Groundwater

Groundwater is underground water found in the pore spaces and cracks of soil, sand, and rock. The source of all groundwater is precipitation, either through direct percolation into the earth's surface, or through replenishment from local surface water including lakes, ponds, wetlands or rivers. Sometimes groundwater also flows into surface water through a process called baseflow. The process of groundwater replenishment is termed recharge.

Groundwater is stored in and moves at varying speeds through aquifers. An aquifer is a water-bearing geologic formation that can store and yield usable amounts of water, and consist of permeable layers of soil, sand, gravel or fractured rock such as granite or limestone. They are classified according to type, areal extent, thickness, yield, and direction of groundwater movement. There are two types of aquifers: consolidated rock and unconsolidated rock. Consolidated rock aquifers are composed of limestone, sandstone or other rock. Some, such as granite, are almost impervious and yield very little water, while others, such as limestone, are very porous and can yield vast amounts of water. Unconsolidated rock aquifers are composed of granular materials such as sand and gravel and typically yield larger amounts of water.

Aquifers are also confined or unconfined. Unconfined aquifers are typically located near the land surface, are composed of permeable materials such as sand or gravel, and recharge quickly, making them susceptible to contamination. The area of the aquifer that is filled with water is termed the saturation (or saturated) zone; the top of the saturation zone in an unconfined aquifer is termed the water table or phreatic surface, where water pressure equals atmospheric pressure. The area between the saturation zone and the land surface is the vadose zone.

Confined or artesian aquifers are typically located at greater depths and below impermeable layers such as rock or clay. They are typified by little or no recharge. For this reason, they often contain what is termed fossil or geologic water, and are thus susceptible to mining. Groundwater mining occurs either when groundwater extraction exceeds recharge (as in unconfined aquifers) or when groundwater will [Page 829]not be recharged naturally as in most confined aquifers. Artificial recharge is also possible either through the direct injection of water into the subsurface as in California or through directed rainwater recharge as is increasingly common in northern India.

Contrary to popular myth, groundwater does not flow in rivers or channels beneath the earth's surface. The one exception is with Karst topography. Karst (an area of Slovenia) topography is where the solution of limestone, dolomite, gypsum, or marble, creates very erodable areas on the land surface or underground. It is possible for water to flow through the underground caverns created through this process. Karst is found in the U.S. states of Florida, Texas, and Kentucky, and in China, Slovenia, and Turkey.

Groundwater flows through aquifers toward lower elevations through the force of gravity. In confined aquifers, however, groundwater can flow up gradients, causing artesian conditions, where groundwater flows to the surface due to pressure created through the confined character of the aquifer. This occurs along the foothills of the Rocky Mountains in the United States, but is also common in other areas.

...