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Drying increases organic colloidal mobilization in the karst vadose zone: Evidence from a 15‐year cave‐monitoring study
Author(s) -
Liao Jin,
Hu Chaoyong,
Li Xiuli,
Ruan Jiaoyang
Publication year - 2021
Publication title -
hydrological processes
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.222
H-Index - 161
eISSN - 1099-1085
pISSN - 0885-6087
DOI - 10.1002/hyp.14163
Subject(s) - vadose zone , karst , geology , hydrology (agriculture) , groundwater , cave , environmental science , soil science , soil water , ecology , paleontology , geotechnical engineering , biology
Understanding the hydrological processes of colloids within the karst vadose zone is vital to the security of karst groundwater and providing appropriate paleohydrological explanations of colloid‐facilitated metals in speleothem. This study addresses the mobilization mechanisms driving colloidal organic matter (COM) transport in the karst vadose zone using a 15‐year long monthly monitoring dataset from a cave drip point (HS4) in Heshang Cave, Qingjiang Valley, China. Variations in COM concentrations were reported as the fluorescence difference values of raw and filtered (<0.22 μm) samples at an excitation wavelength of 320 nm and emission wavelength of ~400 nm. A fluorescence humification index (HIX) lower than 0.8 and an autochthonous index (BIX) higher than 1.2 indicated that the origin of COM was mainly from the karst vadose zone, rather than the soil zone. The COM concentration varied from 0.001 to 0.038 Raman Unit (RU), with evident seasonal fluctuations. Rising limbs for COM values occurred prior to rising limbs within a dripwater hydrograph; moreover, the COM peak values corresponding to the beginning of the increasing hydrograph generally suggested that the mobilization of COM reflected the movement of the air–water interface (AWI) in the karst vadose zone rather than rainfall intensity or flow velocity. COM peak values were positively correlated with the antecedent drying duration and negatively correlated with HIX values. These phenomena may be explained by the increased amount of organic matter that was aggregated and absorbed on the surface of carbonate in the karst vadose zone during a longer drying duration. Moreover, the longer drying duration was also beneficial to autochthonous biological activity, which subsequently decreased the HIX value of the organic matter in the karst vadose zone. The movement of AWI and the drying duration are both controlled by the outside weather conditions. This study is therefore conducive to evaluating the security of karst groundwater in response to climate change, and challenges prevailing paleoclimate interpretations of colloid‐facilitated metal abundance timeseries reported from speleothems.