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CMP Part 3A. Environmental Protection Plan <br /> Oxygen-demanding substances introduced by urban runoff threaten receiving <br /> water bodies with oxygen depletion resulting from biological and chemical <br /> degradation of oxidizable material. Oxygen demand is best represented through <br /> chemical oxygen demand (COD), but biochemical oxygen demand (BOD) has <br /> historically been used as the most convenient means of reporting demand. BOD <br /> and COD concentrations occurring in stormwater runoff are approximately <br /> equal to those of secondary treatment effluent. The degree to which oxygen is <br /> lost as a result of this demand depends upon the amount of degradable material <br /> in the starmwater, benthic oxygen demands and the physical nature of flow in <br /> the receiving water body. <br /> Nutrient (nitrogen and phosphorus) input resulting from urban runoff is <br /> critical, since it becomes a contributing factor in eutrophication of <br /> downstream inactive waters. Nutrient input levels are generally less than <br /> inputs from municipal treatrnent sources, but nevertheless are significant to the <br /> tota] water quality condition. Phosphorus input to water bodies, particularly <br /> lakes, has in most cases been found to be the limiting element in determining <br /> algal productivity. Nutrient character in the storm water runoff is highly <br /> dependent upon oxygen conditions, pH and physical conditions such as <br /> sediment and organic content in the water. <br /> Toxic heavy metal loading from urban runoff inerits attention as a potential <br /> nondegradable aid to stream deterioration. Metals are capable of reaching <br /> critical levels in quiet areas where they are able to accumulate in bottom <br /> sediments. Loading of heavy metals during a storm contributes a significant <br /> portion of the entire load to urban streams. <br /> Other constituents of urban runoff of concern, but of variable severity, include <br /> � bacteria, chlorides (C]) from road salt, oil and grease, pesticides and PCB's. <br /> These pollutants can be locally severe depending upon receiving water <br /> conditions and proximity to source areas. <br /> The major identified sources of urban stormwater pollution include: vehicular <br /> and industrial emissions and leakages, combined sewer overflows, skid control <br /> grit and deicing salts, street and construction litter, vegetation and animal <br /> � droppings, improperly applied pesticides, atmospheric fallout and precipitation <br /> and urban erosion. The largest inputs of urban pollutants within a developed <br /> area come from industrial land uses, with commercial and high density <br /> residential uses following for most pollutants. <br /> Once pollutants are present on the urban surface, how they are moved about, <br /> either suspended in the water or in solution, becomes very important for the <br /> purpose of controlling them. Characteristically, an urban runoff event consists <br /> of the washing-off of accumulated debris from a parking lot, rooftop, street,etc., <br /> rapidly enough to move sand, grit, leaves and the like and dissolve the soluble <br /> chemicals. Extreme stress on the assimilative ability of the receiving water body <br /> is generally caused by an increase in both pollutant concentration and water <br /> runoff volume as the storrn begins and overcomes normal watershed depression <br /> storage. The phenomenon of highest concentration during an event occurring on <br /> the rising limit of a hydrograph (runoff versus time) followed by �radual <br /> tapering-off is commonly called 'first flush'. <br /> City of Orono Community Management Plan 2008-2030 Page 3A-18 <br />