Polluted Runoff

A variety of water pollutants originate from roads and parking lots including hydrocarbons (e.g. gasoline), heavy metals (e.g. zinc, lead, and copper), nutrients from roadside fertilizers (e.g. nitrates and phosphates), and salts for de-icing.

The principle contaminants in stormwater, as outlined in Stormwater Strategies: Community Responses to Runoff Pollution, include:

Category Examples
Metals zinc, cadmium, copper, chromium, arsenic, lead
Organic chemicals pesticides, oil, gasoline, grease
Pathogens viruses, bacteria, protozoa
Nutrients nitrogen, phosphorus
Biochemical oxygen demand (BOD) grass clippings, fallen leaves, hydrocarbons, human, and animal waste
Sediment sand, soil, and silt
Salts sodium chloride, calcium chloride

 

Hydrocarbons

Hydrocarbons include gasoline, diesel, benzene, and other products.  While smog and large-scale oil spills often highlight the negative consequences of hydrocarbon pollution, small-scale motor vehicle leaks can also be detrimental to aquatic and soil environments.  Estimates of as much as 20,000 gallons/year of residual oil have been found in runoff from one square mile of urban roads and parking lots.*  Runoff of oil and gasoline contains heavy metals (e.g. copper, cadmium, lead, zinc, and nickel) which can accumulate in the food chain and lead to illness, including cancer.

Streets often drain to valuable water sources including rivers, lakes, wetlands, or estuaries.

Heavy Metals

As many as 90 different types of metals can be found in road runoff, with zinc, copper, and lead being the most common.  Lead is diminishing due to the elimination of leaded gasoline, however copper is common from wear and tear on brakes and zinc is prevalent from tire degradation.  Metals are generally positively charged which means that they bind easily to negatively charged clay particles.  This bond limits the solubility of metals, but it can mean that they are carried away in runoff.  Ultimately metals can end up as sediment in streams or lakes where they are in contact with aquatic organisms.  Over time at the bottom of water body, or in more acidic conditions, metals become more soluble, which means that they can be absorbed by aquatic organisms and passed through the food chain.  The accumulation of heavy metals by organisms can lead to sickness or even death.

Nutrients

Nitrates and phosphates are common fertilizers used to improve plant growth.  These nutrients can pose serious problems however, if high quantities are allowed to wash into streams.  Excess nitrates or phosphates can lead to algal blooms in rivers.  When the algae dies, bacteria assist to decompose the algae, but consume oxygen in the process.  If too much oxygen is used, it can be fatal to other aquatic organisms such as fish.

Salts

Salt is used for de-icing roads because it lowers the melting point of water.  When ice and snow melts, the runoff is often concentrated in gutters and storm drains before being discharged into arroyos or streams.  Salt can infiltrate and contaminate groundwater at these sites.  It may also negatively impact the growth of plants or reproduction of aquatic organisms.