Analysis of the Three-Dimensional Fluorescence Spectroscopy Characteristics of Dissolved Organic Matter in Groundwater from a Subtropical Cave in Dry Season—Daxiao Cave in South China Karst

Abstract

Groups in dissolved organic matter (DOM) emit fluorescence information at characteristic wavelengths when irradiated by excitation waves, which can reveal the geochemical behavior of dissolved organic matter in the environment and its sources, but there are few relevant studies in cave groundwater systems. In order to investigate the relationship between drip hydrochemistry characteristics and DOM in cave systems after subsurface leakage, in this study, from the perspective of dissolved organic matter in the karst cave water system, the groundwater in the dry season of Daxiao Cave was selected as the research object. Five drip points and one water pool (DX-1, DX-2, DX-3, DX-4, DX-5, and DX-C) in Daxiao Cave were monitored and consecutively sampled for four months. The parallel factor analysis method (PARAFAC), three-dimensional (3D) fluorescence parameters, and excitation-emission matrix fluorescence spectroscopy (EEM), combined with the hydrochemistry characteristics of the drip water and correlation analysis, were used to analyze the 3D fluorescence spectral characteristics of the DOM of the drip water of Daxiao Cave and their influencing factors. The results show that (1) the hydrochemistry type of the drip water in Daxiao Cave was within the Ca–Mg–HCO3 type, and Ca2+, Mg2+, and HCO3− were the dominant ions in Daxiao Cave; (2) the fluorescence fractions of drip water in dry season caves were dominated by C1 (humus-like), C2 (tryptophan-like), and C3 (tyrosine-like), and the fluorescence fractions of drip water DOM were controlled by protein fluorophores; (3) the DOM in the drip water of Daxiao Cave in the dry season was controlled in part by subsurface leakage and was largely the result of microbial degradation; and (4) the DOM of the drip water may be influenced by the chemical composition of the water, but the exact process is not clear.

Characteristics and Influencing Factors of Dissolved Organic Matter in Cave Drip Water—A Case Study from Furong Cave, Southwest China

“…The average BIX value for the four monitoring sites in Furong Cave was 1.46 (Table S4), corresponding to BIX values (>1) associated with the input of newly formed autochthonous DOM into the water [64,65]. Similar to the average BIX value (1.01) observed in Daxiao Cave, indicating that the dominant source of dissolved organic matter (DOM) in cave drip water is endogenous organic matter [45], the fluorescence parameter characteristics in the drip water of Daxiao Cave are similar to those of Heshang Cave (HIX = 0.73; BIX = 1.24) [22]. The HIX values of the DOM at the four monitoring sites were higher in summer compared to other months (Figure 3h), indicating a higher degree of humification for the DOM during the period June to August.…”

“…In the case of Furong Cave, the fluorescence parameters in the drip water predominantly originate from endogenous sources, particularly during the dry season [85]. As precipitation decreases, the microbial activity in the overlying soil correspondingly increases, leading to an elevated production of DOM in the soil, which serves as a source for DOM in drip water [45]. Additionally, the increased residence time of the infiltrating water in the overlying aquifer allows for the transportation of organic matter generated through microbial decomposition processes.…”

“…Previous studies have been conducted on rainwater DOM in karst and non-karst areas [3,43,44], as well as the DOM in drip water [3,27,45,46]. However, the transport mechanism of DOM is especially intricate due to the influence of hydrogeological processes on its composition [45].…”

“…Previous studies have been conducted on rainwater DOM in karst and non-karst areas [3,43,44], as well as the DOM in drip water [3,27,45,46]. However, the transport mechanism of DOM is especially intricate due to the influence of hydrogeological processes on its composition [45]. Research in karst regions on DOM and hydrochemical characteristics primarily focuses on the underground rivers in karst areas and the soil-karst groundwater systems that are sensitive to infiltration events [47].…”

Spatial Distribution and Sources of Dissolved Organic Matter in High-Antimony Shallow Groundwater in the Xikuangshan Mine, Hunan, China

“…The HIX is used mainly to characterize the degree of humification of humic-like substances produced by terrestrial DOM sources. When HIX >16, it indicates a strong humic character and strong contribution from terrestrial sources; when HIX<4, DOM was characterized as having a biological or aquatic bacterial origin; when 4<HIX<6, it had a weak humic character; and 6<HIX<10 indicates an important humic character [45][46][47][48][49][50].…”