11.1 GENERAL PROPERTIES OF SUBSURFACE WATER
Subsurface water occurs by infiltration of rainfall and/ or snowmelt into the ground. Once the water has infiltrated, it can follow one of two paths: (1) move in a general lateral direction within the unsaturated zone close to the ground surface or (2) move in a general downward direction and join the saturated zone. Interflow is flow in the unsaturated zone; groundwater flow is flow in the saturated zone.
The earth's crust is composed of soils and rocks containing pores (i.e., voids) that can hold air and water. The various types of soils and rocks have different relative amounts of pore space and, consequently, can hold different amounts of air and water. In subsurface water evaluations, the earth's crust is divided into two zones: (1) the unsaturated zone, where the pores are filled with both air and water, and (2) the saturated zone, where the pores are filled only with water. The boundary between the unsaturated and saturated zones is the water table.
The distance from the ground surface to the water table varies from place to place. In some places it may be less than 1 m, whereas in others it may be more than 100 m. In general, the water table is not flat, tending to follow the surface topography in a subdued way, deeper beneath the hills and shallower beneath the valleys. In certain cases it may even coincide with the ground surface, as with ponds and marshes, or lie slightly above it, as in the typical exfiltration to perennial streams and rivers.
Extent of Groundwater Resources
Although only a fraction of precipitation infiltrates into the ground, the total amount of subsurface water is far greater than the total amount of land surface water. This is because groundwater flow is characteristically a very slow process, whereas land surface water moves at comparatively faster speeds. The average residence time of surface water (i.e., the time elapsed while flowing on the earth's surface) is estimated at 2 wk. On the other hand, the average residence time of subsurface water has been estimated to vary between 2 wk to 10,000 yr  . Both surface and subsurface water are driven by the force of gravity in their unrelenting movement toward the sea.
To place the relationship between surface and subsurface water amounts in the proper perspective, it is necessary to examine the world's water balance. Studies have shown that about 94 percent of all the world's water is seawater. Of the remaining 6 percent, one-third occurs in solid form in glaciers and polar ice caps, and two-thirds is fresh water, which includes surface and subsurface water. Of the total amount of fresh water, more than 99 percent is groundwater. The water stored in lakes, reservoirs, streams, rivers, the unsaturated zone below the ground surface, and in vapor form in the atmosphere accounts for only a small fraction of the total amount of fresh water .
About half of the groundwater is contained within 800 m of the earth's surface . Not all can be used, either because of its salinity or because of the great depths at which it occurs. The distribution of groundwater varies throughout the land areas of the world. Where it does occur, it can been used to supplement surface water supplies. Furthermore, in regions with little or no surface water resources, groundwater is often the only source of fresh water.
A note of caution is necessary here. While it is true that groundwater quantities exceed surface water quantities by at least two orders of magnitude, surface water has the advantage that is is fully recyclable in the short term (12-14 days), while groundwater is typically not. Thus, depletion of groundwater (sometimes referred to as "overdraft") effectively amounts to mining, since the time required for replenishment would be too long for practical consideration (hundreds to thousands of years). Furthermore, where surface and groundwater systems are interconnected, groundwater overdraft generally leads to loss of baseflow and the progressive aridization (desertification) of the landscape.
The feasibility of extracting water from a groundwater reservoir is determined by the following three properties: (1) porosity, (2) permeability, and (3) replenishment. Porosity is the ratio of void volume to total volume of soil or rock. It is interpreted as a measure of the ability of the soil deposit or rock formation to hold water in sufficiently large quantities.
Permeability describes the rate at which water can pass through a soil deposit or rock formation. Permeable materials are those that allow water to pass through them easily. Conversely, impermeable materials are those that allow water to pass through them only with difficulty or not at all. The permeability value is a function of the size of pores or voids and the degree to which they are interconnectd.
Replenishment relates to the size and extent of the groundwater reservoir and its connection to other surface/groundwater resources of the region. Replenishment is largely controlled by nature, although it can be affected by human activities, both positively (e.g., the artificial recharge of groundwater), and negatively (by paving formerly porous land).
An aquifer is a saturated permeable geologic formation which can yield significant quantities of water to wells and springs. By contrast, an aquiclude is a saturated geologic formation that is incapable of transmitting significant amounts of water under ordinary circumstances.
The term aquitard describes the less permeable beds in a stratigraphic sequence. These beds may be permeable enough to transmit water in significant quantities, but not sufficient to justify the cost of drilling wells to exploit the groundwater resource. Most geologic formations are classified as either aquifers or aquitards, with very few formations fitting the definition of an aquiclude.
Aquifers can be of two types: (1) unconfined and (2) confined. An unconfined aquifer, or water table aquifer, is an aquifer in which the water table constitutes its upper boundary. A confined aquifer is an aquifer that is confined between two relatively impermeable layers or aquitards. Unconfined aquifers occur near the ground surface; confined aquifers occur at substantial depths below the ground surface. Figure 11-1 shows typical configurations of confined and unconfined aquifers.
The water level in an unconfined aquifer rests at the water table. In a confined aquifer, the water level in a well may rise above the top of the aquifer. If this is the case, the well is referred to as an artesian well, and the aquifer is said to exist under artesian conditions. In some cases, the water level may flow above the ground surface, in which case the aquifer is known as flowing artesian well, and the aquifer is said to exist under flowing artesian conditions.
The water level in wells located in a confined aquifer defines an imaginary surface referred to as the potentiometric surface . Several wells can help establish a potentiometric contour map, a map depicting lines of equal hydraulic head in the aquifer. A potentiometric map provides an indication of the direction of groundwater flow in an aquifer.
A perched aquifer is a special case of unconfined aquifer. A perched aquifer forms on top of an impermeable layer located well above the water table. Infiltrating water is held on top of this impermeable layer to form a saturated lens, usually of limited extent and not connected to the main water table. The water table of a perched aquifer is referred to as a perched water table.
Recharge and Discharge.
Typically, groundwater flows from a recharge area, through a groundwater reservoir, to a discharge area. The recharge area is an area of replenishment with infiltrated water. The groundwater reservoir is the main body of the aquifer. The discharge area is the area where the infiltrated water returns back to the surface.
In humid and subhumid climates, aquifer recharge usually takes place in upland slopes, ' with aquifer discharge occurring in the valleys, where the water table is shallow enough to be intercepted by streams and rivers. In arid and semiarid regions, however, the situation may be quite different. In this case, the water table in the valleys is usually much deeper, with aquifer recharge taking place primarily by channel transmission losses in streams and rivers.
Discharge from an unconfined aquifer is accomplished in three ways. First, if
the water table is close to the ground surface, water may be discharged from the aquifer either by vapor diffusion upward through the soil or through evapotranspiration by vegetation. Second, if the water table is intersected by a stream, discharge is accomplished by exfiltration. Third, an aquifer can be discharged by human-induced means, i.e, by pumping through a well, either for agricultural, municipal, or industrial uses.
Discharge from a confined aquifer is accomplished in two ways: first, by fast seepage through a permeable path in the overlying impermeable material, or by slow seepage through aquitards; and second, by human-induced means, i.e., by well pumping as with unconfined aquifers .
11.2 PHYSICAL PROPERTIES OF SUBSURFACE WATER
In an unconfined aquifer, the water table is the surface at which the water pressure is exactly equal to atmospheric pressure. The soil or rock below the water table is generally considered to be saturated with water. Indeed, the water table is the upper limit of a zone of saturation, or saturated zone.
The capillary fringe is located immediately above the water table. Water is held in this fringe by capillarity, at moisture levels close to saturation. However, the capillary fringe differs from the saturated zone in that a well will fill with water only to the base of the capillary fringe, i.e., the water table. Water in the capillary fringe is referred to as capillary water to distinguish it from the water in the saturated zone, or groundwater proper.
The thickness of the capillary fringe varies from one rock formation to anotherdepending on the size of the pores-from a few millimeters to several meters. Due to natural irregularities, the top of the capillary fringe is likely to be an irregular surface, with the moisture likely to decrease gradually in a direction away from the water table.
Lowering of the water table by drainage, pumping, or other means will result in a lowering of the capillary fringe. However, all water cannot be drained out of the soil or rock formation. Surface tension and molecular effects are responsible for a certain amount of water being retained in the pores against the action of gravity.
The total amount of water in an aquifer of area A and thickness b is
in which V = total volume of water, A = surface area of the aquifer, b = aquifer thickness, and n = porosity. However, the total amount of water that will drain freely from an aquifer is
in which Vw = volume of free-draining water and Sy = specific yield, the ratio of freedraining water volume to aquifer volume. Since a certain amount of water is always retained in the pore volume; the specific yield of an aquifer is always less than its porosity.
In coarse-grained rocks with large pores, specific yield will be almost equal to the porosity, with specific retention reduced to a minimum. Conversely, in fine-grained rocks, specific retention can approximate the value of porosity, with specific yield being close to zero.
11.3 EQUATIONS OF GROUNDWATER FLOW
11.4 WELL HYDRAULICS
11.5 SURFACE-SUBSURFACE FLOW INTERACTION
Alley, W. M., T. E. Reilly, and. O. E. Franke. 1999. Sustainability of ground-water resources. U.S. Geological Survey Circular 1186, Denver, Colorado, 79 p.
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