Water Table

The water table is the upper portion of the saturated or groundwater zone beneath the Earth’s surface. The depth of the water table is an important consideration in drilling wells for water supply, building and roadway construction, and septic system and landfill disposal design.

Subsurface Hydrology

The water table forms the boundary between the unsaturated and saturated zones below the Earth’s surface. The unsaturated zone, also known as the zone of aeration and the vadose zone, forms the uppermost layer in the ground. The pore spaces in the subsurface materials—such as sand, silt, clay, gravel, or consolidated rock—contain both air and water in varying proportions. Water in the unsaturated zone, including capillary water, is contained under pressure lower than the atmosphere's, whereas air and gases are generally under atmospheric pressure. Water in this zone may move up and down or sideways. Soil water in this zone can also be evaporated back into the atmosphere.

Starting at the ground surface, the unsaturated zone is divided into the belt of soil water (or root zone), the intermediate belt, and the capillary fringe. The belt of soil water is bounded on top by the land surface and on the bottom by the intermediate zone. It contains plant roots and soil water that is available for plant growth. The depth of the belt varies over the landscape, but it is generally no more than a few meters.

The intermediate belt in the unsaturated zone lies between the belt of soil water and the capillary fringe. The depth of this belt varies substantially, being much deeper in dry climates and much shallower in wet areas. The capillary fringe is the lowermost subdivision of the unsaturated zone. It is located immediately above the water table. The interstices between particles in this zone are filled with water under pressure lower than the atmosphere's. The water in the capillary fringe is continuous with the water in the saturated zone below the water table, but it is held above it by surface tension. The upper boundary of the capillary fringe with the intermediate belt is somewhat indistinct. However, it is sometimes arbitrarily defined as the level at which 50 percent of the pore spaces or interstices are filled with water. The vertical extent of the capillary fringe depends upon the soil texture because capillary rise is greater when the openings are smaller. Thus, the thickness in silty material may be as large as 1 meter, where the openings are very small, to as little as 1 centimeter in coarse sand or fine gravel, where the openings are much larger.

The saturated zone’s pore spaces are completely filled with water or other fluids. The water table is at the top of the saturated zone. It will, therefore, rise or fall as the saturated zone increases or decreases because of variations in precipitation or pumping. A perched water table can exist above the regional water table in some locations. The separation is caused by a relatively impermeable layer, such as a clay lens, which impedes infiltration. The perched water table is generally shallow and of limited areal extent, although it can become much larger in irrigated areas.

Water Table Variations

The word “table” suggests a surface that is flat and static. In the case of the water table, neither term is appropriate. Rather, the water table tends to be a subdued replica of the surface topography. It rises to higher levels under the highest portions of the landscape (hilltops and divides) and is lowest in the valleys, where it approaches the surface close to streams, lakes, or swamps. This topographic configuration is caused by water percolating through the unsaturated zone, which raises the water table, in contrast to streams, lakes, and swamps that receive base flow or seepage from groundwater in the saturated zone, which lowers the water table.

The depth of the water table from the ground surface varies enormously. It is at or close to the surface in swamps and marshes, meaning an unsaturated zone is nonexistent in those places. The depth in arid regions can be measured in tens and even hundreds of meters below the surface. Under natural conditions, the water table will rise after recharge during a period of precipitation and fall during a period of drought. The greater the amount of precipitation and resultant recharge, the larger the rise; conversely, the greater the intensity and length of the dry period, the larger the fall.

The extent of the rise and fall of the water table can be greatly magnified by anthropogenic intervention. One obvious example is the decline in the water table that is caused by groundwater extraction. Pumping for water supplies or dewatering operations at construction sites can radically lower the water table, with the greatest amount of lowering occurring at the point from which water is removed and decreasing with distance around that point. The resulting decline in the water table forms a cone-shaped region around the extraction point called a cone of depression, which can spread out tens and hundreds of meters from the well. In homogeneous subsurface materials, the shape of the cone of depression forms a smooth hyperbolic surface shape. If the subsurface materials are heterogeneous, which is the more common condition, the cone of depression tends to be irregular in shape and spread out over a large area. The vertical distance between the original water table (or static water level) and the new lowered water table (or pumping water level) is known as the “drawdown.” The greater the rate and duration of pumping, the larger and deeper the cone of depression and the greater the drawdown. Water-bearing geologic formations called aquifers vary enormously in their permeability, a measure of the ease with which they support fluid movement. Thus, water-table fluctuations are less in aquifers with high water transmission rates (high permeability) and much more in aquifers with low permeability, resulting in less capability for groundwater movement.

Environmental Issues

Water table fluctuations can present many environmental problems. For example, at the residential level, septic system disposal fields, typically arrays of weeping tiles, require a minimum depth to the water table of at least 1.2 meters for the effluent to be properly absorbed into the soil. If the water table is too shallow or the drainage system is overloaded, microbial decomposition of the effluent may not fully occur, leading to groundwater contamination. Other problems arise with overpumping an aquifer when the declining water table can fall below the depth of the well, resulting in a dry well. Depending upon the magnitude of the decline, the well would have to be deepened, or a new well in another location would have to be drilled. The normal hydraulic gradient generally follows the topography from high locations to lower ones, but it can be affected by pumping so that the gradient is reversed. This means that leachate from landfills or septic-system disposal fields can thus be induced to flow into a nearby well.

Overpumping, in which the pumping rate is more than the natural recharge from precipitation, can create a cone of depression large enough to interfere with other wells and cause them to go dry. This is a problem with large-diameter public wells that supply water to many users in a community. As a result, many governmental units at the local, county, or state level require pumping tests at specified rates and duration so that the decline and recovery of the water table can be measured to minimize interference with neighboring wells. These pumping tests are often seventy-two hours in duration. In areas where the groundwater yield is marginal and the water table decline can be substantial, some municipalities even require four-hour pumping tests for domestic wells that serve only one residential dwelling unit.

One interesting illustration of the environmental impact of pumping and water level change occurred on a large scale in Brooklyn, New York. There were many wells in Brooklyn in the early part of the twentieth century that pumped large amounts of groundwater. The resulting decline in the water table facilitated the construction of subways. When water quality problems developed as urbanization spread in Brooklyn, the pumping stopped, and the water table rise started flooding the subway tracks. A certain amount of pumping had to be resumed to keep the subways working, even if the water was simply fed into storm sewers that drained into the ocean.

Irrigated lands often have problems with perched water tables that develop over a shallow, relatively impermeable bed, such as a clay lens. The water applied to the irrigated soils causes the perched water table to rise toward the surface. As a result, the capillary fringe will finally reach the surface and allow groundwater to be continuously discharged to the atmosphere by evapotranspiration (evaporation plus plant transpiration). The evaporation will produce a buildup of salts in the surface soil as groundwater contains dissolved mineral matter. In addition, the rising water table can drown out plant roots near the surface.

Taken together, these two effects have created numerous problems for farmers in irrigated areas, from the days of the ancient farmers in Mesopotamia (the Tigris and Euphrates Valleys in what is now Iraq) to the vast irrigated fields in the Indus River Valley in Pakistan, the San Joaquin and Imperial Valleys in California, the Nile Delta in Egypt, and the wheat belt of Western Australia. For example, a water table that rises at an average annual rate of 0.3 meters and consequent salinization and water logging of the soil in the lower Indus River in Pakistan have caused 36,000 hectares of land to go out of production yearly. The solution to this problem requires an elaborate system of drains that remove the salt-laden water from the affected area.

Overpumping in a coastal area can cause a decline in the water table, resulting in saltwater or saline intrusion into a freshwater aquifer, thus creating a contaminated aquifer. Proximity to the sea means that saltwater can be drawn into the well, making the water unfit for human consumption. Saline intrusion problems have been well documented in Miami, Florida, which has large wells close to the coast and is located above the extremely permeable Biscayne Aquifer. The solution to this problem is moving the wells inland or installing recharge wells between the coast and the contaminated wells.

On an even larger scale, one of the significant factors in selecting Yucca Mountain in Nevada as a high-level radioactive waste disposal site was the incredible depth of the water table. This area is about 145 kilometers northwest of Las Vegas and has an average annual precipitation of under 200 millimeters. The deep-water table and the great extent of the unsaturated zone meant that waste disposal canisters would presumably not be affected by the movement of groundwater, which could become radioactive and flow down gradient into larger, regional groundwater flow systems. The disposal site is supposed to isolate the waste materials for 10,000 years. Whether the water table would move upward and flood the disposal site during this period due to climatic change is controversial.

Significance

The water table is a critical part of the subsurface portion of the hydrologic cycle. The depth of the table governs the viability of subsurface septic disposal systems and landfills, which requires specified depths to the water table to protect the underlying saturated zone from contamination. Hazardous and radioactive waste disposal also requires placement in an unsaturated zone that is sufficiently large and distant from groundwater movement in the saturated zone. Water-table depths are essential in wetland determinations and agriculture, where most plants require soils with pore spaces containing air and water.

The water table marks the irregularly undulating top of the saturated zone. The importance of groundwater is best illustrated by the fact that it comprises an estimated 25 percent of the total volume of freshwater, as compared to just 0.3 percent in lakes and 0.03 percent in streams. The US Geological Survey estimates that groundwater accounts for 39 percent of the total amount of water withdrawn for public supply purposes in the nation. Wells must go below the water table in order for groundwater to be extracted, and declining water levels resulting from overpumping or extended drought present immediate water supply problems. Ground-level subsidence in the San Joaquin Valley in California has exceeded 9 meters due to overpumping from the underlying aquifers and necessitated the establishment of recharge wells. The decline of the water table in the largest known aquifer in the United States, the High Plains Aquifer spanning the states of Nebraska, Kansas, Oklahoma, and Texas, and the potential demise of farming in this region as a result, is but another instance of the impact of water-table fluctuations.

As the twenty-first century progresses, changes in the water table, primarily based on groundwater, have only become more dire. A global study published in Nature in 2024 found that groundwater levels are declining in 71 percent of aquifers worldwide. This study, conducted in over forty countries, also found that groundwater was being depleted far faster in the first decades of the twenty-first century than in any preceding decade. Dry, arid areas like California, Iran, and some regions of the Mediterranean are most at risk. The impact of climate change on these conditions cannot be understated. However, some countries, such as Thailand and Saudi Arabia, have devised mitigating plans to counteract this trend. 

Principal Terms

capillary fringe: the lowest portion of the unsaturated zone just above the water table

clay lens: a subsurface layer of low-permeability clay soil, extending over a limited area, that traps groundwater and produces a secondary water table perched above the main water table

perched water table: groundwater that occupies an area above the main or regional water table, as would form above a clay lens

pore: a small opening in rock or between soil particles

saturated zone: a subsurface zone in which all of the pore spaces are filled with water

unsaturated zone: a subsurface zone in which the pore spaces are filled with both air and water

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