Turbidity is a measure of the cloudiness of the water, which is related to the shape, size and concentration of particles suspended in the water. Turbidity is measured with an optical probe which shines a beam of light through the water and detects the amount of light that is scattered off the particles that are present. Turbidity is measured in Nephelometric Turbidity Units or NTU. Turbidity typically increases during high flow events (e.g. heavy rain, snowmelt, etc.) as soil particles are washed off city streets, parking lots and agricultural fields. Resuspension of river sediments under high flow conditions can also contribute to higher turbidity levels.
Turbidity measurements can be quite “jumpy” as a result of the nature of the particles in the water. For example, one large particle or piece of debris passing in front of the sensor can result a very high reading even if the water turbidity is generally low. Air bubbles can also interfere with turbidity measurements. The sensor is also very susceptible to fouling as biological and inorganic material can accumulate on the face of the sensor and interfere with the ability of the sensor to send and detect the light signal used to measure turbidity.