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Water Quality

Urban Runoff

Introduction > Source Control BMPs > Treatment BMPs > References


Introduction

Contaminated stormwater runoff is recognized as a leading source of water quality problems in urban settings and often the hydrologic impacts from urbanization can be more harmful than the pollutants it carries (MELP [now WLAP], 1992). In general, urbanization can change the local hydrologic cycle by decreasing:

  • the degree of infiltration and increasing the volume of runoff;
  • the amount of depression storage (i.e., puddles) due to regrading;
  • the amount of evapotranspiration due to vegetation removal; and,
  • the travel time of stormwater to receiving waters because of efficient sewer systems (WEF, 1998).

Occurring in conjunction with these hydrologic changes is the addition of potential contaminants to stormwater resulting from everyday urban living. These contaminants are transported over and through the landscape with stormwater and deposited in nearby waterbodies. Identifying the causes of water quality problems in urban watersheds is difficult because of the many potential pollution sources and many different land use activities. Therefore, a comprehensive watershed management approach is needed in order to address problems in a cost effective manner. The management of urban surface runoff is most successful when undertaken on a watershed basis including issues of flood control, erosion control and pollution control. Flood and erosion control are based on the reduction of peak discharges caused by large storm events. Pollution control, on the other hand, is based on treating runoff from the smaller more frequent storms (MELP, 1992).

Best management practices for urban stormwater include source control and treatment (or structural) approaches. Source control BMPs involve reducing the production of pollutants or minimizing the contact between pollutants and stormwater runoff. Treatment BMPs involve the physical removal of contaminants from stormwater prior to its release to a waterbody. It is important to note that many treatment options require careful site assessment prior to design and ongoing maintenance to ensure effectiveness.

This section provides an overview of both source and treatment BMP approaches to addressing non-point source pollution from urban stormwater. Many of the contributing sources of pollutants in contaminated urban runoff may be industrial; these are not covered in this section, but are under the Industrial Best Management Practices section. The information presented here is taken from a number of sources including the Greater Vancouver Sewerage and Drainage District’s (GVRD) Best Management Practices Guide for Stormwater (1999), MELP’s Urban Runoff Quality Control Guidelines for British Columbia (1992) and Environmental Objectives, Best Management Practices and Requirements for the Review of Land Development Proposals (in draft) and the Capital Regional Districts’ Guide to Existing Best Management Practices and Stormwater Program Development. All of these documents are excellent sources of information.

The US Environmental Protection Agency and the American Society of Civil Engineers are currently developing a database on the performance and effectiveness of individual BMPs. The primary focus of this work will be to determine the efficiency of BMPs and BMP systems and clarify relationships between design and efficiency.

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Common Contaminants on Roadways

Pollutants commonly detected in urban runoff which may be harmful to receiving waters include suspended solids, oxygen demanding substances, toxic metals and trace elements, organic contaminants, nutrients, and pathogenic bacteria. Other constituents and characteristics which may affect the behaviour and fate of the pollutants in urban runoff include major ions such as sodium, chloride, calcium, magnesium and potassium, and the alkalinity, hardness, pH, salinity and temperature of the runoff water (MELP, 1992).

The following is a partial list of contaminants that may be expected in urban stormwater:

  • asbestos from brake linings and clutch linings
  • bacteria from animals and birds, soils, litter, livestock hauling, livestock waste hauling and onsite sewage tanks and fields
  • bromide from auto exhaust
  • cadmium from tire fillers and insecticides
  • chloride from road salts
  • chromium from moving engine parts and brake linings
  • copper from bearing and bushing wear, moving engine parts, brake linings and radiator repair
  • cyanide from de-icing road salts
  • pesticides (fungicides, herbicides and insecticides) from roadside maintenance
  • iron from autobodies, moving engine parts, bridges, guardrails, overpasses, lamp standards, and other structures
  • lead from gasoline, tire fillers, lubricating oil and grease, bearing wear and automotive and radiator repair
  • manganese from moving engine parts and gasoline additives
  • nickel from diesel fuel, lubricating oil, bushing wear, brake linings and asphalt paving
  • nitrogen from the atmosphere, animal wastes, onsite sewage systems, vegetative matter and fertilizers
  • particulates from pavement wear, vehicles, the atmosphere and road maintenance
  • PAHs from automobiles and pesticides
  • PCBs from pesticides, atmospheric deposition and tire catalyst
  • petroleum from paving, fuels spills, engine blow-by, lubricant leaks, antifreeze and hydraulic fluids
  • phosphorus from the atmosphere, animal wastes, onsite sewage systems, vegetative matter and fertilizers
  • potassium from the atmosphere and fertilizers
  • rubber from tire wear
  • sediments from construction sites, stream channel erosion, poorly vegetated lands and motor vehicles
  • sulphate from roadbeds, road salts and fuels
  • zinc from tire fillers, motor oil additives, automotive and radiator repairs, grease and paint manufacturing



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