Turbidity is a measure of water quality that describes the cloudiness or haziness of a fluid. This clarity is determined by suspended particles that scatter light passing through the water. High turbidity indicates potential environmental degradation and health risks. Monitoring this parameter helps ensure the safety of natural aquatic ecosystems and the processed water delivered to consumers.
Defining Turbidity and Its Sources
Turbidity measures how tiny suspended solids interfere with light transmission, not the direct amount of material. These microscopic particles include silt, clay, organic matter, algae, and microorganisms. When numerous, these materials absorb and scatter light, causing the water to appear murky or opaque.
Suspended particles originate from both natural and human activities. Natural sources include the erosion of soil and rock, which introduces fine silt and clay. Decomposition of plant and animal material, runoff from glaciers, and the growth of phytoplankton also contribute to water haziness.
Human (anthropogenic) sources often cause chronic increases in turbidity. These primarily involve land disturbance, such as runoff from agricultural fields and construction sites washing sediment into waterways. Wastewater discharge and urban runoff also carry organic matter and pollutants that increase the concentration of suspended solids.
Ecological Impacts of High Turbidity
High turbidity levels disrupt natural aquatic environments like rivers, lakes, and streams. The immediate effect is light attenuation, as suspended particles scatter and absorb sunlight. This reduction in light penetration inhibits photosynthesis in submerged aquatic vegetation (SAV) and phytoplankton, which form the base of the food web.
Decreased photosynthesis reduces dissolved oxygen production, stressing or suffocating fish and other aquatic organisms. Suspended particles also absorb solar energy, converting it to heat and causing a rise in surface water temperatures. This thermal change is harmful, especially to cold-water fish species adapted to narrow temperature ranges.
The physical presence of these solids creates stress for aquatic life. Fine particles can clog the gills of fish and invertebrates, reducing respiratory efficiency and increasing disease susceptibility. When suspended material settles, it smothers spawning beds and benthic habitats, disrupting life cycles and decreasing biodiversity.
Turbidity and Drinking Water Safety
Turbidity in source water challenges the production of safe drinking water. High turbidity is associated with increased microbial contamination, as suspended particles can harbor harmful microorganisms. These particles, such as aggregates of clay and organic matter, act as physical shields for pathogens like Giardia cysts and Cryptosporidium oocysts.
This “shielding effect” prevents disinfectants, such as chlorine, from making sufficient contact with microbes to neutralize them. Pathogens encased within a particle are protected from chemical action, allowing them to pass through treatment and into the public water supply. Higher turbidity increases the likelihood that these disease-causing agents will evade inactivation.
Turbidity also actively interferes with the disinfection process by increasing the water’s “chlorine demand.” Suspended organic particles consume the added chlorine, depleting the disinfectant available to kill pathogens. Water treatment plants must use higher doses of chemicals to achieve the required disinfection level. This increased chemical use can lead to the formation of undesirable disinfection byproducts. Maintaining low turbidity is a primary operational goal, serving as a reliable indicator of effective filtration and pathogen control.
Measuring and Controlling Turbidity
Turbidity is quantitatively measured using a turbidimeter, which operates on the principle of nephelometry. This method involves shining a light into the water sample and measuring the intensity of the light scattered by suspended particles, typically at a 90-degree angle. The result is expressed in Nephelometric Turbidity Units (NTU), providing a standardized measure of water clarity.
A simpler, less precise field method uses a Secchi disk, which measures water transparency by the depth at which the disk is no longer visible. For municipal water treatment, controlling turbidity is a multi-step process designed to remove suspended solids.
The first step, coagulation, involves adding chemicals to neutralize particle charges, causing them to clump together into floc. These larger clumps then settle out of the water during sedimentation. The final step is filtration, where water passes through media like sand and gravel to physically remove remaining fine particles. Removing these suspended solids ensures the water is clear enough for disinfectants to work properly, safeguarding public health.