pool.The shape of BTC keeps almost constant when the symmetrical pool is located in different positions.The symmetrical pool yields longer tails and higher peak than the asymmetrical pool.But when there are three pools in the system,the peak concentration is higher for the asymmetrical pools than for the symmetrical pools.The parameters calculated using the Qtracer2 program show that more pools added in the pipe yields larger dispersion coefficient and longer mean tracer travel time.Also,the travel time variance and the viscous sublayer thickness increase with the increase of the pools.The two kinds of pools have almost the same mean tracer travel time and the same viscous sublayer thickness.The travel time variance for the symmetrical pool is larger than for the asymmetrical pool.The simulations reveal that the EM can reproduce the BTCs obtained using the single pipe,but it cannot fit the BTC tails caused by the presence of pools.The LGM gives good model fits to the BTCs obtained using the single pipe and the pipe with asymmetrical pools,but it is incapable of replicating the BTCs obtained using the pipe with symmetrical pools.However,the TRM is able to reproduce the BTCs from all of the examined cases.The TRM yields the best fits to experimental BTCs because the model can describe the transient storage in pools by the partition coefficient and the mass transfer coefficient.Fitted parameters using the TRM reveals that the dispersion coefficients and the mass transfer coefficients increase,and the partition coefficients decrease,with the increase of pools.The increased tailing occurs mainly from the decrease of partition coefficient,as the number of pools increases.Because the partition coefficients are virtually identical for the two types of pools and the mass transfer coefficient is obviously larger for the asymmetrical pool,the mass transfer coefficient is the major factor leading to different BTC shapes.The more appreciable BTC tails that occur for the symmetrical pools result mainly from the less intense mass exchange between water in the pools and water in the pipe.Several tracer experiments were carried out in a pipe under nine different flow rates in order to investigate the effect of flow rate variation on solute transport in the karst conduit.Three pipe structures were chosen:a single pipe,the pipe with the symmetrical pool and the pipe with the asymmetrical pool.The experimental results show the peak concentration increases and the tailing of BTC reduces with the increase of flow rate.In addition,the tracer duration time decreases and the width of BTC becomes narrowed.There is no clear trend in the dispersion coefficient(calculated using the Chatwin method)with increasing flow rate.By use of the experiment data,the relations between the quantitative characteristics and discharge were established.Then the value of a given characteristic can be predicted for a given discharge.In addition,we established the relations between other characteristics and the peak time.Other characteristcs can be predicted after the peak time has been predicted.The first method yields a better prediction effect.Three models,the advection-dispersion equation(ADE),the TRM and the transient storage model(TSM),were used to simulate the experimental BTCs and investigate the change characteristics of transport parameters with increasing flow rate.Simulations show that the ADE was capable of replicating the almost symmetrical BTCs of the single pipe but incapable of fitting the appreciable BTC tails for the pool-pipe system.Nevertheless,the TRM and TSM could reproduce all the BTCs for the pool-pipe system very well.In the TRM,a clear positive correlation with discharge emerges for the partition coefficient and the mass transfer coefficient.Moreover,there is a declining trend in the dimensionless mass transfer coefficient with increasing flow rate.The TSM displays a clear increase in main channel cross-sectional area and exchange coefficient with discharge.Additionally,the storage zone area,the dead zone ratio and the fraction of median travel time due to transient storage all exhibit clear negative correlations with the flow rate.As the flow rate increases,the variation of the parameters is more irregular for the asymmetrical pool than for the symmetrical pool.This is mainy related to more complex transient storage in the asymmetrical pool.The asymmetrical pool has a much greater exchange coefficient than the symmetrical pool.The hydraulic uptake length and the storage zone residence time is obviously shorter for the asymmetrical pool than for the symmetrical pool.These results may indicate that more tracer particles were temporarily trapped into the asymmetrical pool,and the solutes detained in the asymmetrical pool would be released back to the main channel more rapidly.The difference between the BTCs of the two pools is mainly related to this.Analysis of the relationship between the model parameters and the pipe structure and flow rate is very important for understanding the physical mechanism of parameters and predicting solute transport.