Study On The Influence Of Pool Volume And Conduit Length On Solute Transport In Karst Conduit

The pool structure developed in karst conduit often leads to the tail of the field breakthrough curve,and the length of conduit also has a significant effect on breakthrough curve.The study on the effect of the pool and conduit length on solute transport in karst conduit is of great significance to the correct understanding of field curve and the prediction of solute transport in karst conduit.Although the influence of pool form on solute transport in karst conduit has been discussed in previous studies,the influence of pool volume and conduit length on solute transport in karst conduit is still lack of understanding.In this paper,the structure of karst conduits and pools in yield are generalized into PVC round pipes and pools.Pools of different sizes,numbers and conduit lengths are designed,and indoor tracer tests are carried out.The relationship between the overall shape of the breakthrough curve and the six characteristic parameters,such as the adjusted peak concentration,the mean transport time,the time variance,the penetration time of the descending solute plume,the penetration time of main solute plume,the tracer duration time and the volume,flow rate and conduit length were analyzed.At the same time,the experimental breakthrough curve is simulated by using the transient storage model,and the relationship between the dispersion coefficient,the cross-sectional area of the main channel,the cross-sectional area of the storage zone,the exchange coefficient and the volume of the storage zone and the volume,flow rate and conduit length is established.Finally,the geometric structure of karst conduit,such as cross-sectional area of conduit and pool volume,is predicted according to the relation.The experimental results show that as the pool volume increases,the BTC tailing increases and the peak concentration decreases with the increase of flow rate,the BTC tailing decreases,the peak concentration of the symmetrical pool increases first and then decreases,and the peak concentration of the asymmetric pool is basically unchanged.With the increase of conduit length,the tail of the curve becomes shorter and the peak concentration decreases slightly,which is closer to the normal distribution.Except that the penetration time of main solute plume in the symmetrical pool is basically constant with the increase of pool size and shows an exponential and rising relationship with the number of pools,the time characteristic parameters basically increase linearly with the pool volume,and the adjusted peak concentration displays a power function relationship with the pool volume.When the pool volume is the same,the time variance,the penetration time of the descending solute plume,and the tracer duration time for different pool sizes are larger than them for different number of pools,while the mean transport time is basically the same for the two pool distributions.The slope of the linear relationships between mean transport time,time variance and pool volume decrease gradually with the increased discharges.All time characteristic parameters show power function relationships with the discharge;the slope of the relationship between the time variance in the two pools,the penetration time of main solute plume in the asymmetric pool and the discharge changed obviously with the increased pool sizes.The mean transport time increases linearly with the increase of conduit length,while the time variance,the penetration time of the descending solute plume,and the tracer duration time decrease linearly with the increase of conduit length.The adjusted peak concentration basically does not change with the increase of conduit length,while the penetration time of main solute plume slightly increases with the increase of conduit length.The cross-sectional area of the storage zone or the calculated volume ofc the storage zone shows a good linear relationship with the pool volume indicating a potential to use this parameter(A_s=0.00766V+2.481E-6,R~2=0.972,V_s=0.8058V+1.923E-4,R~2=0.972),which is almost constant with the discharge,to estimate the actual pool volume.The maximum error of estimating the pool volume using the calibration parameter cross-sectional area of the storage zone was 23.3%.The cross-sectional area of the main channel does not change with the pool volume and changes little with the discharge;it has a close value to the actual cross-sectional area,so this parameter can be used to predict the cross-sectional area of the conduit.The law of dispersion coefficient is different in the two kinds of pool structures,and the law of exchange coefficient is different in the two kinds of pool distribution,and the two parameters gradually increase with the increase of the flow rate,showing a good positive correlation with the flow rate.The relationship between the dispersion coefficient and the exchange coefficient and the volume of the pool is obviously different under different pool structures,different pool distribution conditions and different flow conditions.Therefore,it is difficult to obtain a unified relationship formula,and it is difficult to directly deduce the internal conduit structure.The dispersion coefficient,the cross-sectional area of the storage zone and the exchange coefficient all decrease with the increase of the conduit length,showing a good linear relationship with the conduit length.The cross-sectional area of the main channel and the volume of the storage zone remain unchanged with the increase of the conduit length.In this paper,the relationship between the breakthrough curve and solute transport model parameters and the volume,flow rate and conduit length of pool is analyzed.It is of great significance to the quantitative analysis of the breakthrough curve of karst conduits in the field and the analysis of conduit structure,which provides an important basis for further prediction and control of groundwater pollution.