Methods for Distinguishing between Single and Multiple Discharger Situations

Abstract

Designing programs to control pollutant discharges into streams is easier if the areas of water quality degradation from adjacent discharges do not overlap, so that each discharge may be treated as a single entity. Conservative pollutants accumulate in a stream but decaying pollutants from one source may be of insignificant concentration at the point where the next downstream source is introduced. There may be greater overlap of areas of degraded water quality at high streamflows than at low ones. In assessing whether discharges may be treated separately, all streamflow conditions should be considered. This paper develops and presents a simple set of techniques based on dimensionless variables that may be used to determine whether adjacent discharges of nonconservative pollutants may be regarded as independent. The application of these techniques is demonstrated for an example problem. It is shown, for example, that for dissolved oxygen (DO) deficit, if the reaeration coefficient exceeds the decay coefficient and the residence time between adjacent dischargers is greater than 10 times the reciprocal of the decay coefficient, there is no significant overlap between adjacent dischargers at any streamflow.