Turbidity, suspended sediment, and water clarity: A review

Suspended sediment causes a range of environmental damage, including benthi c smothering, irritation of fish gills, and transport of sorbed contaminant s. Much of the impact, while sediment remains suspended, is related to its light attenuation, which reduces visual range in water and light availabili ty for photosynthesis. Thus measurement of the optical attributes of suspen ded matter in many instances is more relevant than measurement of its mass concentration. Nephelometric turbidity an index of light scattering by susp ended particles, has been widely used as a simple, cheap, instrumental surr ogate for suspended sediment, that also relates more directly than mass con centration to optical effects of suspended matter. However, turbidity is on ly a relative measure of scattering (versus arbitrary standards) that has n o intrinsic environmental relevance until calibrated to a 'proper' scientif ic quantity. Visual clarity (measured as Secchi or black disc visibility) i s a preferred optical quantity with immediate environmental relevance to ae sthetics, contact recreation, and fish habitat. Contrary to common percepti on, visual clarity measurement is not particularly subjective and is more p recise than turbidity measurement. Black disc visibility is inter-convertib le with beam attenuation, a fundamental optical quantity that can be monito red continuously by beam transmissometry Visual clarity or beam attenuation should supplant nephelometric turbidity in many water quality applications , including environmental standards.