Required Tests for Black and Odorous Lake Water

When a lake turns black and emits a foul smell, it indicates severe organic pollution and depletion of dissolved oxygen. Under anaerobic conditions, organic matter decomposes to produce hydrogen sulfide, ammonia, mercaptans, and metal sulfides (e.g., ferrous sulfide, manganous sulfide), which cause the black color and stench. To assess the pollution level, identify causes, and design remediation strategies, a systematic set of tests must be conducted.

Sensory Assessment

Before any instrumental analysis, a simple sensory evaluation should be performed. Visual inspection records the water’s color—black often points to metal sulfides, while reddish or yellowish hues may indicate excess iron or manganese. Olfactory assessment identifies typical odors: rotten‑egg smell (H₂S), pungent ammonia, or musty notes. If both black color and malodor are obvious, the water body can be preliminarily classified as black and odorous.

Core Water Quality Indicators

Four parameters are essential for quantitative classification and routine monitoring:

Transparency (Secchi depth): Measured on site with a black‑and‑white disk. When transparency falls below 25 cm, the water is considered black and odorous; below 10 cm indicates severe blackening.

Dissolved oxygen (DO): Black‑odorous waters are typically hypoxic or anoxic. DO below 2 mg/L defines a black‑odorous state; below 0.2 mg/L is severe. On‑site electrode measurement is required because sample transport alters DO.

Oxidation‑reduction potential (ORP): A value below –200 mV signals a highly reduced environment dominated by anaerobic decomposition. Values between –200 and +50 mV indicate mild black‑odorous conditions.

Ammonia nitrogen (NH₃‑N): Elevated ammonia (>8 mg/L) is a key indicator of organic pollution; >15 mg/L corresponds to severe black‑odor.

Substances Responsible for Black Color and Odor

To identify the root cause, targeted analyses are necessary:

Sulfide (S²⁻): The main contributor to odor. Spectrophotometric methods (e.g., methylene blue) are commonly used.

Iron and manganese (Fe²⁺, Mn²⁺): Their reaction with sulfide forms black precipitates (FeS, MnS). Total and divalent iron/manganese should be measured.

Organic Pollution Load

Chemical oxygen demand (COD) and five‑day biochemical oxygen demand (BOD₅) indicate the total organic content. High values mean abundant organic matter that fuels anaerobic decomposition. Total phosphorus (TP) and total nitrogen (TN) are also necessary: they are the key drivers of eutrophication, which often coexists with black‑odor conditions.

Sediment acts as an internal pollution source. Even if external inputs are cut off, accumulated organic matter in sediment continues to release H₂S, NH₃, and Fe²⁺/Mn²⁺. Important tests on sediment include organic matter content, acid‑volatile sulfide (AVS), iron/manganese speciation, and redox potential. These data guide decisions on dredging or in‑situ remediation.