01-24-2000 Council Work Session - Laserfiche WebLink

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precipitation and urban erosion. The largest inputs of urban pollutants within a developed area come from industrial land uses, with commercial and high density residential uses following for most pollutants. Once pollutants are present on the urban surface, how they are moved about, either suspended in the water or in solution, becomes very important for the purpose of controlling them. Characteristically, an urban runoff event consists of the washing-off of accumulated debris from a parking lot. rooftop, street, etc., rapidly enough to move sand. grit, leaves and the like and dissolve the soluble chemicals. Extreme stress on the assimilative ability of the receiving water body is generally caused by an increase in both pollutant concentration and water runoff volume as the storm begins and overcomes normal watershed depression storage. The phenomenon of highest concentration during an event occurring on the rising limit of a hydrograph (runoff versus time) followed by gradual tapering-off is commonly called 'first flush’. The spring thaw of snow and ice can have an impact as great or greater than rain. Pollutants trapped by ice and snow are released together with the melt-water to produce a flush effect when temperatures rise enough to melt significant amounts of ice and snow. Other factors affecting pollutant migration include the storm pattern; physical factors such as soil type, land slope, and type of vegetation ground cover; infiltration into drainage systems; and amount of temporary storage available in the watershed. The worst time for an urban stormwater runoff to occur is during a period of low flow, high temperature and low wind, when design loads for point source inputs have not taken into account nonpoint inputs. These conditions overstress the ability of the stream or lake to assimilate the pollutants. Pollutants behave in various ways during stormwater runoff events. A typical storm would contribute extremely variable amounts of sediment. The laigest amount of sediment per volume of runoff w ould follow a first flush behavior, that is it would correspond w ith the greatest flow of water and then diminish. Associated with, and often adhered to this sediment are several harmful pollutants, including metals, nutrients, bacteria, pesticides, and PCB’s. Pollutants that adsorb to sediment generally tend to adsorb to the fine fraction, thus increasing mobility and management difficulties. The question of scale between individual runoff parameters becomes important in analyzing the effects of urban runoff. For pollutants such as COD and bacteria, the immediate areas downstream must be considered critical soon after an event. Pollutants such as metals and nutrients, how ever, migrate far downstream and have long-lasting effects." I I I 1 1 I 1 n fc-» CMP3A- 15