fDOM / CDOM

fDOM stands for fluorescent dissolved organic matter, the portion of dissolved organic matter in water that fluoresces under ultraviolet light. The closely related term CDOM (colored, or chromophoric, dissolved organic matter) refers to the fraction that absorbs light and gives water its yellow-brown tint. Both are measured optically and reported in relative fluorescence units (RFU). Dissolved organic matter comes from decaying vegetation, soils, and peat, as well as from sewage and industrial effluent. fDOM is therefore a fast, continuous proxy for the organic content of water (including dissolved organic carbon) that would otherwise require laboratory analysis. It is used to track natural organic loads, to flag organic pollution, and to anticipate problems in drinking-water treatment.

Because it responds to both natural and pollutant organic matter, fDOM is a versatile screening channel in catchment, river, and source-water monitoring.

fDOM is measured by fluorescence: the sensor emits ultraviolet light that dissolved organic matter absorbs and re-emits at a longer wavelength, and the intensity of that fluorescence is proportional to the amount of fluorescent organic matter, reported in relative fluorescence units. It is one channel of a multiparameter sonde connected to a Thiamis gateway, giving a continuous organic-matter proxy without sampling. The reading needs interpretation because it is affected by temperature and by turbidity, which can quench or block the signal, so it is corrected and read alongside the turbidity channel. As an optical sensor it needs a wiper, regular cleaning against biofouling, and calibration checks to counter drift.