Một khi bị ô nhiễm, nước ngầm là khó khăn, nếu không phải không thể, để làm sạch, vì nó có chứa các vi khuẩn phân hủy vài và không tiếp xúc với ánh sáng mặt trời, dòng chảy nước mạnh, hoặc bất kỳ của các quá trình lọc tự nhiên khác làm sạch nước mặt. Eugene P. Odum; Sinh thái học và hỗ trợ cuộc sống bị đe dọa hệ thống của chúng tôi | Chapter 13 Groundwater Once polluted groundwater is difficult if not impossible to clean up since it contains few decomposing microbes and is not exposed to sunlight strong water flow or any of the other natural purification processes that cleanse surface water. Eugene P. Odum Ecology and Our Endangered Life Support Systems GROUNDWATER AND AQUIFERS Part of the precipitation that falls on land may infiltrate the surface percolate downward through the soil under the force of gravity and become what is known as groundwater. Like surface water groundwater is an extremely important part of the hydrologic cycle. Almost half of the people in the . obtain their public water supply from groundwater. Overall the . has more groundwater than surface water including the water in the Great Lakes. Unfortunately pumping it to the surface for use is sometimes uneconomical and in recent years the pollution of groundwater supplies from improper disposal has become a significant problem Spellman 1996 . Groundwater is found in saturated layers called aquifers that lie under the Earth s surface. Aquifers are made up of a combination of solid material such as rock and gravel and open spaces called pores. Regardless of the type of aquifer the groundwater in the aquifer is always in motion. Aquifers that lie just under the Earth s surface are in the zone of saturation and are called unconfined aquifers see Figure . The top of the zone of saturation is the water table. An unconfined aquifer is only contained on the bottom and is dependent on local precipitation for recharge. This type of aquifer is often referred to as a water table aquifer. The actual amount of water in an aquifer is dependent upon the amount of space available between the various grains of material that make up the aquifer. The amount of space available is called porosity. The ease of movement through an aquifer depends upon how well the pores are connected. The ability of an aquifer to pass water is .

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