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<br />pollutional characteristics. Higher runoff concentrations have <br /> <br /> <br />been predicted (EPA, 1974) under the following conditions: 1) <br /> <br /> <br />the early stages of a storm, including first-flush effects, 2) in <br /> <br /> <br />response to intense rainfall periods, 3) after prolonged dry <br /> <br /> <br />periods, 4) in areas of construction activities, and 4) in densely <br /> <br /> <br />settled, highly paved or industrial areas. <br /> <br /> <br />The early portion of a storm tends to dissolve and suspend <br /> <br /> <br />the pOllutional materials that have collected on roofs, lawns, <br /> <br /> <br />and streets. For this reason, a concentration peak for some <br /> <br /> <br />pollutant arrives at the storm sewer outlet ahead of the flow <br /> <br /> <br />peak produced by a rainstorm. This has been termed the first- <br /> <br /> <br />flush effect. It is usually more dominant in combined sewers <br /> <br /> <br />than with separate storm sewers due to the scouring of solids in <br /> <br /> <br />the combined sewer system (Lager and Smith, 1974). <br /> <br /> <br />Weibel, Anderson and Woodward (1964) in a study in Cincinnati, <br /> <br /> <br />Ohio reported average concentrations of various pollutants for <br /> <br /> <br />different time intervals of runoff. These data have been sum- <br /> <br /> <br />marized in Table 11, and include data obtained for all flow <br /> <br /> <br />conditions and seasonal variations. The first-flush effect is <br /> <br /> <br />shown clearly for its impact on constituent concentrations. <br /> <br /> <br />Burn, Kranczyle, and Harlow (1968) also reported average <br /> <br /> <br />annual discharge concentrations at various time increments for <br /> <br /> <br />storm sewer discharges and the "first-flush" effect was evident, <br /> <br /> <br />especially in the case of the different solids parameters. <br /> <br />24 <br /> <br />. <br /> <br />, <br />