Effect of Sewage Outfall on a Stream Ecosystem

Abstract

Human activity has affected profoundly streams and lakes in all parts of the world. Streams have been subjected to additions of gross amounts of domestic sewage, industrial effluents (e.g., wastes from tanneries, pulp mills, creameries, steel mills, and chemical factories), agricultural wastes, oil spills, mining wastes, urban runoff, radioactive materials, pesticides, waste heat, and numerous other pollutants, often because it was considered expedient and economical to have the unwanted materials carried away (“out of sight”) by the flowing water. Likewise, under the guise of “progress,” streams have been channelized, stabilized, dewatered (for irrigation), and super-watered (artificially increased flow for drinking and power plant needs). In most cases, the effects on the aquatic biota are insidiously cumulative. In some cases, the effects are readily apparent [e.g., acid mine drainage; see Parsons (1968)], but in others the effects accumulate more slowly [e.g., accumulations of trace metals; see Whitton and Say (1975)]. In all cases, a longitudinal gradient develops below the point of insult and, given enough time (distance) without further insult, the stream ecosystem generally recovers to a state of well-being.