What it is
Total organic carbon measures the carbon in dissolved and suspended organic matter: decaying plant material, humic substances from peat, algae and anything organic that has got into the supply. It is a single figure for a complex mixture.
Where it comes from
Peaty and upland catchments and surface-influenced supplies carry the most natural organic matter, rising after rain. Sewage, slurry and agricultural run-off add to it. Levels are usually low in deep groundwater.
Why it matters
There is no health-based limit and the guideline is no abnormal change. Organic matter matters for three reasons: it gives water colour and taste, it feeds bacterial growth in the system, and where chlorine is used it forms disinfection by-products. A rise is a prompt to look at the source.
What can be done about it
Organic matter is reduced by activated carbon filtration, by coagulation and filtration on larger supplies, and by protecting the source from run-off. The engineer sizes treatment to the level and to how the water is disinfected.
Information last reviewed 5 October 2026 by Pozzani Pure Water.