Turbidity is a measure of the cloudiness or haziness in a fluid, caused by individual particles suspended within the water. These suspended materials are not dissolved, but include substances such as silt, clay, organic matter, microorganisms, and algae. When water appears murky or opaque, it indicates high turbidity, which is a significant indicator of overall water quality. The clarity of water is affected by both natural processes like erosion and human activities, including industrial discharge and agricultural runoff.
The Physics of Light Scattering
Modern methods for measuring water clarity rely on the interaction between light and suspended particles. When a beam of light passes through a water sample, the suspended solids interfere with light transmission. This interference involves both light absorption and light scattering, with scattering forming the foundation for most common measurement techniques.
Light scattering occurs when particles absorb incident light energy and re-radiate it in all directions. The degree of scattering depends on the particle’s size, shape, composition, and the light’s wavelength. Instruments quantify the intensity of this scattered light, a technique known as nephelometry. This approach is more sensitive than measuring the loss of transmitted light, which is prone to interference from colored substances.
Laboratory Measurement Standards
Precise quantification of turbidity in a laboratory utilizes a specialized instrument called a nephelometric turbidimeter. This instrument works by shining a light source through the sample and measuring the light scattered at a 90-degree angle to the incident beam. This 90-degree detection angle is most sensitive to the light scattered by smaller colloidal particles, which commonly cause low-level turbidity in drinking water.
The standard unit of measurement for this technique is the Nephelometric Turbidity Unit (NTU). The NTU scale is tied to a primary calibration standard, typically a synthetic polymer suspension called Formazin. Formazin is used because it is highly reproducible and provides a reliable reference for calibration. By calibrating the turbidimeter with Formazin standards, the scattered light signal is accurately converted into an NTU value, ensuring consistency across laboratories.
Field Methods for Monitoring
While laboratory turbidimeters provide the most precise data, less complex methods are frequently employed for quick checks and environmental monitoring in the field. These simpler techniques rely on visual observation of light penetration rather than electronic quantification of scattered light. The Secchi disk is a classic tool used in deeper bodies of water, such as lakes and oceans, to assess water transparency.
The Secchi disk is a flat, circular plate, often painted with alternating black and white quadrants. It is lowered until it disappears from view, and then raised until it reappears. The average of these two depths provides the Secchi depth, which measures the water’s clarity. For shallower water bodies, a turbidity tube is used; water is poured into the tube until a Secchi-like pattern at the bottom is no longer visible. These field tools provide a relative measure of clarity useful for tracking trends, but they do not yield the precise NTU values required for regulatory compliance.
Why Turbidity Measurement Matters
Measuring water clarity is important because high turbidity levels impact public health and the environment. In drinking water treatment, elevated turbidity is problematic because suspended particles can shield microorganisms, such as viruses and bacteria, from disinfection chemicals like chlorine. These particles also increase the surface area for pathogens to attach, compromising the treatment process effectiveness. Regulatory agencies, like the U.S. Environmental Protection Agency (EPA), set strict limits on the maximum allowable turbidity in treated drinking water.
For aquatic ecosystems, turbidity affects the health and survival of organisms by reducing light penetration into the water column. When light is scattered by suspended solids, less light reaches submerged aquatic plants, limiting their ability to photosynthesize and produce oxygen. High sediment loads can also physically harm aquatic life by clogging the gills of fish and interfering with respiration. Monitoring turbidity is a standard practice for assessing ecological health and tracking the impact of human activities on waterways.