Particulate Matter, Color, Turbidity, and Light

Abstract

Natural waters contain particulate matter that increases turbidity, imparts apparent color, and interferes with light penetration. These particles originate from erosion on watersheds and within water bodies, vegetative debris from watersheds, and microorganisms produced in water bodies. Suspended particles vary from tiny colloids (1–100 mμ) that remain suspended indefinitely in still water to larger silt and sand particles held in suspension by turbulence. The settling velocity of particles in still water is estimated by the Stoke’s law equation and it depends mainly on particle diameter and density—large, dense particles settle the fastest. Organic particles settle slowly because of their low density, but planktonic organisms also have adaptions that lessen settling velocity. Solar radiation of all wavelengths is quenched quickly—about 50 % is reflected or converted to heat within the first meter. Within the visible spectrum, red and orange light is absorbed most strongly followed by violet, and by yellow, green, and blue. Plants absorb mostly red and orange light and yellow and green light for use in photosynthesis. Turbidity absorbs all wavelengths of light and diminishes photosynthesis. Two common ways of assessing turbidity in natural waters are by measuring the depth of disappearance of a 20-cm diameter disk (the Secchi disk) and by using nephelometry to determine the amount of light reflected at a 90 ° angle from a water sample. The weight of particles retained on a fine filter allows gravimetric assessment of suspended solids.