The document discusses water resources and water treatment. It explains that most of Earth's water is ocean water, while freshwater comes from surface water, groundwater, and the water cycle. It then describes how surface water flows in rivers and collects in lakes and reservoirs, and how groundwater flows through permeable rock layers and can be accessed through wells. The document also outlines water use for residential, industrial, and agricultural purposes, as well as problems like overdraft and habitat degradation. It concludes by explaining processes for desalination and water purification to remove salts and contaminants, making water safe for drinking.