Turbidity is a laboratory measure of the relative clarity or cloudiness of water. It is an indirect measure of the suspended particles in the water and is a general measure of the scattering and absorbing effect that suspended particles have on light. Turbidity–causing particles commonly arise from the weathering of rocks and soils during rainfall events or during rapid snow melt events, which are common during the fall and winter periods in the Island Health region. Turbidity–causing particles can also be present in the summer due to the growth of biological organisms such as algae, cyanobacteria or zooplankton. The higher the turbidity level, the cloudier the water looks. Turbidity is measured in nephelometric turbidity units (NTU). At 1.0 NTU the water is clear, at 5.0 NTU cloudiness may be noted  and at 10 NTU the water is visibly cloudy.

Drinking Water

High turbidity can interfere with the disinfection of drinking water by causing ultraviolet light and/or chlorination processes to become ineffective in destroying pathogens. Turbidity events can also be linked to an increase of disease causing micro-organisms in the source water.  The turbidity objective for drinking water from surface water supplies is less than 1.0 NTU.

The numerous potential sources of turbidity makes it challenging for a water supplier to determine the specific types of particulate matter in the water and the corresponding health risks associated with turbidity. Water suppliers are expected to apply a precautionary approach to protecting the public’s health and provide public notification to their customers accordingly.

As turbidity levels increase water suppliers and Island Health Drinking Water Officers will assess the situation. Based on the assessment public notices may be communicated to the water system users

Boil Water Notice (BWN) –  issued when there is an increased turbidity above 1.0 NTU or there is an adverse microbial health risk associated with water use.

To help determine when it is appropriate to issue a water notice, Island Health has adopted the Ministry of Health  Decision Tree for Responding to a Turbidity Event in Unfiltered Drinking Water .

Drinking Water Officers expect water suppliers using surface water to:

  • Include turbidity as part of their ongoing water quality monitoring program,
  • Include standard operating procedures within their Emergency Response Plans to respond to turbidity events,
  • Use the Ministry of Health Turbidity Decision Tree to help make decisions during turbidity events and,
  • Communicate with the Drinking Water Officer and their customers during turbidity events