Water turbidity - how to measure it and what it means (NTU/FNU)
One downpour is enough to turn a previously clear stream milky or brown. That change, visible to the naked eye, has its number - and people who measure it regularly read the catchment’s story from it: when soil washed off the fields, when a bloom started, when someone stirred up the water upstream. Turbidity is one of the most intuitive and at the same time useful indicators: simple to capture, yet telling.
It is measured with a turbidimeter or a plain turbidity tube that even children can use in a workshop. In both versions it shows the same thing: the water’s response to what is happening in the catchment - often faster than any other parameter.
What turbidity is
Turbidity is the optical “haziness” of water caused by suspended matter - particles of soil, silt, plankton, organic matter. It is not the same as the concentration of suspended solids, but a good, quick indicator of it. The more particles scatter light, the higher the turbidity.
It is given in NTU or FNU units (the infrared-beam method per ISO 7027 gives FNU). High turbidity after rain signals erosion and runoff; persistent turbidity - a bloom or a steady inflow of suspended matter. Turbidity also limits the access of light in the water column, so it matters for life.
How to measure it - two routes
A turbidimeter (nephelometer) - an instrument measuring light scattered at 90°.
- Calibrate the meter with a standard (formazin/standard).
- Take a sample into a clean cuvette, without bubbles; wipe the outside of the wall.
- Mix gently and measure right away - suspended matter settles, so a delay underestimates the result.
A turbidity tube - a field, simple equivalent of the Secchi disk for shallow watercourses: you pour water into a transparent tube with a marker at the bottom and drain the water until the marker becomes visible; you read the height of the column from the scale. Cheap, great for education and citizen science, though less accurate.
What the result means
What says the most is the change relative to the baseline of a given water: you know your watercourse’s typical turbidity, so a sudden jump after rain or a long-persisting haze stands out at once. A single reading is worth less than its trend - and than its link to rainfall or runoff.
| Order of magnitude | Typical situation |
|---|---|
| < 1 NTU | water treated for drinking (US EPA filtration requirement: ≤ 1 NTU, in practice ≤ 0.3 NTU); a clean mountain stream at low flow |
| 1-10 NTU | typical river or lake water at baseline; a large lowland river on rainless days tends towards the upper end |
| > 10 NTU, sustained | prolonged light limitation harms photosynthesis and bottom-dwelling organisms |
| tens to hundreds of NTU | typical short-lived peaks during run-off after a downpour or during a flood wave - this is not a measurement error |
These are indicative orders of magnitude, not a quality standard - there is no single “good” turbidity for a river, because the natural background differs between catchments (sandy vs clayey, lowland vs mountain). The diagnostic value lies in the departure from your own baseline, not in a comparison with the table.
The most common mistakes
- Air bubbles in the cuvette - they overestimate the reading.
- A scratched or dirty cuvette - it falsifies the measurement; keep it clean.
- Delay and settling - measure a fresh, gently mixed sample.
- No calibration or strong water colouration (interference) not accounted for.
How often
What says the most is a steady series supplemented with measurements at events (after downpours, during blooms) - that is when turbidity carries the most information about the catchment. A fixed site and a fixed time of day matter here as much as anywhere else.
Compare turbidity across dates and stations
Have a series of turbidity readings? With every result, record the date, the station, the unit (NTU or FNU), the method and the instrument model. Two instruments reporting the same unit - even two models from one manufacturer both giving FNU - can differ by roughly ten to a few tens of percent, because the reading depends on the optical design. Changing the meter or the method in the middle of a series looks like a change in the water on the chart. Before you switch equipment, measure with both in parallel for a while and mark the moment of the change in the series. Do not mix a turbidity-tube result (centimetres, or indicative NTU from the tube’s scale) with turbidimeter readings in one series. In LimnoLog, import your spreadsheet, compare the series on a chart and prepare a report for the selected period.
The example below (synthetic data, one instrument and one method) shows readings from successive dates, with a chart and PDF for the same range. The comparison shows differences between readings - it will not point to the cause of a change.
See an example comparison of readings
Sources and further reading
- Methodology: the standard EN ISO 7027 (determination of turbidity) and GIOŚ materials on water monitoring.
- Orders of magnitude in the table: the drinking-water filtration requirement per US EPA (Monitoring & Assessment - Turbidity); typical values of streams and large rivers: In-Situ - typical turbidity values.
- Comparability of instruments: Foster G.M. et al. (2021), Technical note - Relative variability of selected turbidity standards and sensors in use by the U.S. Geological Survey, USGS Open-File Report 2021-1009, doi:10.3133/ofr20211009.
- What to pair with turbidity: transparency (the Secchi disk) and total suspended solids - together they give a fuller picture.
See it in the LimnoLog app
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