Measures of Turbidity in Water

Turbidity and Suspended Sediment

Sediment from nonpoint sources is the most widespread pollutant of surface water. Turbidity is the measure of the amount of suspended material in water and is determined by the relative light transmission of the suspension. Turbidity is an important consideration because it greatly reduces algal populations by inhibiting sunlight and slowing photosynthesis, changes heat radiation, has harmful effects on benthic fish and plants, and compromises most of water's major beneficial uses. The concentrations of suspended sediment in streams can be highly variable and are influenced by many factors, including the following: rainfall intensity and duration, soil condition, geology, topography, and present vegetation. Concentrations are measured in milligrams per liter (mg/l)

Testing Information

There are two typical scales used to measure turbidity, percent transmission and optical density. Percent transmission, or transmittance, varies from 0-100% and is based on the amount of light that is able to penetrate through the water sample. Optical density, or absorbance, is based on a logarithmic scale ranging from 2-0 where 2 represents the most turbid and 0 represents the least. A transmittance value of 50% corresponds to about a 0.3 absorbance value.