Description

Coloured Dissolved Organic Matter (CDOM), also known as chromophoric dissolved organic matter, refers to the fraction of dissolved organic carbon (DOC) that absorbs light, particularly in the ultraviolet and blue regions of the electromagnetic spectrum. It originates primarily from the decomposition of plant and animal material in terrestrial and aquatic environments.

Fluorescent Dissolved Organic Matter (FDOM) is a subset of CDOM that has the additional property of fluorescing when exposed to specific wavelengths of light. FDOM can originate from various sources, including terrestrial humic substances, microbial activity, and anthropogenic inputs. Monitoring FDOM often provides a proxy for CDOM and a specific insight into the nature and source of the dissolved organic matter present.

Why is CDOM Monitoring Important?

Monitoring CDOM and FDOM levels in water is crucial for a wide range of environmental and operational reasons:

• Water Quality Indicator: CDOM/FDOM concentration is a direct indicator of organic matter load, which can influence taste, odour, and colour in drinking water.
• Disinfection Byproduct (DBP) Formation: High CDOM levels can lead to the formation of harmful disinfection byproducts (DBPs) when water is treated with chlorine. Monitoring helps optimise treatment processes to minimise DBP formation.
• Carbon Cycling and Climate Change: CDOM is a significant component of the global carbon cycle, influencing how carbon is transported and processed in aquatic systems. Changes in CDOM can reflect broader environmental shifts, including the impacts of climate change on terrestrial runoff.
• Aquatic Ecosystem Health: CDOM absorbs sunlight, influencing light penetration into the water column. This affects primary productivity (photosynthesis) of aquatic plants and algae and can impact the entire food web.
• Tracer for Water Sources and Mixing: The unique fluorescent signatures of different FDOM components (e.g., humic-like vs. protein-like fluorescence) can help identify the origin of water (e.g., terrestrial runoff, wastewater, algal production) and track mixing patterns in estuaries or lakes.
• Optical Interference: High CDOM can interfere with other optical water quality sensors (like those for chlorophyll or turbidity) by absorbing or scattering light, necessitating compensation or specific sensor design.

How CDOM Sensors work

Most Organic Matter will dissolve in water and a proportion of this will become CDOM and a proportion of this FDOM. The FDOM fluoresce when exposed to UV light wavelengths. The sensor operates by emitting a specific ultraviolet wavelength of light (e.g., around 370nm), into the water sample. This particular light excites the FDOM molecules, which then re-emit light at a different, longer wavelength (e.g., around 460nm). A sensitive detector precisely measures the intensity of this emitted fluorescence, with the intensity being directly proportional to the amount of CDOM and FDOM present, thereby providing a real-time measurement of the organic matter present.

The Aquaread CDOM sensor, for use with the AP-6000 or AP-7000 probe, uses an excitation at 365nm and then detects the resultant fluorescence between 450nm and 520nm. This sensor emits high intensity ultraviolet (UV) light, which is harmful to skin and eyes.

When to use the sensor

CDOM/FDOM monitoring is an increasingly vital practice for comprehensive water quality assessment, particularly in areas impacted by organic matter loading or concerned with drinking water quality. It is highly valuable for:

• Drinking Water Source Monitoring: Essential for water treatment plants to predict and manage DBP formation risks and assess raw water quality changes.
• Watershed Management: Tracking the input of terrestrial organic matter from forests, wetlands, and agricultural areas into rivers and lakes, especially during rainfall events.
• Estuarine and Coastal Monitoring: Understanding the mixing of fresh and saltwater and the transport of land-derived organic matter to coastal zones, which impacts marine ecosystems.
• Eutrophication Studies: Assessing the role of organic matter in nutrient cycling and its influence on algal growth and oxygen dynamics.
• Climate Change Research: Investigating changes in carbon export from terrestrial to aquatic environments in response to shifting climate patterns.
• Dam and Reservoir Management: Monitoring organic matter loads that can affect water clarity, oxygen levels, and treatment challenges within reservoirs.
• Research and Ecological Studies: Providing insights into the composition and reactivity of dissolved organic matter, which supports microbial activity and broader ecosystem functioning.

7000-CDOM Electrode

The 7000-CDOM electrode is a Chromophoric (Fluorescent) Dissolved Organic Matter sensor designed for the detection of dissolved organic matter in water, when connected with the AP-6000 or AP-7000 water quality probe from Aquaread. It is designed to give a measure of CDOM or FDOM in water. It achieves this by emitting light into the sample and measuring the Fluoresce light re-emitted by the organic matter in the sample. This fluorescence measurement techniques is not ideal for quantitative measurements, but can be used to get an indication of increasing or decreasing concentrations over time at a given location.

Automatic Cleaning Mechanisms on AquaProbe

Both the AP-6000 and AP-7000 AquaProbes include automatic cleaning mechanisms which are incredibly beneficial for any optical sensor. The brush and wipers are critical because they:

• Prevent Biofouling Build-up: By periodically wiping the optical surfaces, they physically remove algae, biofilm, and other organic matter before it can significantly impact readings. This is especially crucial for long-term deployments.
• Dislodge Air Bubbles: The wiping action effectively dislodges any trapped air bubbles that might cling to the sensor’s surface, ensuring a clear path for light transmission and reception. Air bubbles are a common issue in turbulent waters or when probes are initially deployed.
• Maintain Accuracy: By keeping the optical windows clean, the cleaning mechanism ensures the sensor’s readings remain accurate and reliable over extended periods, reducing the need for frequent manual cleaning and site visits.