Water Education - Water Quality

The Quality Difference between Water Supplies in Urban Area and Rural Area - Page 2

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Major categories of urban pollutants include sediments, nutrients, microbes, and toxic metals and organics. Sediment concentrations in urban runoff are particularly problematic because of their ubiquitous nature, and the fact that many other pollutants occur in a solid-state associated with sediment particles. Sediment loadings occur primarily from soil erosion and runoff from construction sites in urban areas. Road sanding can also be a major source of sediments. Increased sediment in urban runoff may cause significant biological, chemical, and physical changes in receiving waters including loss of water clarity, clogging of gills and filters of aquatic organisms, and aquatic habitat degradation including the smothering of spawning beds and benthic communities. Sources of nutrients such as nitrates and phosphates include chemical fertilizers applied to landscaped areas, lawns, and gardens, failed septic systems, soil erosion, and atmospheric deposition. Excessive nutrients in urban runoff can stimulate algal growth and cause nuisance algal blooms.

Urban runoff may also contain high levels of organic matter that can lead to depleted oxygen levels in water and sediment when it decomposes. This in turn may cause excessive odors and fish deaths in receiving waters. Microbes include hundreds of different kinds of bacteria, protozoa, and viruses that are ubiquitous in the natural environment. Many are beneficial, while others can cause diseases in aquatic biota, and illness or even death in humans. Some types of microbes are pathogenic (e.g., Giardia spp.), while others indicate a potential risk for water contamination (e.g., fecal coliform bacteria) and may limit swimming, boating, and consumption of fish and shellfish in receiving waters. Microbes are almost always found in high concentrations in urban stormwater, but are highly variable in nature and very difficult to eliminate.

Primary sources of microbes include failed septic systems, and waste products from pets, birds, and wild mammals commonly found in urban areas. Toxic pollutants commonly found in urban runoff include trace metals such as lead, copper, zinc, and organic compounds including oils, grease, phthalates, and chlorinated hydrocarbons. Copper, lead, and zinc were detected in more than 90% of stormwater samples from the NURP studies, and 14 toxic organic compounds were detected in more than 10% of the samples (U.S. EPA, 1983). Sources of toxics include the breakdown of metal products, vehicle fuels and fluids, vehicle wear, industrial processes, and the use of industrial and household chemicals such as paints, preservatives, and pesticides. Trace metals and organic compounds may be highly toxic to aquatic organisms and can bioaccumulate in fish and shellfish.

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