Groundwater
From Water Wiki
Leaving aside the glaciers, most fresh water on the planet is stored as groundwater...water beneath the surface of the ground. Except that--and this is an important exception--some groundwater is discharged above the surface in streams, as the base flow, the flow that continues even when there is no runoff from recent precipitation. In other words, a substantial percentage (30-100%) of the water in a typical southeastern stream is groundwater. In yet other words, streams are where the ground surface dips below the water table--the level of soil or rock that is saturated with water.
So the question arises: how much effect does the withdrawal of groundwater (from the surface aquifer) have on surface water? This is a question that, at least in the Piedmont and western parts of North Carolina, can only be answered with studies of specific areas. Some legislators and others in the southeast are very reluctant to regulate all groundwater, believing it important to maintain a more property-like legal status for "deep aquifers" or some other way of expressing groundwater that is isolated. Some states, notably Nebraska, have followed this line by separately regulating "hydrologically connected groundwater," meaning groundwater that has a significant interaction with surface water.
The aquifers and confining beds that underlie any area comprise the ground-water system of the area. Hydraulically, this system serves two functions : it stores water to the extent of its porosity, and it transmits water from recharge areas to discharge areas. Thus, a ground-water system serves as both a reservoir and a conduit. With the exception of cavernous limestones, lava flows, and coarse gravels, ground-water systems are more effective as reservoirs than as conduits. Water enters ground-water systems in recharge areas and moves through them, as dictated by hydraulic gradients and hydraulic conductivities, to discharge areas.
In the humid part of the country, recharge occurs in all interstream areas-that is, in all areas except along streams and their adjoining flood plains. The streams and flood plains are, under most conditions, discharge areas.
Recharge rates are generally expressed in terms of volume (such as cubic meters or gallons) per unit of time (such as a day or a year) per unit of area (such as a square kilometer, a square mile, or an acre). When these units are reduced to their simplest forms, the result is recharge expressed as a depth of water on the land surface per unit of time. Recharge varies from year to year, depending on the amount of precipitation, its seasonal distribution, air temperature, land use, and other factors. Relative to land use, recharge rates in forests are much higher than those in cities.
The rate of movement of ground water from recharge areas to discharge areas depends on the hydraulic conductivities of the aquifers and confining beds, if water moves downward into other aquifers, and on the hydraulic gradients. A convenient way of showing the rate is in terms of the time required for ground water to move from different parts of a recharge area to the nearest discharge area. The time ranges from a few days in the zone adjacent to the discharge area to thousands of years (millennia) for water that moves from the central part of some recharge areas through the deeper parts of the ground-water system.
Natural discharge from ground-water systems includes not only the flow of springs and the seepage of water into stream channels or wetlands but also evaporation from the upper part of the capillary fringe, where it occurs within a meter or so of the land surface. Large amounts of water are also withdrawn from the capillary fringe and the zone of saturation by plants during the growing season. Thus, discharge areas include not only the channels of perennial streams but also the adjoining flood plains and other low-lying areas.
One of the most significant differences between recharge areas and discharge areas is that the areal extent of discharge areas is invariably much smaller than that of recharge areas. This size difference shows that discharge areas are more "efficient" than recharge areas. Recharge involves unsaturated movement of water in the vertical direction; in other words, movement is in the direction in which the hydraulic conductivity is generally the lowest . Discharge, on the other hand, involves saturated movement, much of it in the horizontal direction-that is, in the direction of the largest hydraulic conductivity. [1]
"The Piedmont and mountain regions are underlain by igneous and metamorphosed igneous and sedimentary rocks that are referred to collectively as bedrock. They form broad northeast-southwest trending zones in which the rocks are of similar composition and origin. Most of these rocks were formed in the Precambrian and Paleozoic Eras of the Earth’s history and thus are at least several hundred million years old. The bedrock in the Piedmont and mountains is exposed at the surface along steep hillsides and stream channels and in roadcuts. In most other areas they are covered by unconsolidated material formed from the breakdown of the bedrock in the process referred to by geologists as weathering. This layer of weathered material is referred to as saprolite or residuum. The Coastal Plain region is also underlain by the same types of igneous and methamorphic rocks as those present in the Piedmont. However, in the Coastal Plain they are covered by unconcolidated sedimentary deposits which range in thickness from a few feet along the Fall Line to about 10,000 ft. at Cape Hatteras. (See the geologic section in PHYSICAL SETTING OF THE GROUND-WATER SYSTEM.) The sediments underlying the Coastal Plain include sand, clay, beds composed of seashells and limestone." [1]
The rate at which groundwater is rechargedvaries significantly over the different geologies of North Carolina.
