Research on the scale effects of solute transport in a bended karst conduit

Research on the scale effect of solute transport in karst conduits is very important for the correct interpretation of breakthrough curves (BTCs), but the scale effect of solute transport in a bended conduit has not been examined. In this paper, a karst conduit and a pool developed within the conduit are generalized into the transparent hose and cubic pool, respectively. Based on the previously established pool-pipe system, the bended pipes with different lengths are arranged downstream of the pool, and the tracer experiments are conducted to study the effect of the transport scale on the BTCs in conduits. We use the transient storage model to simulate the experimental curves. The results show that with the increasing pipe length downstream the pool, the peak concentration gradually decreases slowly, the rising slope of the BTCs does not change significantly, and the BTC tails gradually shorten, indicating that the longer transport distance exerts a larger effect on solute transport than the conduit bend. The coefficient of skewness (CSK), breakthrough time of posterior solute plume (t_re) and breakthrough time of solute plume (t_d) are well negatively correlated with the length of the pipe system (R²>0.96). When the length of the pipe system with the symmetrical or asymmetrical pool increases to 154.5 m and 164.3 m, respectively, the CSK is close to zero, and the BTC is nearly symmetrical. The dispersion coefficient (D), cross-sectional area of the storage zones (A_s) and exchange coefficient (α) have a good negative correlation with the length of the pipe system. When the length of the pipe system with the symmetrical or asymmetrical pool increases to 159.9 m and 178.1 m, respectively, the A_s is close to zero. Then, the solute retention caused by the pool basically disappears. The results have certain indications for the interpretation of the BTCs in field karst conduits.