Impacts. Metals in stormwater are toxic to many forms of life. In extreme situations, metals can contaminate public water supplies. Since metals can accumulate in aquatic animals, toxic amounts can move up the food chain, eventually affecting humans who consume fish or shellfish with high levels of accumulated metals. The metals of most concern in stormwater are lead, copper, cadmium, chromium and zinc; however, many other metals have been identified in stormwater.

Generally, the concentrations of metals are too low to pose a threat to human health, but the concentrations regularly exceed recommended levels for protection of aquatic life. Lead concentrations in certain metropolitan areas have dropped markedly in recent time, possibly due to the use of unleaded gasoline. This reduction is very important since recent toxicological research has shown that lead is much more toxic than was originally thought.

Most metals in stormwater attach to sediment and settle to the bottom, thus limiting toxicity to aquatic life. However, metals can be redissolved into the water by changes in the water chemistry or biological activity, or resuspended by high incoming flows, thus posing a threat of future contamination.

Indicator parameters. Low or high pH of the stormwater may release metals into stormwater.

Source reduction. Motor oil should be taken to a proper disposal facility and oil leaks in vehicles should be repaired. Industrial and commercial activities should not dump waste solutions and other chemicals into storm sewers or other improper disposal areas. All illegal dumping and spills caused by residents, industries and commercial operations should be reported to prevent major contamination of water bodies. Facilities which store dead automobile batteries should prevent leaking acid and lead from entering runoff.

Removal. In many cases, metal pollutants can be removed through settling in dry detention basins or wet retention ponds because most metals are attached to sediment particles. Sand filters and infiltration techniques will also remove metals attached to sediment by filtering the stormwater. However, if the metal is dissolved in the storm-water, chemical treatment of the runoff may be needed.

Return to: Surface Water Pollutants and Stormwater Runoff