The Effect of Dissolved Organic Matter on Turbidity Measurements in Colored Water

Turbidity is commonly treated as a direct indication of suspended sediment, but optical instruments respond to the light environment created by the whole water sample. Dissolved organic matter can change that environment even when it contains no particles. In rivers, wetlands, reservoirs and coastal waters, coloured dissolved organic matter may therefore influence a reported turbidity value without representing an equivalent mass of suspended solids.

This matters wherever measurements are used to track dredging plumes, stormwater movement, erosion, drinking-water catchments or ecological change. A dark brown reading from a tannin-rich wetland may have a different optical meaning from the same reading in pale mineral-rich water. The distinction becomes especially important when data are compared between sites or used to trigger operational controls.

A dependable monitoring programme separates optical response from physical sediment concentration. That requires attention to wavelength, sensor geometry, calibration material, sample colour, natural background conditions and the purpose of the measurement. The result may be a corrected turbidity estimate, a separate colour indicator, or a decision to interpret the signal as a combined optical index.

Why colour changes optical readings

Dissolved organic matter, often abbreviated as DOM, is a broad mixture of carbon-based compounds produced by soils, plants, algae and microbial activity. The fraction that gives water a yellow, tea-coloured or brown appearance is commonly called coloured dissolved organic matter, or CDOM. Humic and fulvic substances are important contributors, particularly after rainfall flushes organic material from forested soils and wetlands into streams.

Turbidity instruments usually illuminate a water sample and measure light scattered by particles. The detector may be positioned at 90 degrees to the incident beam, as in a nephelometric design, or may measure transmitted, backscattered or forward-scattered light. DOM absorbs some wavelengths as the beam travels through the sample. This reduces the light available to reach a detector and can alter the balance between scattered and unscattered light.

The direction and size of the effect depend on the instrument. In a conventional nephelometer, strong colour can suppress the detected signal or change the relationship between particle concentration and reported NTU. In another optical arrangement, absorption may increase the apparent response because the detector sees a changed contrast between the illuminated volume and its background. There is no universal correction factor for all turbidity sensors.

Absorption, scattering and sensor geometry

Particle scattering and DOM absorption are different optical processes. Clay, silt, organic particles and plankton redirect light, while dissolved compounds primarily remove selected wavelengths through absorption. A water sample can therefore have low suspended solids but a strong colour signal, or high suspended solids with little dissolved colour. If the instrument measures only one optical response, it may not distinguish these conditions.

Wavelength is central to the problem. Many humic substances absorb strongly in the ultraviolet and blue parts of the spectrum, with absorption generally declining toward longer wavelengths. A near-infrared sensor may be less affected by visible brown colour than a blue-light device, although particle composition, path length and detector sensitivity still matter. Near-infrared measurements can also respond differently to black carbon, algae and mineral sediment.

Sensor geometry adds another layer. A long optical path gives dissolved colour more opportunity to absorb light. Small changes in window fouling, bubbles or mounting angle can then become difficult to separate from the water’s natural optical properties. Backscatter sensors, transmissometers and nephelometric probes should therefore be treated as measurement systems with distinct interference patterns rather than interchangeable turbidity meters.

The D & A Instruments site provides background on optical water-quality and sediment-monitoring equipment, including applications in marine and freshwater environments. That broader context is useful when selecting a sensor for a site where colour, sediment and biological material vary together.

Where the error appears in the field

Australian catchments provide many examples of coloured water. In parts of the Murray–Darling Basin, dry periods can be followed by runoff carrying fine sediment and dissolved organic material from floodplains, wetlands and farm soils. In tropical Queensland, intense rainfall can deliver leaf-derived compounds and sediment to waterways at the same time. A turbidity trace may rise sharply, but its change cannot automatically be assigned to suspended mineral particles.

Dredging near ports and estuaries presents a different challenge. A plume may contain resuspended mud, organic detritus and naturally coloured water. Around the Great Barrier Reef, monitoring teams may need to distinguish a dredging-related increase from the background optical variability of tidal creeks and coastal waters. A sensor reading that is technically consistent can still be misinterpreted if the baseline was established during an unusually clear or unusually dark tide.

Reservoirs supplying cities such as Sydney, Melbourne and Brisbane can also develop strong colour after catchment runoff. Water utilities may measure colour, absorbance, dissolved organic carbon and turbidity as separate parameters because each provides different information about treatment demand and catchment condition. A standard turbidity result is not a substitute for a colour or organic-carbon measurement.

The issue is especially visible during field sampling. A clear bottle can look amber or tea-coloured while showing little visible sediment. Conversely, fine clay can keep water visually pale while producing a high scattering response. Human visual assessment is useful for documenting conditions, but it cannot replace instrument-specific validation.

Designing a reliable monitoring method

The first step is to define what the measurement is intended to represent. If the objective is regulatory turbidity control, the project should follow the method and instrument requirements specified by the relevant approval, contract or laboratory protocol. If the objective is sediment transport research, a calibrated suspended-solids estimate may be more useful than a single turbidity number. If the objective is plume mapping, a stable relative optical index may be adequate when paired with site-specific validation.

Baseline observations should cover the natural range of colour and sediment. Measurements taken during dry weather alone will rarely describe an Australian catchment adequately. Include rainfall events, changing tides, reservoir turnover or wetland inflows where those conditions are relevant. Record water depth, weather, flow, salinity, algae, bubbles and recent disturbance beside the optical data.

Paired sampling is essential. Collect water for laboratory suspended-solids analysis while recording the sensor output at the same location and time. Measure colour or absorbance, and where possible include dissolved organic carbon or an equivalent DOM indicator. A relationship that is strong in one season may weaken after a flood because the mixture of mineral particles and organic compounds has changed.

Multiple wavelengths can help separate absorption from scattering. A reference channel that is relatively insensitive to DOM may be compared with a colour-sensitive channel, provided the instrument has been designed and calibrated for that purpose. Technical discussion of multiple optical wavelengths illustrates why wavelength selection is valuable when water composition changes with depth or location.

Calibration for Australian waters

Formazin and polymer bead standards are widely used for turbidity calibration, but they do not reproduce every optical property of natural sediment. A standard can verify that an instrument responds consistently while failing to predict its response to dark, organic-rich water. Calibration checks and field validation answer different questions and should not be treated as substitutes.

A site-specific calibration can relate the sensor signal to laboratory suspended solids across the expected range. Samples should include low and high colour, different particle types and the conditions most likely to affect decisions. Regression models may be linear over a narrow range and strongly non-linear over a wider range. In some projects, separate relationships are needed for baseflow, stormflow and dredging periods.

Reference materials and procedures should be documented carefully. Record the sensor model, optical path, firmware, calibration standard, date, temperature, sample handling and laboratory method. If a probe is cleaned or its optical windows are replaced, verify the response again. Biofouling can create a bias that resembles a change in DOM or sediment concentration.

Australian environmental decisions often sit within state-based approvals and national guidance. The Australian and New Zealand Guidelines for Fresh and Marine Water Quality, commonly called the ANZG guidelines, support risk-based assessment, while state agencies and project approvals may specify local trigger values. A monitoring plan should state whether the reported value is instrument turbidity, laboratory turbidity, suspended solids, or a corrected estimate derived from several measurements.

Practical checks for field teams

Field crews can reduce ambiguity by combining optical readings with simple observations and repeatable sampling. The following checks are particularly useful when coloured water is expected:

• Record water colour, recent rainfall, flow, tide and visible algae.
• Collect paired samples for suspended solids and colour or absorbance.
• Inspect optical windows for fouling, scratches, bubbles and sediment deposits.
• Compare readings at more than one wavelength where the instrument allows it.

A clean instrument does not automatically produce a correct result, but a dirty instrument makes interpretation much harder. Rinse probes with site water before deployment, remove trapped air, allow readings to stabilise and avoid placing the sensor immediately beside a bank, boat hull or disturbed bed. In shallow Australian creeks, wading can create a local sediment cloud that is mistaken for a natural event.

Laboratory checks should use consistent mixing and subsampling because fine particles settle quickly. Dark water can also make visual endpoint methods unreliable, so instrument settings and sample path length need to be recorded. When samples are transported, note temperature and holding time, particularly if microbial or algal activity may alter the organic fraction.

For projects requiring manufacturer or product-management information, the support contact page can help direct technical enquiries to the current Campbell Scientific support arrangements. Keeping the sensor history, site notes and laboratory results together gives future analysts a much stronger basis for interpreting unusual readings.

Separating turbidity from water colour

A useful reporting system may present several variables rather than forcing all optical behaviour into one number. Turbidity can describe the instrument response to scattered light, suspended solids can describe gravimetric particle mass, and colour or absorbance can describe dissolved optical material. These measurements overlap in the field but answer different questions.

Data interpretation should consider ratios and patterns as well as absolute values. A simultaneous rise in turbidity, suspended solids and absorbance after rain is consistent with a mixed runoff event. A strong rise in colour with little change in suspended solids suggests DOM or CDOM is influencing the water without a comparable increase in particles. A turbidity rise with stable colour may point more strongly toward mineral resuspension, though biological particles and bubbles still need consideration.

Time series analysis can reveal these distinctions. Storm hydrographs often show an early organic flush followed by mineral sediment, or the reverse depending on catchment conditions. Estuarine sites may show tidal changes in salinity and particle aggregation that alter scattering without a proportional change in DOM. Looking at coincident flow, rainfall, salinity and optical channels is more reliable than interpreting a single trace in isolation.

For compliance work, the final decision should follow the approved monitoring method and reporting threshold. For research and operational control, a multi-parameter interpretation can prevent unnecessary alarms and improve estimates of sediment movement. A coloured-water effect does not make turbidity data useless; it defines the conditions under which those data need qualification or supplementary measurements.

Using the result in water-quality decisions

The effect of dissolved organic matter on turbidity measurements becomes manageable when the optical signal is linked to the physical and chemical character of the site. Instrument selection, wavelength choice, calibration material and sensor geometry should be considered before deployment, not after an unexpected time series appears.

Projects in Australia also benefit from documenting local background conditions. A monitoring record from a Sydney reservoir, a tropical Queensland creek and a muddy estuary near Perth may use the same unit, yet the relationship between NTU, colour and suspended solids can differ substantially. Catchment geology, vegetation, rainfall intensity, tidal exchange and water-treatment objectives all shape the result.

The most defensible reports state what the sensor measured, how it was calibrated, which interferences were assessed and how the value was used. When DOM is high, a turbidity reading may remain valuable as an operational indicator while requiring a separate suspended-solids or colour interpretation. That clear distinction protects data quality and supports sound decisions about dredging, catchment management, aquatic habitat and water treatment.