Water pollution occurs when harmful substances are introduced into bodies of water in high enough quantities to cause harm to organisms or humans. There are two main types of water pollution: point-source pollution which comes from a single location like a factory discharge pipe, and nonpoint-source pollution which comes from many scattered sources. Water resources can also be divided into surface waters like oceans, lakes and rivers, and groundwater stored underground in aquifers, both of which can become polluted. Reducing water pollution requires education on the issue, laws to regulate pollution, and economic incentives.