Understanding NTU Readings: What Site Teams Need to Know

Turbidity is a measure of how cloudy or unclear water is, caused by suspended particles such as silt and sediment. On a construction site, turbidity readings are one of the most straightforward ways to keep an eye on surface-water quality — particularly at designated monitoring points feeding into local drainage or watercourses.

Readings are measured in NTU (Nephelometric Turbidity Units). The higher the number, the more turbid — or cloudy — the water. A turbidity tube gives a quick visual reading: water is poured in until a marker at the base is no longer visible, and the level at that point gives the NTU result.

As a general guide, an NTU reading of 60 is considered to indicate a lower silt volume. That said, every site is different — your project's environmental permit will set out the maximum suspended solids allowed to discharge, so always check the specific limits that apply to your site rather than relying on a single rule of thumb.

Whatever the reading, it's worth recording it properly. Note the date, time, location, weather conditions and any relevant site activity in your monitoring log alongside the result — this record is often just as important as the reading itself when it comes to demonstrating compliance.