Investigation of Blackening Substances of “Black Water” Occurrence in the Reservoirs With <i>Eucalyptus</i> Plantations, Southern China

The frequent occurrence of "black water" in water source reservoirs with Eucalyptus plantations in Guangxi, China, during winter poses a risk to local drinking water safety. Knowledge of the key blackening substances and their molecular compositional characteristics responsible for reservoir water blackening is essential for black water remediation in drinking water reservoirs of Eucalyptus forest areas. This study utilized advanced techniques, including Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), to analyze the molecular composition of soluble organic compounds in both Eucalyptus leaf soaking solutions and water from Jinwo Reservoir and Tianbao Reservoir, which are representative of Eucalyptus forest areas. The findings indicate that the Eucalyptus leaf soaking solution is predominantly composed of aromatic polyphenols, including pyrogallol and gallic acid as parent compounds, along with their polymerized derivatives, among which ellagic acid (C₁₄H₆O₈) is the most typical compound. Ellagic acid concentrations are higher in winter reservoir water compared to summer, particularly in surface water versus bottom water. This suggests that the aromatic polyphenols released from fallen Eucalyptus leaves during autumn and winter are the primary substances responsible for the blackening of reservoirs, with ellagic acid being the key blackening substance. Gallic acid reacts with iron ions under neutral conditions to form a black solution without precipitation. The resulting complex intermediate was identified as [C₁₄H₈O₁₀Fe]^- using FT-ICR MS. This finding further confirms, at the molecular level, that aromatic polyphenols (e.g., ellagic acid and gallic acid) leached from Eucalyptus leaves can bind with iron ions to form black metal-organic complexes, thereby contributing to the blackening of reservoir water. The findings offer crucial insights for effective management and ensuring the safety of drinking water in such reservoirs.