An aquifer is a porous rock or sediment layer filled with groundwater. This groundwater originates from precipitation that filters through the soil. Wells are drilled into aquifers to access this water. Depending on the surrounding land use, contaminants such as animal manure, pesticides, and herbicides from nearby farms can infiltrate the aquifer, even if they're located miles away. Similarly, factories in the vicinity may leach hazardous heavy metals and other pollutants into the groundwater. Additionally, various bacteria inhabit well water, further emphasizing the importance of water quality testing and filtration.