Black and odorous water bodies in urban rivers are a visible symptom of oxygen depletion caused by excessive pollutant loading from domestic sewage, industrial effluent, stormwater runoff, and contaminated sediment. A rigorous source analysis program, combining standardized water and sediment testing with advanced tracing techniques such as stable isotopes and microbial source tracking, is essential for designing effective and durable remediation strategies. This article explains the formation mechanisms, sampling plans, analytical parameters, testing methods, and practical applications of source apportionment for black and odorous river water.
Technical Overview
A black and odorous water body is an urban river, channel, or pond that exhibits abnormal dark coloration and offensive smell as a result of pollutant inputs exceeding the water body's self-purification capacity. It is a typical manifestation of severe organic and nutrient pollution and is classified as either mild or severe based on standardized indicators.
Formation Mechanism
When biodegradable organic matter and nutrients accumulate faster than the dissolved oxygen supply can be replenished, the water column and sediment-water interface become anoxic or anaerobic. Under these conditions, sulfate-reducing bacteria convert sulfate to sulfide, which reacts with iron(II) and manganese to form iron sulfide and other dark metal sulfide particles that blacken the water and sediment. At the same time, anaerobic decomposition of nitrogenous and sulfur-containing organic matter releases hydrogen sulfide, methanethiol, dimethyl sulfide, dimethyl disulfide, ammonia, and volatile amines, which are responsible for the characteristic offensive odor.
Identification Criteria
According to the national technical guide for urban black and odorous water remediation, four indicators are used for on-site identification. Mild black-odorous conditions are indicated by transparency of 10 to 25 cm, dissolved oxygen of 0.5 to 2 mg/L, oxidation-reduction potential between -200 mV and 50 mV, and ammonia nitrogen of 8 to 15 mg/L. Severe conditions are indicated by transparency below 10 cm, dissolved oxygen below 0.5 mg/L, oxidation-reduction potential below -200 mV, and ammonia nitrogen above 15 mg/L.
Classification of Pollution Sources
- External point sources: direct discharge of untreated or partially treated domestic sewage, illicit sewer connections and misconnections, combined sewer overflows, pumping station discharges, and industrial wastewater from food processing, printing and dyeing, papermaking, chemical, and electroplating facilities.
- Non-point (diffuse) sources: urban rainfall runoff carrying street deposits and particulate pollutants, agricultural runoff containing fertilizers and pesticides, and scattered rural domestic pollution.
- Internal sources: release of nitrogen, phosphorus, iron, and sulfide from contaminated sediment under anaerobic conditions, sediment resuspension, and decomposition of accumulated algae and aquatic plant residues.
- Other direct inputs: dumping of solid waste along riverbanks, wastewater from restaurants and markets, aquaculture effluent, and vessel discharges.
Testing Samples
A scientifically designed sampling program is the foundation of reliable source apportionment. Sampling locations and frequency must capture spatial gradients, dry and wet weather conditions, and all suspected source categories.
River Water Column Samples
Water samples are collected at multiple cross-sections along the river, covering upstream reference points, pollution-affected reaches, confluences, stagnant zones, and the downstream outlet. Vertical profiles at surface, middle, and bottom depths are obtained where stratification is suspected. Samples are stored in precleaned polyethylene or glass containers and preserved in accordance with ISO 5667-3 and national sampling specifications such as HJ 91.2.
Sediment and Pore Water Samples
Surficial sediment (typically 0 to 30 cm) is collected using gravity corers or grab samplers to evaluate internal loading. Pore water is extracted by centrifugation or in-situ porous samplers to measure the diffusive flux of ammonia, phosphate, ferrous iron, and sulfide across the sediment-water interface.
Outfall and Source Discharge Samples
A census of all outfalls is performed during both dry and wet weather. Representative samples are taken from sewage pumping stations, combined sewer overflows, industrial discharges, stormwater outlets, and livestock farms. These samples establish the chemical fingerprint library required for receptor modeling and mass balance calculations.
Biological and Biofilm Samples
Water and biofilm samples are collected for chlorophyll-a, algal density, fecal indicator bacteria, and microbial community analysis, which support trophic status assessment and microbial source tracking.
Testing Items
The analytical parameter suite is organized to quantify blackening and odor formation, evaluate pollution severity, and discriminate among candidate sources.
Conventional Physical and In-Situ Indicators
- Color, chromaticity, and transparency
- Water temperature, pH, and conductivity
- Dissolved oxygen (DO) and oxidation-reduction potential (ORP)
- Suspended solids (SS) and turbidity
Organic Load and Nutrient Indicators
- Chemical oxygen demand (COD) and biochemical oxygen demand (BOD5)
- Total organic carbon (TOC) and dissolved organic carbon (DOC)
- Ammonia nitrogen (NH3-N), nitrate, nitrite, and total nitrogen (TN)
- Total phosphorus (TP) and orthophosphate
Blackening and Odor-Related Indicators
- Total sulfide and dissolved sulfide
- Total iron, ferrous iron, and manganese
- Hydrogen sulfide and volatile organic sulfur compounds, including methanethiol, dimethyl sulfide, and dimethyl disulfide
- Ammonia and volatile amines
Source Tracing Indicators
- Stable isotopes, including delta-15N and delta-18O of nitrate and delta-13C of dissolved inorganic carbon, for distinguishing sewage, fertilizer, atmospheric, and soil nitrogen sources
- Anthropogenic tracers of domestic sewage, such as acesulfame-K, sucralose, caffeine, and selected pharmaceutical residues
- Anionic surfactants (linear alkylbenzene sulfonates) as laundry detergent markers
- Heavy metal profiles for industrial source discrimination
- Petroleum hydrocarbons and fluorescent dissolved organic matter signatures
Biological and Microbial Indicators
- Chlorophyll-a and phytoplankton density
- Fecal coliform, Escherichia coli, and enterococci
- Microbial community composition by 16S rRNA gene sequencing
- Human-associated and livestock-associated genetic markers quantified by qPCR for microbial source tracking
Sediment Indicators
- Moisture content, loss on ignition, and total organic matter
- Acid volatile sulfide (AVS) and simultaneously extracted metals (SEM)
- Total nitrogen, total phosphorus, and heavy metals
- Particle size distribution and sediment oxygen demand
Testing Methods
Field and In-Situ Testing
Dissolved oxygen is measured on site using the electrochemical or optical probe method in accordance with GB 11913 or equivalent portable instrument procedures. ORP is measured with a portable platinum redox electrode following HJ 506. Transparency is determined with a Secchi disk, and multiparameter sondes are deployed for continuous monitoring of diurnal oxygen variation, which reveals photosynthesis-respiration imbalance.
Laboratory Chemical Analysis
- COD by the dichromate reflux titration method (HJ 828) or rapid digestion spectrophotometry (HJ/T 399)
- BOD5 by the dilution and inoculation method (HJ 505)
- Ammonia nitrogen by Nessler's reagent (HJ 535), salicylate spectrophotometry (HJ 536), or distillation-titration (HJ 537)
- Total nitrogen by alkaline potassium persulfate digestion with UV detection (HJ 636)
- Total phosphorus by ammonium molybdate spectrophotometry (GB 11893)
- Sulfide by the methylene blue spectrophotometric method (GB/T 16489)
- Heavy metals by ICP-MS or ICP-OES after acid digestion (HJ 700, HJ 766)
- Anionic surfactants by the methylene blue active substances method (GB 7494)
- Fecal coliform by the enzyme substrate or membrane filtration method (HJ 347.1, HJ 347.2)
- Chlorophyll-a by spectrophotometry (HJ 897)
- Nitrate isotopes by isotope ratio mass spectrometry using the bacterial denitrification preparation technique
Source Apportionment Techniques
- Pollution load inventory and mass balance: quantifying contributions from domestic sewage, industrial discharge, surface runoff, and sediment release per unit river length.
- Wet/dry weather comparative monitoring: separating continuous sewage inputs from rainfall-driven runoff and combined sewer overflow events.
- Stable isotope mixing models: Bayesian isotope mixing approaches apportion nitrate and carbon sources among sewage, fertilizer, and soil end-members.
- Receptor modeling: chemical mass balance and positive matrix factorization applied to the fingerprint library of water quality and tracer data.
- Microbial source tracking: qPCR quantification of host-associated Bacteroidales markers to distinguish human, poultry, swine, and ruminant fecal inputs.
- Coupled hydrodynamic and water quality modeling: simulating pollutant transport and degradation to validate apportionment results under different hydrological scenarios.
Applications
- Development of one-river-one-policy remediation plans with quantified, ranked source contributions
- Outfall investigation, rectification prioritization, and sewage interception network design
- Determination of dredging scope, depth, and urgency based on internal sediment loading
- Selection of in-situ treatment measures such as aeration, ecological floating beds, constructed wetlands, and dosing systems
- Post-remediation effectiveness evaluation against transparency, DO, ORP, and ammonia nitrogen thresholds
- Compliance demonstration for water function zones under GB 3838 surface water quality standards
- Support for total pollutant load control, water environment capacity calculation, and long-term digital monitoring platforms
FAQ
What are the quantitative criteria for identifying a black and odorous water body?
The four core indicators are transparency, dissolved oxygen, oxidation-reduction potential, and ammonia nitrogen. A water body is generally classified as black and odorous when transparency falls below 25 cm, dissolved oxygen falls below 2 mg/L, ORP falls below 50 mV, and ammonia nitrogen exceeds 8 mg/L, with stricter thresholds defining the severe category.
Why must source analysis precede remediation engineering?
Remediation measures are only effective when matched to the dominant pollution pathway. If external inputs are not identified and intercepted first, dredging or aeration alone will produce only temporary improvement because pollutants will continue to enter the channel and regenerate black-odorous conditions.
How are domestic sewage and industrial wastewater contributions distinguished?
A combination of indicators is used. Artificial sweeteners, caffeine, anionic surfactants, and human-associated microbial markers indicate sewage inputs, while characteristic heavy metal profiles, petroleum hydrocarbons, and specific organic compounds indicate industrial discharge. Receptor modeling integrates these fingerprints into quantitative source contributions.
What role does contaminated sediment play as an internal source?
Under anaerobic conditions, sediment releases ammonia, phosphate, dissolved iron, and sulfide into the overlying water, sustaining black-odorous conditions even after external inputs are controlled. Pore water flux measurements and AVS analysis quantify this internal loading and inform dredging decisions.
How long does a complete source analysis program take?
A typical program requires at least one hydrological cycle covering dry and wet seasons, usually three to six months, including outfall census, multi-round sampling, laboratory testing, and modeling validation. Accelerated programs of four to eight weeks can support preliminary screening when historical data are available.
Is wet-weather sampling necessary?
Yes. Dry-weather sampling captures continuous sewage inputs and sewer leakage, while wet-weather sampling reveals stormwater runoff, combined sewer overflows, and resuspension effects. Both conditions are essential for a complete and defensible source apportionment.