Accumulation, conversion and storage of solute from sinkholes to karst spring under concentrated recharge conditions

Abstract

Concentrated recharge processes can define water quantity and quality in karst aquifer systems. Hydrological and hydrochemical dynamics at Qinglongkou (QLK) spring and two sinkholes were synchronously monitored to elucidate solute migration processes under concentrated recharge conditions. The results show that the hydrological and hydrochemical processes of the sinkholes and karst spring have corresponding impulse responses after rainfall events. The components of the leachate including Ca2+, Mg2+, Sr2+ and HCO3− in QLK spring decrease due to dilution during the periods of concentrated recharge, but their fluxes increase by 4% to 271% compared to the sinkholes input owing to the continuous water–rock interactions. Most chemical indicators are diluted along the hydrograph peaks, but the pollutants including nitrogen and phosphorus are aggregated to higher values after rainstorms, both at sinkholes and spring outlet, with total inorganic nitrogen and PO43− concentrations up to 11.34 mg/L and 1.16 mg/L. During heavy rainfall events, the nutrients from agricultural activities are easily dissolved into overland flow, and nitrification mainly controls the nitrogen conversion in karst aquifer system. Pollutant fluxes of karst spring are lower than that of sinkholes due to solute storage, and pollutant concentrations are 2∼3 times higher than their natural background values in spring outlet after rainstorms. Accumulation and storage of pollutants are problems that cannot be underestimated for the clean utilization of karst water resources and the prevention, control, and repair of karst groundwater pollution, which pose severe threats to the karst water quality.