Aquifers

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Aquifers

Introduction

An aquifer refers to an underground source of water. The water can be present within cracks and crevasses of rock, sand, clay, gravel, or other material, and in spaces between adjacent particles of material. In a productive aquifer, the water is accessible and can be withdrawn for drinking, irrigation of crops, and industrial uses.

Because the water has to percolate down from the surface to the region of the aquifer, it is filtered during its passage. This removes large and small particles. Furthermore, the water is protected from sources of contamination that can be detrimental to surface waters. In contrast, surface water is more prone to contamination from disease-causing microorganisms and noxious chemicals.

In many instances, the clean water that is withdrawn can be a reliable source for a long time. However, the withdrawal of water at a greater rate than the aquifer can be replenished depletes the aquifer. Not only does this restrict the water that can be obtained, but, if the aquifer is near the surface, collapse of the overlying material into the underground hole can occur.

Aquifer depletion is a serious problem in major urban areas such as Mexico City, and in agriculture-rich regions such as the midwestern United States.

Historical Background and Scientific Foundations

Aquifers can be located close to the surface or far beneath the surface. Those nearer to the surface are easier and less expensive to gain access to, and so will be utilized as a water source in preference to deeper aquifers. Additionally, when aquifers are closer to the surface, precipitation and runoff from higher elevations and other sources has less distance to percolate down into the ground.

Aquifers tend to be present, not as an interconnected series, but as isolated entities. The image of an aquifer as a vast underground lake within a gigantic cavern is incorrect. Instead, an aquifer is more typically like a sponge, with the available spaces in the rock, gravel, or other support being filled by the water.

A reasonably accurate model for an aquifer is the appearance of water at a certain depth of a hole dug into the sand at a beach. The water has percolated into the spaces between the sand grains and, when the hole is dug into the saturated zone, water will flow into the hole. In this model, the hole represents a well, the water-saturated sand represents the aquifer, and the level of the water in the hole represents the water table.

In an actual aquifer, the water table is a boundary layer. Above the line of the water table, the cracks, crevasses, and pores between the particles of material are not completely filled with water. This region is described as being unsaturated. Below the line of the water table, the spaces are completely filled with water; this region is saturated.

When a well is drilled into some aquifers, the underground pressure can be sufficient to force the water to the surface without the necessity of pumping. This type of well is termed an artesian well. In the majority of aquifers, the water needs to be pumped to the surface.

An aquifer can consist of regions in which the water is more accessible, since it can move more freely through the pores in the rock, gravel, or other material. These regions are known as higher permeability zones, and are the regions that empty first. New water can flow in to replace the water that has been withdrawn. This is termed recharging. Some of this water can come from low permeability zones—other areas of the aquifer that contain water that moves less freely. The result can be a net loss of water with water being withdrawn at a faster

WORDS TO KNOW

HYDROLOGY: The study of the distribution, movement, and physical-chemical properties of water in Earth’s atmosphere, surface, and near-surface crust.

RECHARGE: Replenishment of an aquifer by the movement of water from the surface into the underground reservoir.

SPRING: The emergence of an aquifer at the surface, which produces a flow of water.

WATERSHED: The expanse of terrain from which water flows into a wetland, water body, or stream.

rate than the aquifer is being replenished. This is referred to as groundwater drawdown. In cases where aquifer depletion becomes serious, new wells may have to be drilled to better access the lower permeability zones.

The wells may have to be drilled to a greater depth than before, since the base of the aquifer can slump downward, creating what is called a cone of depression. Wells that are not drilled to the depth of the cone will be dry.

Drawdown can be dramatic. For example, the excessive drawdown of the Mexico City aquifer due to the intensive pumping of water for the cities’ millions of resident caused the water level to drop by almost 35 feet (11 m) in some areas from the early 1980s to the early 1990s.

The variability in the movement of water in different regions of a single aquifer, and the variation between different aquifers with respect to the terrain (hilly versus flat, for example), composition of the underground material (granite contains fewer cracks and crevasses than does limestone, for example), and ease of extracting water means that different aquifers will recharge at different rates.

Recharging is also influenced by urbanization. When land is paved over for features of the urban landscape such as parking lots, water will be impended from diffusing down to the water table.

Impacts and Issues

Freshwater aquifers that are located close to the surface represent an attractive source of water for drinking, agricultural use, and industrial use. Although it makes sense to ensure that the rate of removal of the water is less than the rate at which water is replenishing the aquifer, this has not been done in the past. Indeed, in regions where many individuals access the aquifer via their own wells, such control on water removal is all but impossible.

Overexploitation of an aquifer can cause the overlaying material to collapse. In urban areas, the resulting “sinkhole” can be destructive to property, and dangerous. As well, in coastal regions, the depletion of a freshwater aquifer can cause nearby saltwater to be drawn into the aquifer. This phenomenon, which is known as saltwater intrusion, can destroy the usefulness of the aquifer as a drinking water and agricultural resource. Coastal aquifers, including the Biscayne Aquifer (the main source of water for two major counties of the city of Miami and southern Palm Beach County in Florida) and the New Jersey Coastal Plain aquifer (a 4,200 square mile [10,878 square km] region) are experiencing problems with saltwater intrusion. This is serious for the millions of people in these regions who depend on water obtained from the aquifers.

Another problem region is the Ogallala Aquifer, a huge aquifer that underlies eight midwestern states in the United States. One of the world’s largest aquifers,

spanning an area of 800 miles (1,290 km) from north to south and 400 miles (645 km) from west to east, the Ogallala is losing water at a net rate of 12 billion m³ every year. If this rate continues, the aquifer could be completely depleted by 2025. The land overlying the aquifer is among the nation’s most productive for livestock, and for the growth of crops such as corn, soybeans, and wheat. The aquifer’s depletion would be disastrous for the “breadbasket of America.”

Although less susceptible to contamination than surface water, toxic compounds can also be drawn into an aquifer, especially if water is being withdrawn faster than the rate of replenishment. A particularly tragic example unfolded in Bangladesh. In an effort to lessen one of the highest rates of diarrhea-related infant death in the world, which was largely due to the consumption of contaminated surface water, organizations including UNICEF and the World Bank supported the use of wells to gain access to groundwater in various aquifers throughout the country. During the 1970s and 1980s, over eight million wells were constructed. In the short-term, the initiative was successful, with diarrhea-related illness and infant death dropping by 50%. However, with more time, it became evident that some of the aquifers were contaminated with arsenic. As of 2008, about 30 million people were at risk of arsenic poisoning, representing, in WHO’s words “the largest poisoning of a population in history.”

Many other examples of aquifers at risk are found around the world. As the world’s population grows, and the need for drinking water and food (much of which needs to be irrigated) grows, aquifer depletion will become even more serious.