Turbidity causes a reduction in water clarity due to light attenuation. Turbidity
is an optical property of water since all natural waters contain some light-attenuating
materials. Turbidity is caused by two general types of substances. Particulate
materials interfere with light penetration by scattering. Scatter reduces light
exponentially as shown by the equation
Il/Io= e-σ l
Io= incident light intensity
Il= light intensity after traversing a path l meters long
e is the natural log base (2.718...)
σ= scattering coefficient
Residence time of particulates (the time in suspension) is largely a function of particle size so that silt (defined as particles less than 63 µm in diameter) will remain suspended far longer than sand-size particles. Particulates can be removed by filtration with approriate mesh size.
Absorption is another mechanism by which turbidity reduces light transparency. Absorption results when light is captured, usually by organic material. Tannic acid or chlorophyll (from plant degradation) will reduce light penetration makedly but are completely unfilterable. Absorption follows Beer's Law (as in spectrophotometry) and like scattering is an exponential function determined by the equation Il/Io= e-α l where α is the absorption coefficient
The two equations combine to produce a light attenuation equation (a function of both absorption and scattering) as follows Il/Io= e-(σ+α) l
Why measure turbidity? Light attenuation is an important environmental impact resulting from river or canal effluent, or construction projects such as beach renourishment. In the tropics in particular, seagrasses, benthic algae and invertebrates with photosynthetic symbionts (e.g., reef-building corals) are all affected by light reduction. It should be evident from the attenuation equation above that turbidity measurement should take all sources of light attenuation into account. State agencies concerned with water quality have adopted turbidity standards which cannot be exceeded. The Florida Department of Environmental Regulation requires turbidity monitoring using the Nephelomentric Turbidity Units or NTU's so that during construction, the permitee "shall not exceed 29 NTU's above the associated background turbidity levels as prescribed in the 'Monitoring Required' section pursuant to Rule 62-302, of the Florida Administrative Code".
What is an NTU?
Many states, including Florida, prescribe turbidity standards in terms of the NTU. A sample is placed in a turbidity meter or nephelometer (from the greek 'nephelos' for cloud) which measures the reduction in light due to scattering as a light beam passes through the sample and then to a detector or detectors of varying design that are set at a fixed angle (typically 90o). Such detectors are optimized for particle sizes of 1 micron or less, a size much smaller than natural silt. In addition, the detectors usually read at a fixed wavelength which does not take light attenuation by absorption into account. The scale of measurement can range from < 1 to >1000, using a standard called "Formazin", a chemically prepared synthetic material. Since Formazin has become the standard by which NTU's are measured, NTU's are usually expressed in terms of Formazin Turbidity Units (FTU's), so that 1 NTU = 1FTU. The advantage of Formazin is its particle uniformity and stability that gives reproducible results (as long as the same type of detector is used). There are EPA standards for particulates in drinking water and other applications in which the use of NTU measurements seem to be reasonable, but in natural waters where the concern is for light attenuation, the use of NTU's is questionable. We can now ask "what is 29 NTU and why not some other number?" The short answer is: I don't know. The source of, or rationale for the State of Florida's 29 NTU standard is itself a nephelometric. Some perspectives are in order. (1) Using naturally-occurring fine, carbonate muds or commerically prepared kaolin (a fine aluminum siicate clay), our lab could not physically maintain anything above 20 NTU's in aquaria even with mechanical stirring and pump jets. (2) During mechanical stirring of the bottom in 1 meter of water in Florida Bay, the largest value we could obtain was ~10 NTU, but the reduction in light was nearly 50% (Goldberg, unpublished data). (3) Of all the states and commonwealths within the United States possessing coral reef resources (e.g., Guam, Puerto Rico, the Virgin Islands, Hawaii and others) Florida stands alone with a standard that is 6 or more times higher than the others.
How is light attenuation measured? Devices that measure biologically significant light are generally referred to as quantum meters since they are designed to measure the quanta of visible radiation that is useful in photosynthesis (400-700 nm= violet-red light). This range is referred to as PAR or Photosynthetically Active Radiation. By measuring incident light just below the surface, and at depth x, the attenuation of PAR can be measured directly. Since the sensors measure PAR rather than a single wavelength, the attenuation from both scattering and absorption is measured, and because PAR attenuation is an actual physical feature of natural water, no artifical standards are required once the instrument is calibrated. Quantum meters may use different units of light measurement, but typically, readings are given in photon flux density per unit area per unit of time. The Avogadro number of photons/m2/sec is equal to one mole of light, and this is the most common unit of light measurement.1-2 moles of light/m2/sec is bright in the terrestrial world or at the ocean's surface, but measurements in the marine environment below the surface are a fraction of those figures. Marine light levels are often measured in micromoles (10-6moles). Bright light a few meters below the surface may be 400 micromoles, while 100 micromoles would be considered dim. It is the amount of light measured in these terms that matters most to aquatic organisms, not the oblique measure of NTU's or FTU's. Nonetheless, the latter remains the standard method for assessing turbidity.