Growth Cycles

Point source pollution

Stream

Point source pollution

Septic zone

Clean zone

Zone of recent pollution

Septic zone

Recovery zone

Clean zone

DO normal

DO normal

Dissolved oxygen

DO decreases

(DO) curve

Figure 8.16 Stream zones of pollution.

Zone 1. Degradation

• Wastewater enters the body of water.

• Solids begin to settle, forming sludge banks on the bottom.

• Dissolved oxygen levels in the stream decrease rapidly.

• Water takes on the characteristic color of the wastes.

• Fish population decreases rapidly.

• Bacterial population increases rapidly. Zone 2. Active decomposition

• Concentrations of bacteria and other sewage-related organisms are high.

• Odor is rotten egg odor of hydrogen sulfide.

Note: This zone may not occur if the oxygen demand of the waste discharged does not exceed the aeration rate of the body of water.

Zone 3. Recovery

• Oxygen level begins to increase rapidly.

• Color begins to return to normal.

• Fish population increases.

• Bacterial/microorganism population decreases.

Zone 4. Clean water

• Oxygen levels are at or near saturation.

• Fish populations have returned to normal.

• Color and odor are returning to normal.

Note: The self-purification process removes solids that can settle and organic materials that can be removed by biological activity. It will not remove toxic materials, and it will not remove organic matter when toxic material is present until dilution reduces the concentration of the toxic material enough to eliminate the toxic effect. The process does not remove: (1) disease-causing organisms, (2) dissolved inorganic solids, (3) toxic material, or (4) inorganic dyes. The process can take a long time to complete (up to 30 river miles or more), and the condition of the stream can return to degradation or decomposition due to the addition of more wastes before the process is complete.

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