What is turbidity?
There are three primary ways in which suspended solids are measured: turbidity, total suspended solids, and water clarity:
- Turbidity (measured in nephelometric turbidity units, NTUs) is an optical property of water, where suspended and dissolved materials scatter light and therefore block the transmission of light through the water column.
- Total Suspended Solids (TSS, reported in units of weight per volume) is determined by passing water through a standard filter, and then drying and weighing the residue.
- Water Clarity is a direct measure of visible distance through the water column and is normally measured with a Secchi disk (a black and white disk submerged vertically into the water until it can no longer be seen)
The strength of correlation between each of these measurements is variable; being affected by the size, shape, and refractive index of sediment. Much of the existing research has been conducted using TSS for the measure of suspended sediment; hence TSS provides very clear indications of impacts to salmonids. However, TSS measurements require lab analysis of water samples. As such, guidelines based on TSS are problematic since in-situ determinations of compliance are not possible. Turbidity, on the other hand, can be measured in the field using relatively inexpensive equipment.
How were the turbidity thresholds set?
A comprehensive study undertaken by Newcombe (2003) involved a review of available research papers and consultation with practitioners in the field of sediment impacts. This study generated a model that can be used to determine the severity of impact on rearing success of clear water fish as a function of turbidity and duration of exposure. The 25 NTU and 100 NTU thresholds were extracted from this model based on the following impact information:
At 25 NTUs, rearing success is:
- slightly impaired after one day of exposure (i.e. feeding and other behaviours start to change);
- significantly impaired (i.e. fish growth rates and/or habitat size is reduced) after one week; and
- severely impaired after more than a year.
At 100 NTUs, rearing success is:
- slightly impaired after one to two hours;
- significantly impaired after seven hours; and
- severely impaired after 7 weeks.
Fisheries and Oceans Canada (DFO) supports these turbidity thresholds. A summary of the impacts of sediment on the aquatic ecosystem is provided here.