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Managing Wastewater in Coastal Urban Areas
The engineering and scientific capability needed to account for these variations has developed significantly over the past 20 years.
Nutrients in Coastal Waters
Finding: Nutrient enrichment, primarily due to nitrogen, is an important problem in many estuarine and some coastal marine systems.
Recommendation:Greater attention should be focused on preventing excess regional enrichment of nitrogen and other nutrients at levels that are harmful to ecosystems.
Discussion: Nutrient enrichment can cause oxygen depletion, reduced fish and shellfish populations, nuisance algal blooms, and dieback of seagrasses and corals. While not known to be a problem along much of the open Pacific coast, excess nutrient enrichment, or eutrophication, is a persistent problem in many estuaries, bays, and semi-enclosed waterbodies along the Atlantic and Gulf coasts, and may even be of concern over a large scale in some more open areas along these coasts. Nitrogen controls primary production and eutrophication in most temperate estuaries and coastal waters, although phosphorus can be of concern in many tropical waters and perhaps in some temperate estuaries. By contrast, in freshwater systems, phosphorus is almost always the nutrient limiting growth. It may be important to keep nitrogen and phosphorus concentrations low relative to silicon to avoid causing nuisance algal blooms such as red and brown tides.
Both the sources of nutrients to coastal waters and the associated effects occur at the regional scale making them difficult to measure, assess, and manage. Nutrient inputs to coastal waters come from both point and diffuse sources including wastewater treatment plants, agricultural runoff, urban runoff, ground water seepage, atmospheric deposition, and release of previously accumulated nutrients from bottom sediments.
Source Control and Water Conservation
Reduction or elimination of pollutants at their sources is an effective tool for managing both point and diffuse sources. For example, for trace metals and toxic organics, source control is more efficient than removal at central plants, which may then have problems of safe disposal of large volumes of contaminated sludge.
Water conservation reduces the volume of sewage requiring collection and treatment, however, it does not change the total mass of wastewater pollutants; in fact, pollutant concentrations may actually be increased. The benefits of water conser-