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California Water Blog

US Mayors – Public Infrastructure Budgets: Increasingly Growing Blue


“Impacts of Runoff:

The impacts of runoff vary:

  1. The combination of frequent and intense storm events and impervious areas brings higher risks of flooding.With higher flow rates this urban runoff will demand higher capacities from the local storm drain system.  If these capacities are deficient, flooding with attendant property losses may occur.
  2. Runoff that is directed to storm drains has minimal chance for infiltration into groundwater aquifers, limiting local water supplies and storage potentials.
  3. High flow rates of runoff increase erosion, as well as the risks of sediments moving to new locations in the local watershed.
  4. As runoff volumes increase the potentials for natural water filtration diminish. These natural processes occur as water is filtered through sediments or soil particles or exposed to microbes.

Pollution Sources and Land Use:

Besides the effect on flow, land use directly affects water quality in many other ways. To understand these effects we need to differential between point source pollution and nonpoint source pollution.

Point source pollution refers to contaminants that enter a watershed usually through a pipe. The location of the end of that pipe is documented and the flow out of that pipe is subject to a discharge permit issued by a Regional Water Quality Control Board. Examples of point source pollution are discharges from sewage treatment plants (the wastewater is treated but under the terms of its permit this water still has permissible levels of pollutants in the discharge) and industrial facilities. Because point sources are much easier to regulate than nonpoint sources they were the initial focus of the 1972 Clean Water Act. Regulation of point sources since then has dramatically improved the water quality of many rivers and streams throughout the country.

Unlike point source pollution, nonpoint source pollution, also known as “polluted runoff,” has a defused identity. Nonpoint pollution comes from everywhere in a community and is significantly influenced by land uses. A driveway or the road in front of a home may be sources of pollution if spilled oil, leaves, pet waste or other contaminants leave the site and runoff into a storm drain. Nonpoint source pollution is now the major water quality problem in the U.S.

Common nonpoint source pollutants in urban areas are sediment, pathogens, nutrients, oxygen-demanding substances, heavy metals, and oil and other petroleum products:

  1. Sediment is a frequent pollutant associated with development activities. It affects  aquatic  life,  shortens reservoir life, and complicates water treatment. Its sources are agricultural land erosion, construction sites, washoff from streets and other impervious areas, and streambank erosion.
  2. Pathogens include E. coli (a bacteria used to indicate the presence of fecal waste) and other viruses, bacteria, and protozoa. The source of most pathogens is fecal material from any warm-blooded animal.  In rural or agricultural areas, sources include wildlife, livestock manure, and malfunctioning septic systems. In urban areas the major sources are pet wastes, wildlife that may be present in high numbers (such as birds), septic systems in unsewered areas, and sewage treatment plant discharges (which are considered a point source).
  3. Nutrients of concern are primarily nitrogen and phosphorus. High concentrations of nitrate in drinking water are toxic to infants and may be harmful to pregnant women. Phosphorus leads to overproduction of algae that clog lakes and reservoirs. Sources of nutrients in agricultural areas include fertilizer, livestock manure, and septic systems. Sources of nutrients in urban areas are fertilizer used on lawns, gardens, and golf courses; pet waste runoff; and discharge from sewage treatment plants or industry.
  4. Pesticides can be a concern in drinking water supplies that use surface water. Sources of pesticides are simpler to identify than sources of pathogens or nutrients. They are limited to pesticide application, either in agricultural or urban areas. Studies show that pesticides like diazinon, an insecticide for lawns and gardens, are found frequently in urban areas.
  5. Oxygen-demanding substances consist of organic matter that depletes dissolved oxygen when decomposed by microorganisms. Dissolved oxygen is critical to maintaining water quality and aquatic life. Urban runoff with high concentrations of decaying organic matter (such as leaves, grass clippings, and other organic debris) can severely depress dissolved oxygen levels after storm events, impairing the water quality on which plants and fish depend.
  6. Metals include lead, copper, cadmium, zinc, mercury, and chromium. They can accumulate in fish tissues and affect sensitive animal and plant species. Sources of metals are automobiles (copper is lost from brake pads, for example), industrial activities, illicit sewage connections, and atmospheric deposition (for example, mercury that is released into the air from combustion and then falls to earth in rainfall at another location).
  7. Oil and other petroleum products degrade the appearance of water surfaces, impair fish habitats, and may be toxic to sensitive species. Sources are oil leaks; auto emissions coming off parking lots, roads, and driveway; and improper disposal of waste oil. Concentrations of petroleum-based hydrocarbons are often high enough to kill aquatic organisms.

Imperviousness and Water Quality:

Buildings, roads, sidewalks, and other impervious surfaces define the urban/suburban landscape. Impervious surfaces alter the natural hydrology and prevent the infiltration of water into the ground. Impervious surfaces change the flow of stormwater over the landscape. In undeveloped areas, vegetation holds down soil, slows the flow of stormwater over land, and filters out some pollutants, by both the slowing the flow of the water and trapping some pollutants in the root system. In addition, some of the stormwater filters down through the soil, replenishing groundwater sources.

As land is converted to other uses such as commercial developments, many of these natural processes are eliminated as vegetation is cleared and soil paved over. As more impervious surface coverage is added to the landscape, more stormwater flows faster off the land. The greater volume of stormwater increases the possibility of flooding, and the high flow rates of the stormwater does not allow for pollutants to settle out, meaning that more pollution gets concentrated in the stormwater runoff.

Research on urban stream protection finds that stream degradation occurs at relatively low levels of imperviousness of 10% to 20%. Wetlands suffer impairment when impervious surface coverage surpasses 10%. Fish habitat, spawning, and diversity suffers when imperviousness is greater than 10% to 12%. Wetland plants and amphibian populations diminish when impervious surfaces are higher than 10%. The higher the percent impervious surface coverage becomes, the greater the degradation in stream water quality tends to be. Based on this research, streams can be considered stressed in watersheds where the impervious coverage exceeds 10% to 15%.

The link between impervious surfaces and degraded water quality argues for careful comparisons between dispersed and compact development strategies. On a regional or watershed level, greater overall water quality protection is achieved through more concentrated or clustered development. A clustered approach will decrease the overall impervious cover, resulting in greater protection for the overall watershed, as a much larger percentage of the watershed will be left in its natural condition, preserving water quality. In addition, such centralized development can be directed away from sensitive areas such as stream banks to minimize the negative impact on water quality.”

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