1. Summary
Water is a renewable resource that circulates continually between the atmosphere and the Earth’s surface.
The energy for the hydrologic cycle is provided by the sun.
There are two basic kinds of aquifers: unconfined (receiving water from the top) and confined (between two impervious layers and under pressure).
The way in which land is used has a significant impact on rates of evaporation, runoff, and infiltration.

2. Summary
The four human uses of water are domestic, agricultural, in stream, and industrial.
Water use is measured by either the amount withdrawn or the amount consumed.
Domestic water is in short supply in many metropolitan areas.
The largest consumptive use of water is for agricultural irrigation.
Major sources of water pollution are municipal sewage, industrial wastes, and agricultural runoff.

3. Summary
Organic matter in water requires oxygen for its decomposition and therefore has a large biochemical oxygen demand (BOD).
Point sources of pollution are easy to identify and resolve.
Nonpoint sources of pollution are more difficult to detect and control.
Thermal pollution occurs when an industry returns heated water to its source. Temperature changes in water can alter the kinds and numbers of plants and animals that live in it.

4. Summary Wastewater treatment consists of:
Primary treatment, a physical settling process;
Secondary treatment, biological degradation of the wastes;
Tertiary treatment, chemical treatment to remove certain components.
Groundwater pollution comes from a variety of sources.
Reduced water quality can seriously threaten land use and in-place water use.