Stormwater Turbidity and Its Aquatic Life

Toxicity Turbidity is a measure of the amount of suspended material in a liquid. In stormwater or a natural waterbody (e.g., river, lake, or estuary), turbidity depends on the amount of suspended sediment, dissolved organic matter, and plankton in the water. Turbid stormwater entering a natural waterbody can significantly degrade the habitat of fish and other aquatic life. Reductions in light levels may reduce submerged aquatic vegetation that provides the cover necessary for survival of the prey species. Or reduced visibility may make it difficult for predators to find evasive prey. Gravel on the bottom of a riverbed, which is necessary for salmon to spawn successfully, may be covered with sediments. Often it’s not just a few species but the whole food chain that’s affected. One of the references on page 7 (Meager, 2013) is an article for non-scientists on how turbidity affects the growth, reproduction, and survival of fish. Another reference (Meager, 2006) lists over 185 technical publications, which thoroughly document the toxic effects of stormwater turbidity on aquatic life.

The instrument used to measure the turbidity of a liquid is called a nephelometer. It works by passing a light beam (source beam) through a sample of the liquid and then measuring the light scattered by the suspended particles with a light detector set to the side (often 90°) from the source beam. The particle density is a function of the light scattered toward the detector by the suspended particles in the liquid. The units of turbidity measured by a calibrated nephelometer are called Nephelometric Turbidity Units (NTU). Contractors can use a hand-held nephelometer to measure the turbidity of their construction site’s stormwater runoff.


Polymer Flocculation for Reducing Stormwater Turbidity and Its Aquatic Life Toxicity

Flocculation is the process where a chemical agent (flocculant) is used to reduce the turbidity of a liquid by binding suspended particles in the liquid together to form larger particles (flocs) that are heavy enough to settle to the bottom of the liquid. When the liquid is stormwater runoff, this particle binding and settling process reduces soil erosion and the runoff’s turbidity, as well as the aquatic life toxicity associated with turbidity. Some polymers are good flocculants. Polymers are chemical compounds that have very large molecules composed of one or more structural units that are joined together in a repeating pattern to form long chain-like macromolecules. The two red wavy ribbons in Figure 1 represent polymer molecules dissolved in water, and the brown circles represent suspended soil particles. Cationic polymer molecules have positive charges, and many soil particles (particularly clays) have negative charges. The negatively charged soil particles are attracted to the positively charged polymer molecules, and this causes the soil particles to bind with the polymer chains as shown in Figure 1. Many of the soil particles form ionic bridges between the polymer chains, and some bind to the outside of the polymer chains. This binding process continues until many thousands of polymer chains and soil particles combine to form a floc having sufficient mass to settle to the bottom, thereby reducing the water’s turbidity. Although cationic polymers are effective flocculants and do reduce turbidity, their positive charges make them toxic to aquatic organisms when dissolved in water. Consequently they should not be used as flocculants in stormwater that runs off the land into natural waterbodies. However, anionic polymers, which carry a negative charge, are not toxic. If they’re added to stormwater along with some positive ions, the soil particles will bind onto these anionic polymer molecules and form the ionic bridges shown in Figure 2. Adding positive calcium ions (Ca++) to the anionic polymer enables anionic polymer flocculation, which can reduce the turbidity without harming the aquatic life


Downloadable Files

US EPA Polymer Flocculation Handout

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