Water Body Features under Permanganate Index Exceedance

The permanganate index (COD-Mn) is a crucial indicator of organic pollution in water bodies. When its concentration exceeds the established environmental standards, it signifies a deterioration in water quality. 

This paper analyzes the physical, chemical, and ecological characteristics of water bodies where the permanganate index is elevated, providing a systematic overview of the consequent changes in water quality parameters and aquatic ecosystem structure.

1. Physical Characteristics

Water bodies with a significantly elevated permanganate index typically exhibit distinct physical alterations. The most apparent change is a reduction in water transparency. High concentrations of organic matter, both suspended and colloidal, scatter and absorb light, leading to a turbid or colored appearance. 

Depending on the source of organic pollution—such as humic acids from decaying plant matter or domestic sewage—the water may display a yellowish-brown, dark green, or greyish hue. Furthermore, the presence of organic films and increased particulate matter often results in visible surface foam or scum, particularly near discharge points or in stagnant areas.

2. Chemical Characteristics

From a chemical perspective, an elevated permanganate index is strongly correlated with several interrelated parameters. Primarily, it indicates a high load of readily oxidizable organic substances. The biodegradation of this organic matter by aerobic microorganisms leads to a significant depletion of dissolved oxygen (DO). In severe cases, hypoxic or anoxic conditions may develop, particularly in bottom waters or during stratified periods.

Under anoxic conditions, the biogeochemical cycles are altered. Sulfate-reducing bacteria become dominant, leading to the production of hydrogen sulfide (H₂S), which imparts a characteristic rotten-egg odor. Simultaneously, the redox potential of the water body decreases substantially. 

The reduction in oxygen also facilitates the release of reduced substances from sediments, such as ferrous iron (Fe²⁺) and manganese (Mn²⁺), further contributing to the chemical oxygen demand. Additionally, the decomposition of organic matter releases nutrients like nitrogen and phosphorus, exacerbating eutrophication and creating a positive feedback loop that sustains high levels of organic productivity and subsequent decomposition.

3. Ecological Characteristics

The ecological structure of a water body with a chronically high permanganate index undergoes a fundamental shift. The degradation of water quality favors pollution-tolerant species while eliminating sensitive ones. In aquatic macroinvertebrate communities, sensitive taxa such as Ephemeroptera (mayflies), Plecoptera (stoneflies), and Trichoptera (caddisflies) disappear, while tolerant taxa, particularly oligochaetes (aquatic worms) and certain chironomid larvae, proliferate.

In terms of phytoplankton, conditions often shift from a diverse community to one dominated by a few opportunistic, eutrophic-indicator species, such as cyanobacteria (blue-green algae). These organisms can form dense blooms, further increasing the permanganate index through the release of extracellular organic matter and, upon senescence, their own biomass. 

For fish populations, hypoxic conditions cause stress, leading to reduced growth rates, reproductive failure, and, in extreme cases, mass mortality events. Consequently, overall biodiversity and ecosystem complexity are markedly reduced.