Effects of Illicit Discharge


An illicit discharge is any flow or dumping to a municipal separate storm sewer of any substance that is not composed entirely of stormwater; exceptions are non-stormwater discharges allowed by the NPDES Permit and discharges resulting from emergency firefighting activities. One goal of the stormwater permit is to eliminate illicit discharges. 


Some pollutants that may or may not be easily identifiable include:

Bacteria: High bacteria levels may be found in stormwater runoff as a result of leaking systems, garbage, pet waste, etc. The impacts of bacterial on surface waters may pose health risks, affect recreation uses, and harm aquatic life.

Nutrients & Organic Matter: Nutrients most often identified in stormwater runoff are phosphorus and nitrogen. In surface waters, these nutrient loads can lead to heavy algae growth, euthrophication (an overabundance of nutritional salts in water, which in turn leads to ecological instability) and low dissolved oxygen levels.
Nutrients enter the storm sewer system in a variety of ways, including but not limited to landscaping particles (commercial and residential), car washing detergents and animal wastes. Decomposition of organic matter by organisms in surface waters results in depleted oxygen levels. Low levels of dissolved oxygen can impact water quality and life within surface waters. Sources of organic matter include leaking septic systems, garbage, yard wastes, etc.

Oil & Grease: Numerous activities release oil, grease, and lubricating agents that are readily transported by stormwater.
The intensity of activities,  within the City all contribute the majority of these pollutants present in adjacent surface waters including: 

  • Leaks and spills
  • Maintenance and fueling activities
  • Manufacturing processes
  • Vehicle traffic

Sediment: This is often the largest and most obvious pollutant load associated with stormwater runoff.

Sediment discharges have been shown to be exceptionally high around construction activity. Sediment is associated with numerous impacts to surface waters, including increased turbidity, effects on aquatic and benthic habitat, and reduction in capacity of impoundments. A number of other pollutants often attach to and are carried by sediment particles.

Toxic Substances Including Heavy Metals: Many toxic substances potentially associated with stormwater include metals, pesticides, herbicides, and hydrocarbons.

Toxic compounds may affect biological systems, and accumulate in bottom sediments of surface waters. Heavy metals may be typically found in urban stormwater runoff. Some of these metals include:

  • Arsenic
  • Cadmium
  • Chromium
  • Cooper
  • Lead
  • Zinc

Metals in stormwater may be toxic to some aquatic life and may accumulate in aquatic animals.


Stormwater runoff generally increases in temperature as it flows over impervious surfaces. In addition, water stored in shallow, unshaded ponds and impoundments can increase in temperature. Removal of natural vegetation (such as tree canopy) opens up water bodies to direct solar radiation. Elevated water temperatures can impact a water body's ability to hold dissolved oxygen to support fish and other aquatic organisms.