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United Nations Environment Programme
Division of Technology, Industry and Economics
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Newsletter and Technical Publications

Lakes and Reservoirs vol. 2

The Watershed: Water from the Mountains into the Sea


Groundwater: Water Flowing Under the Land Surface

Groundwater comprises water that flows under the land surface (Fig. 8). Most people only encounter groundwater within the context of water wells and, accordingly, it has been characterized by some as being “out of sight, out of mind ”. Nothing is further from the truth. Groundwater represents the largest single source of fresh water in the hydrological cycle available for beneficial human uses, being greater in volume than all the water in rivers, lakes and wetlands combined. The groundwater in the United States alone, for example, exceeds the total water volume contained in all its lakes, reservoirs, rivers and wetlands. On the other hand, approximately half of the world ’s groundwater resources are located underground at depths too deep to be economically exploited for human use.

Fig. 8: Diagram of the hydrological cycle showing groundwater and surface water relationships along with and groundwater pollution risks. detail

Groundwater represents the portion of precipitation that seeps (“infiltrates ”) into the land surface, entering the empty spaces between soil particles. The larger the soil particles (Fig. 9), the larger the empty spaces, and the greater the potential for water infiltration. Soils composed of large soil particles are more permeable than soils composed of small particles. Thus, they can hold more water than the latter. The infiltrating water sources include natural precipitation or snowmelt, streams, lakes, reservoirs and wetlands.

a.
b.
c.
Fig. 9: Some types of unconsolidated materials in the soil:
a.) sand; b.) rocks; c.) limestone. The last can hold more water than the others.

There are several groundwater properties that fundamentally affect how and where humans interact with it. Water exists under the land surface in permeable geological formations known as aquifers. In some geographic settings (e. g., valleys between mountain ranges), the physical boundaries of an aquifer can closely coincide with the surface stream watershed. However, in limestone and sandhill areas, the physical boundaries of aquifers and surface watersheds can have very different configurations and can be completely unrelated.

If a person digs a well to a depth where water is encountered, the depth at which the water is first encountered identifies the uppermost boundary of the water-saturated soil. Stated another way, it is the upper boundary at which all the spaces between the soil particles are filled with water. Thus, all the soil below this depth comprises the “saturated ” soil zone. The water surface in the well coincides with the upper boundary of the groundwater, and is commonly called the “water table ”.

Photo 42:
Figeh Spring in Syria which provides water to Damascus.
Photo 43:
A typical oasis in Oman

In contrast to the saturated zone, the spaces between the soil particles above the water-table contain no water. This upper soil layer represents the unsaturated zone. The water-table will rise as more water enters the saturated zone (e. g., during periods of precipitation), and it will fall as water is withdrawn from the aquifer (e. g., via wells and pumps) faster than it can be replenished by natural precipitation or other water inputs. Thus, the bottom boundary of the unsaturated zone rises or falls, conversely to the dynamics of the saturated zone. As previously noted, wetlands constitute areas in which the water- table is at, or higher than, the land surface, resulting in a perpetually water-saturated soil condition. Springs (Photos 42 and 43), and artesian wells represent hydrologic conduits or discharge areas through which groundwater can reach the land surface.

Fig. 10: Basic elements of groundwater flow in humid regions (left) and semi-arid regions (right). detail

Groundwater is constantly in motion (Fig. 10). However, its velocity is highly variable, ranging from as little as a few metres per year to as much as a few metres per day. During periods of little precipitation or land surface runoff, groundwater can seep into overlying stream and river channels, thereby providing most or all of the streamflow during such periods (so-called “base flow ”, or dry- weather flow of perennial streams). In fact, groundwater comprises the only readily available natural freshwater supplies in semi- arid and arid regions. This reality has water supply implications in that excessive water withdrawal from a groundwater aquifer can directly affect water availability throughout an overlying watershed. It is noted that the replenishment of groundwater supplies in arid and semi-arid regions is very slow, even though these water resources are the most important and critical in such regions.

Only about half of the world’s groundwater resources is located sufficiently close to the land surface to allow its withdrawal to be economically feasible (Photo 44). Some have identified this limit as being no more than about 800m below the land surface, although the ease of underground water extraction also depends on other factors. The majority of drinking water supplies on a global scale is from groundwater sources. Agricultural production also uses large quantities of groundwater. This latter water use is of special significance in that large quantities of water used for agricultural irrigation undergo transpiration back into the atmosphere via irrigated crops, thereby short-circuiting its passage through the hydrologic cycle. This water “consumption ” is in contrast to water used for drinking water supply and industry. The latter is eventually discharged back into streams, rivers and other receiving water systems, thereby allowing its reuse, usually after pre-treatment of some type.

Photo 44: Caves and cenotes originating from near surface groundwater flowing through limestone, Mexico.

The present volume of water being withdrawn annually from aquifers around the world is approximately 600-700 cubic kilometres. Unfortunately, many aquifers are currently being overexploited to meet human water demands. A prominent example is the Ogallala Aquifer, which underlies much of the central part of the United States. Continuing over-exploitation of this massive aquifer over many years, primarily for agricultural irrigation, has caused its water table to sink to hundreds of metre below the land surface. In some cases, artificial recharge efforts have been used to attempt to augment the replenishment rate of groundwater aquifers. These comprise constructions or other methods of enhancing the infiltration of surface water into aquifer recharge zones, including water spreading, flood areas, permeable surfaces, pits, wells, etc. An important part of aquifer recharge in arid and semi-arid regions is provided by river losses and underground seepage during floods.

Groundwater use in Germany, Belgium, France and Sweden has been relatively constant over the years, while groundwater withdrawals have decreased in Canada and the United States over the last 20 years. In contrast, groundwater use has increased significantly in many developing countries, including China, India, Mexico, Iran and Pakistan. The greatest increases are in developing countries in arid and semi-arid regions with increasing populations.

Because of its relatively slow movement below the land surface, groundwater is especially susceptible to water pollution. The volume and flow of surface waters allows for the possibility of rapid flushing rates for pollutants contained in them. In contrast, the typically slow movement of groundwater insures that a much longer time is required for a groundwater aquifer to flush or otherwise wash-out pollutants contained in its water. The extent of pollution of an individual groundwater aquifer will obviously depend on the types and quantities of the pollutants contained in the water entering the aquifer via its recharge zone. The major pollutant sources include runoff from different types of land use and waste disposal practices, including excessive agricultural fertilizer use, deep well injection of pollutants, etc. The polluted groundwater can result in significant health problems if it is subsequently consumed by humans and/or livestock.

Photo 45: Tilted churches in Mexico City as a result of groundwater overexploitation resulting in subsidence.

Land subsidence from excessive groundwater withdrawal is also a problem, particularly in coastal-plain regions. Areas of significant land subsidence have been observed in Mediterranean coastal-plains, the Texas High Plains in the United States, Libya, Saudi Arabia, and the Chinese Hebei Plain. The water table in some coastal plains has dropped below sea- level, including areas in Germany, Denmark, Netherlands, Italy, Spain and the United Kingdom. Major cities experiencing varying degrees of subsidence include Venice, Milan, Berlin, London, Denver, Houston, Las Vegas, San Francisco, Mexico City (Photo 45), Shanghai and Hanoi. Major effects include negative impacts on the base flows of rivers and springs, changes in the freshwater/sea water balance in some cases, and enhanced salt water intrusion in some coastal rivers and groundwater aquifers.

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