Groundwater is part of the hydrologic cycle. Rain and snow fall on the land surface and infiltrate the soil or run off the land into streams, lakes, and oceans. Evaporation and transpiration (release of water through the leaves of plants) carry the water back into the atmosphere where it once again condenses and falls to earth.
The water, which doesn’t run off or is not evaporated or transpired, filters through the soil and becomes groundwater. Groundwater eventually surfaces at an area of discharge such as a spring, stream, lake, or wetlands where it can evaporate back into the atmosphere.
Water filtering down through the subsurface first moves through an unsaturated zone where the spaces (pores) between solid particles or rocks contain both air and water. Water in this zone is called soil water and some of it will be taken up by plants. The rest of the water continues moving downward and eventually reaches the zone of saturation where the pore spaces are completely filled with water. Water in this zone is called groundwater. The top of the zone of saturation is called the water table. Rock or soil layers within the zone that can readily store and transmit usable amounts of water are called aquifers. Aquifers may be as large as several states combined or as small as a few acres and may be found a few feet or hundreds of feet below the surface. The vertical thickness may vary from a few feet to hundreds of feet.
Aquifers are classified as either Consolidated or Unconsolidated aquifers. Consolidated aquifers occur within rocks such as limestone, sandstone, basalt, etc. The groundwater in these aquifers is present within the interconnected fractures, cracks, pore spaces, rock layers surfaces, and/or solution channel openings. Unconsolidated aquifers occur within uncemented earth material such as sand, gravel, silt, broken or weathered rocks, etc. The groundwater in these aquifers is present between the individual grains or rock particles.
How much water is contained in an aquifer depends on the type of aquifer, the number and size of openings, and how connected the openings are. For example, clay and silt may hold a lot of water but release it slowly because the openings between grains are very small or are not well connected (in the case of clay). On the other hand coarse-grained sand and gravel may hold less water but the water moves more freely because of the larger interconnected openings between the grains. The amount of water held and yielded by consolidated aquifers depends on the size of the rock’s openings and how well connected they are. In general, limestone aquifers yield substantial amounts of water, sandstone aquifers yield moderate amounts and granite aquifers yield smaller amounts.
Several different aquifers, separated by aquitards, can exist within the zone of saturation. Aquitards are lower permeable layers such as clay or nonporous rock that restricts the flow of water from one aquifer to another. An aquifer located below an aquitard is called a confined or artesian aquifer and is often under pressure. If such an aquifer is tapped with a well, the pressure forces the trapped water to rise in the well to an elevation higher than top of the aquifer. An aquifer with no aquitard above it is called an unconfined aquifer or water table aquifer. Water levels in wells penetrating this type of aquifer are the same as the water level in the aquifer (the water table).
Groundwater discharge and recharge
Groundwater, like surface water, is constantly on the move. However, groundwater moves much slower --at rates ranging from feet per day to inches per year depending on the type of soil or rock through which it is moving. The natural movement of groundwater is from upland recharge areas to lowland discharge areas. Discharge areas are points where the water table meets the land surface, such as rivers, springs, lakes, streams, and wetlands. Groundwater discharging into streams provides the water that helps to keep streams flowing year round. This is called base flow.
Many human activities can negatively affect groundwater quality as well as quantity. Those activities can include waste disposal, resource extraction (such as mining), agricultural practices, and urbanization. Once contaminated, groundwater is very costly and difficult to clean up. The best approach to maintaining groundwater quality is to prevent contamination in the first place.