Why Does the Water Color in a Natural Pool Turn into Reddish-Brown “Pumpkin Soup”?

Donglin Li (), Mingyang Zhao, Qi Liu, Lizeng Duan, Huayu Li, Yun Zhang, Qingyan Gao, Haonan Zhang and Bofeng Qiu ()
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Donglin Li: Institute of Geography and Tourism, Qujing Normal University, Qujing 655011, China
Mingyang Zhao: Institute of Geography and Tourism, Qujing Normal University, Qujing 655011, China
Qi Liu: Institute for Ecological Research and Pollution Control of Plateau Lakes, School of Ecology and Environmental Science, Yunnan University, Kunming 650504, China
Lizeng Duan: Institute for Ecological Research and Pollution Control of Plateau Lakes, School of Ecology and Environmental Science, Yunnan University, Kunming 650504, China
Huayu Li: Institute for Ecological Research and Pollution Control of Plateau Lakes, School of Ecology and Environmental Science, Yunnan University, Kunming 650504, China
Yun Zhang: College of Life Science, Hubei Normal University, Huangshi 435002, China
Qingyan Gao: Institute of Geography and Tourism, Qujing Normal University, Qujing 655011, China
Haonan Zhang: Institute of Geography and Tourism, Qujing Normal University, Qujing 655011, China
Bofeng Qiu: Institute of Geography and Tourism, Qujing Normal University, Qujing 655011, China

Sustainability, 2025, vol. 17, issue 16, 1-17

Abstract: Inland aquatic ecosystems, encompassing lakes, reservoirs, and ponds, serve as vital repositories of water resources and provide essential ecological, social, and cultural services. Water color, a key indicator of water quality, reflects the complex interactions among physicochemical, biological, and environmental drivers. Heilong Pool (HP) in Southwest China, which consists of a Clear Pool (CP) and a Turbid Pool (TP), has recently exhibited an anomalous reddish-brown “pumpkin soup” phenomenon in the CP, while the TP remains unchanged. This unusual phenomenon has raised widespread public concern regarding water resource security and its potential association with geological disasters. To elucidate the ecological and geochemical mechanisms of this phenomenon, we employed a multifaceted analytical approach that included assessing nutrient elements, quantifying heavy metal concentrations, analyzing dissolved substances, characterizing algal community composition, and applying δD-δ 18 O isotope analytical models. Our findings illustrated that while Bacillariophyta predominate (>79.3% relative abundance) in the algal community of HP, they were not the primary determinant of water color changes. Instead, Fe(OH) 3 colloidal particles, originating from groundwater–surface water interactions and controlled by redox environment dynamics periodically, emerged as the principal factors of the reddish-brown discoloration. The genesis of the “pumpkin soup” water coloration was attributed to the precipitation-induced displacement of anoxic groundwater from confined karst conduits. Subsequent exfiltration and atmospheric exposure facilitate oxidative precipitation, forming authigenic rust-hued Fe(OH) 3 colloidal complexes. This study provides new insights into the geochemical and hydrological mechanisms underlying water color anomalies in karst-dominated catchments.

Keywords: water color change; karst landscape; geochemical characteristics; hydrological cycle; Heilong Pool (HP) (search for similar items in EconPapers)
JEL-codes: O13 Q Q0 Q2 Q3 Q5 Q56 (search for similar items in EconPapers)
Date: 2025
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