Identifying solute loss from karst conduit to fissures under concentrated recharge conditions

The aquifer within karst areas exhibits significant heterogeneity, accompanied by intricate runoff processes and swift hydrological responses. The solute transport process within karst conduits is influenced by various factors, including karst development characteristics and variations in hydrodynamic conditions, thereby displaying a high level of complexity. This complexity plays a crucial role in governing the safe utilization of karst water resources and the prevention of groundwater pollution. In this paper, a typical karst conduit system in southern China is chosen to investigate the influence mechanism of hydrodynamic conditions on the solute transport process within the karst conduit. This investigation is conducted through multiple sets of field artificial tracer tests. An intriguing phenomenon was observed: When the mean flow velocity of the conduit flow is either low (<200 m/h) or high (>1000 m/h), the solute concentrations and recovery rates are low. Conversely, the solute concentration and recovery rate are highest at a medium flow velocity (560 m/h), with the recovery rates ranging from 0.13 % to 92.44 %. A theoretical formula has been derived to estimate the solute loss from conduit flow to fissure flow. When the conduit is filled with water, an increase in the mean flow velocity leads to an augmentation in the amount of water recharged from the conduit into the fissures, as well as an increase in the stored solute mass, which results in a decrease in the solute recovery rate. The condition that poses the highest risk of groundwater pollution occurs at a medium flow velocity, where the solute concentration and recovery rate are all at their maximum levels. When the recharge rate at the sinkhole is very small, both the cross-sectional area and the mean flow velocity of the karst conduit are reduced. The rough bottom of the karst conduit leads to an increase in dispersivity, resulting in low solute concentration and recovery rate. The results elucidate the reasons behind the significant variations in solute recovery rates under different hydrodynamic conditions. They provide novel evidence for comprehending the water and solute exchange processes between conduits and fissures under concentrated recharge conditions. Furthermore, these findings offer a valuable reference for assessing the risk of groundwater pollution in karst areas.