Storage and release of conservative solute between karst conduit and fissures using a laboratory analog

The contaminant transport processes in karst water systems have a direct impact on the quality and utilization of karst water resources. The storage and release of contaminants or conservative solutes during the solute transport process is a common phenomenon in karst aquifers. The impact of the storage and release is more prevalent after focused recharge events. In this study, laboratory experimental and numerical studies were conducted to quantify the storage and release processes of conservative solute. The results showed that, conduit water recharges into fissures under high water head conditions, and the fissure water drains back into the conduit while the hydraulic gradient reverse. The conservative solute storage-release process controlled by hydrodynamic conditions produces strong asymmetry, long tailing, or bi-peak in the breakthrough curves (BTCs). The BTCs change from single peak to bi-peak with enhanced hydrodynamics under focused recharge conditions. The dual heterogeneous domain model was calibrated to simulate the long-tailing of the BTCs and their noticeably bimodal characteristics. The flow velocity and dispersion coefficient are the major variables that regulate the bimodal structure of the BTCs, which also control the solute storage-release differences between the conduit and fissures. The bimodal structure of the BTCs becomes more pronounced for large discrepancies in flow velocities. The total BTCs are a superposition of solute transport in the conduit path and storage-release path. A method to evaluate the mass of conservative solute transport in storage-release paths was proposed by segmenting the transport curve in the conduit from the total BTCs, thus quantifying the effects of the groundwater storage-release mechanism on the solute transport process in the karst water system.