Citizen Science - Turbidity (Part 1)

The Wye Alliance is a network of citizen scientists monitoring pollutant levels in the River Wye and its tributaries and working closely with the Environment Agency (EA). Citizen Science (CS) projects often work with basic instrumentation/equipment collecting vast quantities of data that regulatory bodies, such as the EA, could only dream about. For example, there are over 400 citizen scientists collecting twice-weekly data in the River Wye watershed. All the information is uploaded using the Epicollect App and is available to one and all via the WyeViz database.

Turbidity is an important measure of water quality and clarity that is widely used in both freshwater & marine ecological studies as well as in many industrial processes (e.g. sewage and potable water treatment, breweries, pharmaceutical preparations). In simple terms, it provides a measure of the haziness or cloudiness of a fluid caused by the presence of suspended microscopic particles.

Here is a recent photo (Photo 1) of Belmont Pool, a fishing lake in a country park on the outskirts of Hereford. In addition to the avian population, coarse fish, insects and plant life, there is also a colony of terrapins. Unfortunately, at the time this photograph was taken (October 2024), the turbidity of the water was excessively high which is bad news for all the flora & fauna tenants.

Photo 1: Belmont Pool (October 2024)

There are a variety of units used to quantify turbidity which is extremely inconvenient and confusing. However, the most widely used are Nephelometric Turbidity Units (NTU) so we will stick with these units. So what are good and bad levels of turbidity?

• Drinking water should always be < 1 NTU and, typically, <0.3 NTU
• Freshwater will vary from <10 NTU (visibility to depths of 50 cm or more) to >240 NTU (visibility to depths of 10 cm or less)
• Turbidities > 10 NTU cause short-term stress to aquatic life
• Turbidities >100 NTU are unsafe for aquatic life

The turbidity level in Photo 1 will be in excess of 100 NTU.

Why is high turbidity water bad?

1. higher water temperatures because the suspended particles trap more heat from the sun
2. reduced photosynthesis at lower depths due to less light penetration
3. reduction in dissolved oxygen content due to points 1. and 2.
4. clogs fish gills making breathing more difficult
5. sediment can smother fish eggs and affect larval development of aquatic life (insects)
6. stressed fish (points 1 - 4) are more susceptible to disease
7. predators (e.g. birds such as the kingfisher) cannot see their prey
8. Bottom feeders such as swans, ducks cannot see their food source
9. silts up ponds, river courses, etc
10. may encourage/sustain toxic algal blooms (Photo 2)

Photo 2: Toxic Blue-Green Algae

What causes high turbidity?

1. In these parts, the main source (> 70 %) of high turbidity is soil run-off due to poor or incompetent agricultural practices. The coffee-coloured streams contain the nutrient-rich red topsoil that is the lifeblood of Herefordshire's agricultural heritage (Photo 3).
2. Sewage discharges from CSOs and poorly maintained or leaking septic tanks
3. Road run-off after heavy rainfall plus other urban sources
4. Algal blooms
5. Erosion
6. Chemical pollution (Photo 4)

Photo 3: Typical Coffee-coloured Soil Run-off

Photo 4: Chemical of Natural Foam?

In part 2, I'll describe how we, as citizen scientists, measure turbidity.