Recharge processes on typical karst slopes implied by isotopic and hydrochemical indexes in Xiaoyan Cave, Guilin, China

Summary Measurements of the hydrogen and oxygen isotopic composition of precipitation and drip stream water at drip site XY5 in Xiaoyan Cave in Guilin City were conducted to investigate the infiltration and vadose flow processes, and to characterize precipitation–discharge relationship in the overlying karst hillslope area. A marked seasonal variability of oxygen and hydrogen isotopes was observed in precipitation ranging from −18.7‰ to −0.6‰ for δ 18 O and −139‰ to +5‰ for δD, respectively during the study period from November 2013 to October 2014. However, the narrow range observed for cave drip stream water (δD of −43‰ to −22‰, δ 18 O of −7.3‰ to −4.3‰) was attributed to homogenization via mixing of meteoric waters in the unsaturated zone above the cave. Although drip stream waters plot close to or slightly above the local meteoric water line, the weighted average δ 18 O value of precipitation is depleted than in dripwater, which is attributed to evaporation predominantly in the rainy season when cave air circulation is most active. The relationship between surface infiltration or leakage and cave discharge is connected by the distinct models of flow through the epikarst zone. Vadose flow showed high peaking hydraulic responses but buffered rain isotope responses with an attenuate magnitude of 5.0–9.4‰ for δ 18 O. The sudden hydrochemical proxy variability was related closely to increase of discharge and the flushing out of old water with higher dissolved calcite. A threshold of flow generation expressed by precipitation amount must be exceeded to create a peak in discharge, as a function of excessing the field capacity of soil and residual capacity of epikarst zone. The sources of vadose seepage are soil and epikarst reservoir water with the attenuation of the isotopic fluctuations (−4.28‰ to −5.92‰ for δ 18 O) and slight hydrological response, indicating mixed signals of infiltration water in epikarst storage reservoir. The total discharge volume of percolating water is 2084 L derived from piston flow due to the increased hydraulic head in the dry season, which provides a quantitative evidence of the potential adaptive capacity of the system to deal with the extreme droughts.