Groundwater Quality

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Groundwater Quality

Introduction

Groundwater quality refers to the state of water that is located beneath Earth’s surface. Groundwater can gather in cracks in subsurface rocks and in between soil particles.

Since many compounds can dissolve in water and others can be suspended in water, there is a potential for contamination with toxic compounds. These include petroleum, hydrocarbons (oil), pesticides, minerals, and disease-causing (pathogenic) microorganisms.

Groundwater tends to be less prone to contamination than surface waters such as streams, rivers, and lakes because the contaminants have to pass down through the ground to reach the water. Nevertheless, contamination can occur, especially if there are cracks in overlying soil and rock through which the toxic compounds can more easily flow.

The contamination of groundwater can be serious, especially if the water is a supply of drinking water or water for crop irrigation. In 2008, an ongoing example is the contamination of groundwater in parts of Bangladesh by naturally occurring arsenic. The increased use of the groundwater has increased the amount of this poisonous mineral leaching into the water from the surrounding ground. Tens of millions of people have been poisoned.

Groundwater quality will become even more important as global climate change increases the frequency of drought in some areas of the world.

Historical Background and Scientific Foundations

Groundwater, which makes up about 20% of the world’s freshwater supply, is part of a water cycle in which water falls to the surface, moves down into the subsurface, and moves back to the surface (naturally, given enough subsurface pressure, or by being pumped). Not all groundwater will recycle. If not deliberately tapped, some groundwater can remain in place indefinitely. However, the need for water for drinking, agriculture, and industrial uses continues to lead to the discovery and exploitation of available groundwater sources.

Water that is far underground is less prone to contamination. This is mainly because the water that is percolating down from the surface is filtered as it passes through soil. Toxic compounds can bind to soil particles or can be too large to pass very far into the subsurface. This is particularly true for bacteria.

However, if the soil is dry and cracked, and if underlying rock has fractures running through it, contaminated water can reach farther underground. As well, groundwater that lies near the surface has a greater chance of being contaminated.

The filtering capacity of the soil can also be overloaded if a large amount of pollutants is present. Although some of the toxic material can be filtered out as it moves downward, not all will be. Another example is the contamination of groundwater by a well that has been drilled down to tap into the groundwater source. If the well is not constructed properly or has a crack, then contaminants entering the well from the surface (as can happen if the well is flooded in a heavy rainstorm) can be a concentrated source of contaminant that does not have to move downward through the soil before contacting the groundwater; the well is a conduit that directly connects the surface with the groundwater. It is important that wells not be drilled close to the source of contaminants such as a septic tank, livestock field, manure pile, or petroleum tank.

Even if groundwater is free from microorganisms, the water usually contains minerals that have leached out from the surrounding soil and rock. Some minerals are not harmful or are even beneficial. For example, dis-

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.

solved minerals including calcium (CA), magnesium (MG), and potassium (K) can give the water a taste that is appealing.

However, if minerals are present in excessive concentrations, the water quality can deteriorate. One example is elevated salt (NaCl), which makes the water too saline to drink. Infiltration of saltwater into coastal groundwater is occurring in states such as Florida, where the availability of water for drinking and agriculture is becoming a concern. As freshwater is pumped out from the subsurface, saltwater that has infiltrated into coastal sediment can be drawn farther inland.

As well, groundwater elevated in sodium can be a concern for people with heart ailments. Much of the continental United States is undercut by groundwater that contains high levels of sodium. As groundwater nearer to the surface is used up, the very salty water may be drawn closer to the surface, which could threaten the nation’s freshwater supplies.

In urban areas and regions where agricultural use is intensive, groundwater quality can be degraded by the addition of a variety of contaminants. These include human and industrial waste, fertilizers, pesticides and herbicides, leakage from landfills, petroleum (such as from leaking underground fuel storage tanks), and oil.

Impacts and Issues

Groundwater quality is vital to the water supply of many nations. According the National Ground Water Association (NGWA) and the Environmental Protection Agency (EPA), of the approximately 341 billion gallons (1.2 trillion liters) of freshwater withdrawn for use every day in the United States, groundwater makes up about 83 billion gallons (314 billion liters), almost one-quarter of the total. In 2005, there were nearly 16 million wells in the United States. Maintaining groundwater quality is vital to the continued health of the nation.

However, groundwater quality is under threat. As one example, in coastal regions where saltwater can contaminate groundwater, the loss of freshwater is a concern. This concern will grow as sea levels continue to rise with the warming atmosphere, as was forecast in the 2007 report of the Intergovernmental Panel on Climate Change (IPCC). In Florida, for example, modeling studies done by the EPA have shown that a sea level rise of only 2 ft (0.6 m) would submerge the southern part of the state including Miami and threaten the Everglades with saltwater contamination.

Because pollutants can remain in the water for a long time, pollution of groundwater represents an even more serious threat to human health. Radioactively contaminated water, for example, can be unusable for thousands of years.

Pollution of groundwater is not rare. For example, the EPA has records of over 400,000 cases of petroleum-based fuel spills from underground storage tanks. Although detection and remediation of groundwater pollution is a priority in the United States as mandated by the Resource Conservation and Recovery Act and the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA, which is

also known as the Superfunnd), it can be less important in developing countries.

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. An especially tragic example has occurred in Bangladesh. To lessen one of the highest rates of diarrhea-related infant death in the world, due mostly to contaminated surface water, UNICEF and the World Bank supported a well-drilling campaign. During the 1970s and 1980s, over eight million wells were drilled. As water was withdrawn, the remaining water became increasingly contaminated with arsenic. As of 2008, about 30 million people are at risk of arsenic poisoning, “the largest poisoning of a population in history,” according to the World Health Organization.

In the United States, aquifer depletion is also a concern. For example, the huge Ogallala aquifer that lies underneath eight midwestern states is being depleted because water is being withdrawn for use faster than it can be naturally replaced. If this continues, the aquifer could dry up by 2025, which would be disastrous for the United States and global food supply.