Water is a vital resource that sustains life. Freshwater sources include groundwater, which infiltrates underground, and surface water. Groundwater depletion can occur when withdrawals exceed recharge, causing water tables to fall and land to subside. Increasing supplies involves desalination or reverse osmosis. Water pollution reduces water quality and harms organisms. Major pollutants include pathogens, nutrients, chemicals, sediments and heat. Pollution can be from point sources like factories or nonpoint sources like runoff. Treating sewage reduces pollution levels. Large-scale water diversions for uses like irrigation and cities can deplete rivers and harm ecosystems.