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Groundwater: the story on HearLore | HearLore
Groundwater
Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations, yet it remains largely unseen to the human eye. This hidden reservoir holds about 30 percent of all readily available fresh water in the world, making it a critical component of the global water supply. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water, serving as the underground storage tank for this vital resource. The depth at which soil pore spaces or fractures and voids in rock become completely saturated with water is called the water table, marking the boundary between the unsaturated zone above and the saturated zone below. Groundwater is recharged from the surface through precipitation, streams, and rivers, and it may discharge from the surface naturally at springs and seeps, forming oases or wetlands that support diverse ecosystems. Despite its importance, groundwater is often thought of merely as water flowing through shallow aquifers, but in the technical sense, it can also contain soil moisture, permafrost, immobile water in very low permeability bedrock, and deep geothermal or oil formation water. The study of the distribution and movement of groundwater is hydrogeology, also called groundwater hydrology, a field dedicated to understanding the complex interactions between water, rock, and human activity.
The Great Artesian Basin
The Great Artesian Basin in central and eastern Australia is one of the largest confined aquifer systems in the world, extending for almost 2 million square kilometers. By analyzing the trace elements in water sourced from deep underground, hydrogeologists have been able to determine that water extracted from these aquifers can be more than 1 million years old. This ancient water, often referred to as fossil water, infiltrated into the ground millennia ago and has been trapped beneath the surface since before human civilization began. Where water recharges the aquifers along the Eastern Divide, ages are young, but as groundwater flows westward across the continent, it increases in age, with the oldest groundwater occurring in the western parts. This means that in order to have traveled almost 1000 kilometers from the source of recharge in 1 million years, the groundwater flowing through the Great Artesian Basin travels at an average rate of about 1 meter per year. The slow movement of this water highlights the long-term nature of groundwater systems and the potential risks associated with over-extraction, as the natural recharge rate cannot keep pace with human demand. The Great Artesian Basin serves as a testament to the vast scale and ancient history of groundwater, providing a unique window into the Earth's hydrological past.
Common questions
What percentage of the world's fresh water is groundwater?
Groundwater makes up about 30 percent of all readily available fresh water in the world. This amount represents roughly 0.76 percent of the entire world's water including oceans and permanent ice. About 99 percent of the world's liquid fresh water is groundwater.
How old is the water in the Great Artesian Basin?
Water extracted from the Great Artesian Basin in Australia can be more than 1 million years old. This ancient water infiltrated into the ground millennia ago and has been trapped beneath the surface since before human civilization began. The oldest groundwater occurs in the western parts of the basin.
Which cities experience subsidence due to groundwater removal?
Cities such as Bangkok, Mexico City, Venice, and New Orleans experience subsidence due to groundwater removal. The San Joaquin Valley in California experienced subsidence of up to 9 meters in the first half of the 20th century. Mexico City has experienced rates of subsidence of up to 30 centimeters per year.
How does saltwater intrusion affect coastal aquifers?
Saltwater intrusion occurs when too much groundwater is pumped near the coast allowing denser seawater to penetrate freshwater aquifers. This contamination renders potable freshwater supplies unusable once salt amounts to more than 2 to 3 percent of the reservoir. Many coastal aquifers including the Biscayne Aquifer near Miami face problems with saltwater intrusion.
What is the average rate of groundwater flow in the Great Artesian Basin?
Groundwater flowing through the Great Artesian Basin travels at an average rate of about 1 meter per year. This slow movement means water can take 1 million years to travel almost 1000 kilometers from the source of recharge. The slow flow highlights the long-term nature of groundwater systems and risks associated with over-extraction.
Human use of groundwater causes environmental problems that are often invisible until it is too late. Polluted groundwater is less visible and more difficult to clean up than pollution in rivers and lakes, making it a silent crisis that threatens public health and ecosystems. Major sources of groundwater pollution include industrial and household chemicals and garbage landfills, excessive fertilizers and pesticides used in agriculture, industrial waste lagoons, tailings and process wastewater from mines, industrial fracking, oil field brine pits, leaking underground oil storage tanks and pipelines, sewage sludge and septic systems. Additionally, groundwater is susceptible to saltwater intrusion in coastal areas and can cause land subsidence when extracted unsustainably, leading to sinking cities like Bangkok and loss in elevation such as the multiple meters lost in the Central Valley of California. These issues are made more complicated by sea level rise and other effects of climate change, particularly those on the water cycle. Earth's axial tilt has shifted 31 inches because of human groundwater pumping, a startling fact that underscores the profound impact of human activity on the planet's physical structure. The consequences of groundwater depletion and pollution are not limited to local areas but have global implications, affecting food security, economic stability, and environmental resilience.
The Hidden Temperature
The high specific heat capacity of water and the insulating effect of soil and rock can mitigate the effects of climate and maintain groundwater at a relatively steady temperature. In some places where groundwater temperatures are maintained by this effect at about 10 to 15 degrees Celsius, groundwater can be used for controlling the temperature inside structures at the surface. For example, during hot weather relatively cool groundwater can be pumped through radiators in a home and then returned to the ground in another well. During cold seasons, because it is relatively warm, the water can be used in the same way as a source of heat for heat pumps that is much more efficient than using air. This geothermal potential of groundwater is increasingly recognized as a sustainable energy source, playing an important role in reducing CO2 emissions and thus mitigating climate change. In pioneering nations, such as the Netherlands and Sweden, the ground and groundwater are increasingly seen as just one component in district heating and cooling networks. Deep aquifers can also be used for carbon capture and sequestration, the process of storing carbon to curb accumulation of carbon dioxide in the atmosphere. The ability of groundwater to regulate temperature and support sustainable energy solutions highlights its multifaceted role in addressing both water scarcity and climate change.
The Global Dependence
Groundwater makes up about 30 percent of the world's fresh water supply, which is about 0.76 percent of the entire world's water, including oceans and permanent ice. About 99 percent of the world's liquid fresh water is groundwater, and global groundwater storage is roughly equal to the total amount of freshwater stored in the snow and ice pack, including the north and south poles. This makes it an important resource that can act as a natural storage that can buffer against shortages of surface water, as in during times of drought. Groundwater provides drinking water to at least 50 percent of the global population, and 2.5 billion people depend solely on groundwater resources to satisfy their basic daily water needs. In India, 65 percent of the irrigation is from groundwater, and about 90 percent of extracted groundwater is used for irrigation. The Asia-Pacific region is the largest groundwater abstractor in the world, containing seven out of the ten countries that extract most groundwater, including Bangladesh, China, India, Indonesia, Iran, Pakistan, and Turkey. These countries alone account for roughly 60 percent of the world's total groundwater withdrawal. The reliance on groundwater is increasing, mainly due to growing water demand by all sectors combined with increasing variation in rainfall patterns, making it a critical resource for the future of global water security.
The Sinking Cities
Subsidence occurs when too much water is pumped out from underground, deflating the space below the above-surface, and thus causing the ground to collapse. The result can look like craters on plots of land, and in severe cases, this compression can be observed on the ground surface as subsidence. The city of New Orleans, Louisiana is actually below sea level today, and its subsidence is partly caused by removal of groundwater from the various aquifer/aquitard systems beneath it. In the first half of the 20th century, the San Joaquin Valley experienced significant subsidence, in some places up to 9 meters due to groundwater removal. Cities on river deltas, including Venice in Italy, and Bangkok in Thailand, have experienced surface subsidence; Mexico City, built on a former lake bed, has experienced rates of subsidence of up to 30 centimeters per year. For coastal cities, subsidence can increase the risk of other environmental issues, such as sea level rise. For example, Bangkok is expected to have 5.138 million people exposed to coastal flooding by 2070 because of these combining factors. The loss of elevation is not only permanent but also reduces the capacity of the aquifer to hold water, creating a vicious cycle of depletion and damage. The physical consequences of groundwater extraction are a stark reminder of the need for sustainable management practices to protect both human infrastructure and natural ecosystems.
The Saline Threat
Aquifers near the coast have a lens of freshwater near the surface and denser seawater under freshwater. Seawater penetrates the aquifer diffusing in from the ocean and is denser than freshwater. For porous aquifers near the coast, the thickness of freshwater atop saltwater is about 1 meter for every 1 meter of freshwater head above sea level. This relationship is called the Ghyben-Herzberg equation. If too much groundwater is pumped near the coast, saltwater may intrude into freshwater aquifers causing contamination of potable freshwater supplies. Many coastal aquifers, such as the Biscayne Aquifer near Miami and the New Jersey Coastal Plain aquifer, have problems with saltwater intrusion as a result of overpumping and sea level rise. Sea level rise causes the mixing of sea water into the coastal groundwater, rendering it unusable once it amounts to more than 2 to 3 percent of the reservoir. Along an estimated 15 percent of the US coastline, the majority of local groundwater levels are already below the sea level. The threat of saltwater intrusion is compounded by climate change, which increases the risk of coastal flooding and reduces the availability of freshwater. The salinization of groundwater is a major environmental problem that affects local economies and environments, requiring careful management and innovative solutions to ensure the sustainability of coastal water resources.
The Future of Water
The impacts of climate change on groundwater may be greatest through its indirect effects on irrigation water demand via increased evapotranspiration. There is an observed decline in groundwater storage in many parts of the world, due to more groundwater being used for irrigation activities in agriculture, particularly in drylands. Some of this increase in irrigation can be due to water scarcity issues made worse by effects of climate change on the water cycle. Direct redistribution of water by human activities amounting to about 24,000 cubic kilometers per year is about double the global groundwater recharge each year. Climate change causes changes to the water cycle which in turn affect groundwater in several ways: There can be a decline in groundwater storage, and reduction in groundwater recharge and water quality deterioration due to extreme weather events. In the tropics intense precipitation and flooding events appear to lead to more groundwater recharge. However, the exact impacts of climate change on groundwater are still under investigation, as scientific data derived from groundwater monitoring is still missing, such as changes in space and time, abstraction data and numerical representations of groundwater recharge processes. The future of groundwater depends on our ability to adapt to these changing conditions, develop sustainable management practices, and invest in research and technology to ensure the long-term availability of this vital resource.