Aquifers form when precipitation (rain, snow) and water from rivers, lakes and streams seeps through porous surface rock and collects underground. After years and years of accumulation enough water can accumulate to be welled and provide a water source. Another benefit of aquifers is that, depending on the porous surface rock material, the water is naturally filtered and often suitable for direct drinking. This is also why it is dangerous to dump chemicals or use fertilizers around aquifers because those chemicals will eventually seep into the groundwater and contaminate the water source.