Drying increases organic colloidal mobilization in the karst vadose zone: Evidence from a 15‐year cave‐monitoring study

Abstract

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

The impact of seasonal and event‐based infiltration on transition metals (Cu, Ni, Co) in tropical cave drip water

“…We also note that we cannot exclude the interannual effects, for example, multiyear droughts, which may additionally modulate the relationship between precipitation and runoff, and, hence, potentially the transport of organic ligands. 26,66,67 Nevertheless, even though the attribution of a single event is challenging, the overall picture drawn by our results from the four drip sites suggests that drip water transition metals in Larga Cave respond not only to seasonal variations but may also have the potential to trace event-based high-flux infiltration.…”

“…While our record is too short to make similar inferences, we note that the length of the dry seasons during or directly before the monitored time interval was not significantly different; nor did the region experience pronounced water deficits as indicated by the estimated P-PET values (Figure 5), which is in line with the data from the U.S. Drought Monitor. 63 Given that Heshang Cave typically experiences larger dry season water deficits than Puerto Rico, we expect that the effect described by Liao et al (2021) 26 is of minor importance at Larga Cave.…”

“…18,19 This soil flushing increases the concentration of both dissolved organic matter (DOM) and the elements that show strong binding affinities not only to soilderived organic ligands, such as Y, Zn, and Pb, but also to the first-row transition metals Cu, Co, or Ni in the drip water. [20][21][22][23][24][25][26] In particular, Hartland et al (2012) 21 found that high flux of metals (Cu, Ni, Zn, Ti, Mn, Fe) and particulate natural organic matter (NOM) occurred in rapid, short-lived pulses following peak infiltration events, whereas low flux of metals (Co and V > Cu, Ni, and Ti) and nanoscale, colloidal NOM were offset from infiltration events. The proposed mechanism is that small organic colloids (1-100 nm) and solutes (< 1 nm) migrate more slowly through the porous soil and matrix porosity of karst rocks than particles, suggesting that changes in the ratios of trace metals (e.g., Cu/Ni and Cu/Co) in drip waters and speleothems may help archive information on NOM composition and infiltration events.…”

“…Recent work has shown that particulate and colloidal organic matter may be transported very fast from the soil to the cave in response to strong infiltration events 18,19 . This soil flushing increases the concentration of both dissolved organic matter (DOM) and the elements that show strong binding affinities not only to soil‐derived organic ligands, such as Y, Zn, and Pb, but also to the first‐row transition metals Cu, Co, or Ni in the drip water 20–26 . In particular, Hartland et al (2012) 21 found that high flux of metals (Cu, Ni, Zn, Ti, Mn, Fe) and particulate natural organic matter (NOM) occurred in rapid, short‐lived pulses following peak infiltration events, whereas low flux of metals (Co and V > Cu, Ni, and Ti) and nanoscale, colloidal NOM were offset from infiltration events.…”

“…However, the results of Baldini et al (2012)22 and Hartland et al (2012)21 support the interpretation of increased flux of organic complexes following increased infiltration. In a 15-year-long cave-monitoring study in Heshang Cave (China), Liao et al (2021)26…”