by Dr. Alyre Chiasson

Turbidity is a relatively common concept even when not referring to water. An idea that is unclear is often said to be “turbid” or “murky”.  However, as a water quality parameter it refers to the scattering of light. In common language this describes the clarity of water. The scattering  of light can be due to material in suspension, like silt, or dissolved material like  organic acids that produce the tea-coloured water associated with bogs. Perhaps because its presence is readily detected by the human eye, it was one of the first water quality parameters to be measured after temperature.

Turbidity was first measured around 1900, using the “Jackson Candle Method (Figure 1). A glass tube was suspended over a candle flame. The tube was calibrated using a solution of diatomaceous earth (silica) at a concentration of 1,000 parts per million (ppm). The units as indicated by the scale on the tube were called “Jackson Turbidity Units”. The procedure was simple. You filled the tube until you could no longer see the candle flame. The method was crude for several reasons. The diatomaceous earth was inconsistent in particle size depending on the source. The candle itself and human eyesight were additional factors that made consistency and accuracy highly variable.

Figure 1: Figure 1. A glass tube was suspended over a candle and the amount of light passing through the sample was assessed by the human eye.1

The use of formazin, in the place of the distomaceous earth was a significant step forward  as the particles were more uniform in size and closer to what might be encountered in drinking water. As a polymer it could be produced in the lab under controlled conditions. Although formazin is still in use today, it is a suspected carcinogen and has a short shelf life. Plastic polymer beads have largely replaced it in most applications. However, the most accurate way to measure turbidity is using the “nephelometric method”. This consist of shining a beam of light through the sample and measuring the degree of scatter of the light at a 90 degree angle to the source (Figure 2).. The amount of scattered light is measured in NTU (nephelometric turbidity units).

Figure 2: Figure. The basis of modern turbidity measurements.2
Figure 3: A modern turbidimeter (Hach)3

Turbidity in drinking water is visually unappealing.  Viruses and bacteria can also cling to the particles that are causing the cloudiness and thus present a health risk. In lakes or reservoirs, turbidity can block sunlight and impede photosynthesis which in turn can disrupt the food chain. A positive relationship has been shown between the presence of loons and water clarity. Loons appear to favour lakes that have clear water, presumably because prey is easier to see.3

In rivers and streams, the main threat is the deposit of sediments on the bottom, smothering fish eggs and aquatic insects. Aquatic insects are an important food source in rivers and streams. Like lakes, high turbidity can block sunlight and reduce the growth of plants which insects and some fish depend upon. Directly, sediments can clog the delicate gills of aquatic invertebrates and in some cases lead to their abrasion, the loss of the ability to extract oxygen from the water, and eventually death4.

The sources of turbidity in water can be multiple. This can include soil erosion, runoff, wastewater, construction projects, mining, logging and even fires.  Preventing erosion and controlling runoff are probably the best means of preventing damaging levels of turbidity in our lakes, rivers and stream.

In the next blog I will look at nutrients, mainly phosphorus and nitrogen and how they are linked to blooms of blue-green algae, also  know as cyanobacteria.

References

1Sadar, M.J. (1998). Understanding turbidity science. Hach Company, Loveland, Colorado (Technical Information. Series, Booklet II).

2https://ca.hach.com/parameters/turbidity

3Baker, R. J., Anderson, Y. C., Gelvin-Innvaer, L., Hamady, M., Haws, K., & Perry, P. S. (2004).

Tenth anniversary report — Minnesota Loon Monitoring Program 1994–2003.

Nongame Wildlife Program, Minnesota Department of Natural Resources Report

12 pp.

4 Anonymous. No date. Impacts of sediment to aquatic habitats. https://www.abbotsford.ca (retrieved November 27, 2023)