2.2 The Water Cycle and Oxygen-Demanding Waste

properties of water

composition of water molecules

• water molecules are cohesive, meaning that there are strong forces of attraction between molecules of water

water will remain a liquid over a wide variety of temperatures

• it has a high boiling point and a low freezing point
• it also has a high heat capacity, so liquid water changes temperature very slowly; it can store large amounts of heat without a change in temperature
• it has a high heat of vaporization, so it takes a lot of heat to evaporate liquid water
• water is the universal solvent and can dissolve a variety of compounds (eg. nutrients, waste, pollutants)
• has high surface tension, meaning that its surface behaves like a membrane (its surface is often similar to that of a solid)
• ice is less dense than liquid water — water expands when it freezes

causes for concern with water

• too much (flooding, eg. Pakistan)
• too little (droughts, eg. California)
• too poor of quality (contaminated, eg. Flint)

the water cycle

• the vast majority of water on earth is saltwater; of the freshwater, most of it is located in glaciers/permanent snow cover and groundwater

oxygen-demanding waste

• oxygen demanding waste: organic waste that can be decomposed by aerobic bacteria
  • aerobic bacteria: bacteria that use oxygen
  • because the bacteria use so much oxygen, the level of oxygen dissolved (DO) in the water decreases
  • fish and other animals need high levels of DO to survive
• origins of oxygen demanding waste
  • main cause — failing infrastructure
  • eg. leaking pipes, failing/overflowing sewage systems
  • results in the release of oxygen demanding waste, eg. raw sewage, manure, decomposing algae, paper factories, meat-processing plants

streams

• stream: a body of water that flows
  • eg. rivers, creeks
• usually recover quickly from oxygen demanding waste, but if they are overloaded, the breakdown of degradable waste by bacteria depletes DO and produces an oxygen sag
  • oxygen sag: the reduction in dissolved oxygen plotted over a distance along a water body from a point at which sewage or other pollutants have been discharged
• biochemical oxygen demand (BOD): the amount of oxygen consumed by bacteria while they decompose organic matter

sediment

• a pollutant
• sediment/suspended matter: insoluble soil particles which make water turbid
  • turbid: cloudy, opaque, or thick with suspended matter

\