Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles suspended in the water column. These microscopic particles scatter light, making the water appear murky. Measuring this optical property indicates water clarity and overall quality, helping determine the potential presence of pollutants and the effectiveness of water treatment processes.
Defining Turbidity and Measurement Units
Turbidity is a physical characteristic of water, quantifying suspended matter rather than chemical contaminants or pathogens. These particles can include silt, clay, organic detritus, plankton, and tiny microorganisms. Higher levels of suspended matter result in greater light scattering and cloudier water.
The standard unit of measurement is the Nephelometric Turbidity Unit (NTU). This unit is quantified using a turbidimeter, which shines a beam of light through a water sample. A detector measures the intensity of the light scattered by the suspended particles, with a higher NTU reading indicating higher turbidity.
This precise method contrasts with older, less accurate techniques, such as the Secchi disk. The Secchi disk is lowered into a water body until it is no longer visible, providing a measure of water transparency. While useful for simple monitoring, the Secchi disk method is subjective and cannot accurately measure the low turbidity levels required for modern drinking water standards.
Common Causes of Increased Turbidity
Increased turbidity primarily results from the introduction of fine, suspended solids into water systems, often accelerated by human activity. Sediment runoff from disturbed land is a frequent source, particularly from construction sites where exposed soil is prone to erosion. Agricultural practices, such as tilling and over-grazing, also expose topsoil, allowing silt and clay particles to wash into nearby streams and rivers.
Riverbank erosion contributes significant sediment, especially where streamside vegetation has been removed. Losing these root systems destabilizes the banks, making them susceptible to scouring. Urban stormwater discharge is another source, as runoff washes over impervious surfaces like roads and parking lots, picking up fine particles before draining into waterways.
Organic material also elevates turbidity levels, particularly during algae blooms. When water bodies receive excess nutrients like phosphorus and nitrogen, phytoplankton populations rapidly increase. These microscopic organisms become suspended particles, and when they die and decompose, the decaying organic matter further contributes to the water’s cloudiness.
Impacts on Water Quality and Health
High turbidity impacts both aquatic ecosystems and public health. The cloudiness reduces the depth to which sunlight can penetrate the water column. This light blockage hinders photosynthesis in submerged aquatic plants, leading to a reduction in the dissolved oxygen (DO) they produce. When DO levels drop below two parts per million, it creates a stressful or fatal environment for most aquatic life.
Suspended solids also cause physical harm to aquatic organisms. Fine sediment particles can irritate and clog the gills of fish, causing their delicate structures to develop protective but damaging morphological changes. These responses include epithelial hyperplasia and lamellar fusion, which reduce the surface area available for gas exchange, making it difficult for the fish to absorb necessary oxygen. Furthermore, as the suspended particles settle, they can smother fish eggs, larvae, and the bottom-dwelling invertebrates that form the base of the food chain.
From a public health perspective, turbidity is directly linked to the risk of waterborne disease transmission. The suspended particles provide microscopic hiding places for harmful pathogens, particularly the protozoan parasite Cryptosporidium. This parasite is known for its resistance to common disinfectants, and when it or other microbes are physically encapsulated within a particle, the chemical disinfectant cannot make direct contact to inactivate them. This shielding effect compromises the disinfection process in water treatment facilities, increasing the potential for disease outbreaks, which often manifest as gastrointestinal illnesses.
Strategies for Turbidity Management
In drinking water treatment facilities, turbidity is managed through a sequence of physical and chemical processes to ensure the water is safe for consumption. The process begins with coagulation, where positively charged chemicals, such as aluminum sulfate, are added to neutralize the negative charges on the tiny suspended particles. This chemical destabilization allows the particles to overcome their mutual repulsion.
Coagulation is immediately followed by flocculation, a gentle mixing process that encourages the newly destabilized particles to collide and stick together, forming larger, heavier aggregates called flocs. These flocs are then removed during sedimentation, where they settle out of the water column by gravity in large basins. The final step is filtration, where the water passes through a medium, typically layers of sand and gravel, to physically trap any remaining fine particles.
Because of the direct link between turbidity removal and pathogen inactivation, the U.S. Environmental Protection Agency (EPA) regulates turbidity as a treatment technique. For water systems using conventional filtration, the treated water must maintain a turbidity level of $0.3$ NTU or less in at least 95% of the samples taken each month, and never exceed 1.0 NTU. Beyond treatment, preventative measures like erosion control are implemented to manage turbidity at the source. These measures include installing silt fences, creating sediment basins to capture runoff on construction sites, and stabilizing exposed riverbanks with native vegetation.