Surface water colority testing quantifies the chromatic intensity of rivers, lakes, reservoirs, and other natural water bodies, providing a rapid indicator of humic contamination, organic pollution, and industrial discharge. Using standardized techniques such as platinum-cobalt colorimetry in accordance with GB 11903-89 and ISO 7887, accredited third-party laboratories can determine whether a water body complies with the classification limits specified in GB 3838-2002. This article presents the technical principles, sampling requirements, testing procedures, and typical applications of surface water colority measurement.
Technical Overview
Colority, also referred to as chromaticity or color degree, describes the coloration of water caused by dissolved organic matter, metal compounds, and suspended particulates. In surface water, the dominant contributors include humic and fulvic acids, tannins and lignins from decaying vegetation, iron and manganese species, phytoplankton pigments, and effluents from industries such as pulp and paper, textile dyeing, and leather tanning. Although colority itself is rarely toxic, elevated values frequently signal the presence of natural organic matter, decomposition processes, or anthropogenic inputs that may affect treatment cost, disinfection by-product formation, and ecological health.
A fundamental distinction in colority measurement is made between true color and apparent color. True color refers to the color of water after removal of suspended solids by filtration through a 0.45 micrometer membrane and reflects only dissolved chromophoric substances. Apparent color is determined on the original unfiltered sample and includes the optical contribution of turbidity. Regulatory assessment and most engineering applications are based on true color, expressed in degrees on the platinum-cobalt (Pt-Co) scale, where one degree corresponds to the color produced by 1 milligram of platinum per liter in the form of potassium chloroplatinate.
The principal standards governing this testing include GB 11903-89, which specifies the platinum-cobalt visual comparison method, ISO 7887, which covers both visual and spectrophotometric determination of color, and ASTM D1209, which defines the platinum-cobalt scale for clear liquids. Under GB 3838-2002, the environmental quality standard for surface water, colority limits are assigned according to the designated function class of the water body:
- Class I and Class II waters: colority not exceeding 5 degrees
- Class III waters: colority not exceeding 15 degrees
- Class IV and Class V waters: colority not exceeding 25 degrees
Testing Samples
Rivers and Streams
Representative grab samples or depth-integrated samples are collected at established monitoring cross-sections, away from riverbanks, backwater zones, and direct discharge points. Sampling frequency typically aligns with routine environmental monitoring programs, with additional samples collected during high-flow events, algal bloom periods, or pollution incidents.
Lakes and Reservoirs
Samples are taken at fixed stations at various depths to capture thermal stratification and algal distribution effects. Deep-water reservoirs used as municipal sources are often sampled at the intake depth to ensure the data reflect the water actually entering the treatment plant.
Ponds, Wetlands, and Estuarine Waters
Shallow, vegetated water bodies frequently exhibit naturally elevated colority due to humic substances leached from surrounding soil and plant litter. Testing these systems establishes baseline conditions and helps distinguish natural background color from contamination.
Drinking Water Sources and Raw Water Intakes
Raw water collected at intake structures is tested to guide coagulant dosing, activated carbon application, and other decolorization processes, and to verify that the source meets sanitary requirements for potable water production.
Testing Items
Depending on the project objective, surface water colority testing may include the following items, reported in degrees or Hazen units (Pt-Co):
- True colority, determined on the sample after filtration through a 0.45 micrometer membrane
- Apparent colority, determined on the original unfiltered sample
- Colority of highly colored samples after quantitative dilution
- Complementary optical and pollution indicators such as turbidity, suspended solids, UV254 absorbance, chemical oxygen demand, and total organic carbon, which assist in interpreting the origin of the color
Testing Methods
Platinum-Cobalt Visual Colorimetry
This classical method specified in GB 11903-89 and ASTM D1209 uses a standard stock solution prepared from potassium chloroplatinate and cobalt chloride in hydrochloric acid. A graded series of color standards is prepared in 50 milliliter matched Nessler tubes, and the filtered sample is compared against the standards under defined viewing conditions against a white background. The reported result is the value of the matching standard or an interpolated value between adjacent standards.
Dilution Method for High-Color Samples
When the colority of a sample exceeds the upper limit of the visual comparison range, typically around 70 degrees, the sample is diluted quantitatively with colorless distilled water until its color falls within the standard range. The measured value is multiplied by the dilution factor to obtain the original colority.
Spectrophotometric Method
ISO 7887 permits instrumental determination of color by measuring absorbance at a defined wavelength such as 436 nanometers using a calibrated spectrophotometer with matched cuvettes. The absorbance is converted to Pt-Co equivalents through a calibration curve. This approach eliminates inter-operator visual bias and supports automated, traceable data recording in high-throughput laboratories.
Sample Handling and Quality Assurance
Samples are collected in clean glass containers, protected from light, and analyzed as soon as possible, preferably within 24 hours of collection, since biological activity and oxidation can alter color over time. Quality assurance measures include reagent blanks, duplicate determinations, verification of the standard color series, and consistent filtration procedures for true-color determination. All testing should be performed by accredited third-party laboratories operating under documented quality management systems.
Applications
Surface water colority data support a wide range of environmental and engineering decisions:
- Assessment and classification of surface water quality against the limits of GB 3838-2002
- Evaluation of drinking water sources and optimization of coagulation, adsorption, and advanced oxidation processes for color removal
- Compliance monitoring of effluents from textile, dyeing, pulp and paper, and tanning operations that discharge to receiving waters
- Baseline surveys and long-term trend monitoring in limnological and ecological research
- Evidence collection for pollution incident investigation and verification of remediation performance
When interpreted together with turbidity, organic matter surrogates, and hydrological data, colority provides a cost-effective early warning parameter for watershed management.
FAQ
What is the difference between true color and apparent color?
True color is measured after suspended solids are removed by filtration through a 0.45 micrometer membrane and reflects only dissolved substances, whereas apparent color is measured on the unfiltered sample and includes the influence of turbidity. Regulatory limits for surface water are generally based on true color.
Which standards govern surface water colority testing?
The most commonly applied standards are GB 11903-89 for the platinum-cobalt visual comparison method, ISO 7887 for visual and spectrophotometric color determination, and ASTM D1209 for the platinum-cobalt color scale. The corresponding surface water quality limits in China are defined in GB 3838-2002.
What units are used to report colority?
Colority is reported in degrees on the platinum-cobalt scale, sometimes called Hazen units or PCU. One degree equals the color of a solution containing 1 milligram of platinum per liter as potassium chloroplatinate with a defined cobalt chloride content.
How should samples be collected and preserved?
Samples should be collected in clean glass containers, filled with minimal headspace, protected from light and temperature extremes, and analyzed as soon as possible, ideally within 24 hours. Chemical preservatives are generally not added because they may alter the color of the sample.
Why do some natural waters show high colority without pollution?
Wetlands, forest streams, and peatland-fed rivers often contain elevated humic and fulvic substances that produce yellow to brown coloration at levels well above those of impacted urban waters. Site-specific background data are therefore essential when interpreting results.
Can colored samples with high turbidity be tested directly?
For true-color determination, the sample is first filtered through a 0.45 micrometer membrane to remove suspended solids. Unfiltered measurement is possible for apparent color, but strong turbidity interferes with visual comparison, making the spectrophotometric approach preferable in such cases.
