The communities that depend on the Colorado River are facing a water crisis. Lake Mead, the river’s largest reservoir, has declined to levels not seen since the construction of the Hoover Dam nearly a century ago. Arizona and Nevada are facing their first mandatory water cuts, while water is being released from other reservoirs to keep Colorado River hydroelectric plants running.
If the mighty Colorado and its reservoirs are also not protected from worsening heat and drought from climate change, where will the West get its water?
There is a hidden answer: underground.
As rising temperatures and droughts dry up rivers and melt mountain glaciers, people are increasingly dependent on the water beneath their feet. Groundwater resources currently supply drinking water to nearly half of the world’s population and account for about 40% of the water used for irrigation globally.
Many people don’t know how old – and how vulnerable – much of that water is.
Most of the water stored underground has been around for decades, and much of it has been lying around for hundreds, thousands or even millions of years. Older groundwater tends to remain deep underground, where it is less easily affected by surface conditions such as drought and pollution.
As shallow wells dry up under pressure from urban development, population growth and climate change, old groundwater is becoming increasingly important.
drinking ancient groundwater
If you bite into a piece of bread that is 1,000 years old, you’ll probably notice.
The taste of underground water over a thousand years may also have been different. It leaks natural chemicals from the surrounding rock, changing its mineral content. Some of the natural contaminants associated with groundwater age – such as mood-boosting lithium – may have positive effects. Other contaminants such as iron and manganese can cause trouble.
Old groundwater is also sometimes too salty to drink without expensive treatment. The problem may be worse near coasts: Overpumping creates space that can draw seawater into aquifers and contaminate drinking supplies.
It can take thousands of years for ancient groundwater to be replenished naturally. And, as California saw during its 2011-2017 drought, natural underground storage spaces shrink when empty, so they can’t refill their previous capacity. This compaction in turn causes the land above to crack, buckle and sink.
Yet people today are digging deeper wells in the West because droughts deplete surface water and make farms more dependent on groundwater.
What does it mean for water to be ‘old’?
Let’s imagine a typhoon hit central California 15,000 years ago. As the storm now rolls into San Francisco, most of the rain falls over the Pacific Ocean, where it will eventually evaporate back into the atmosphere. However, some rain also falls on rivers and lakes and on dry land. As rain seeps through the soil layers, it slowly enters the “flow path” of underground water.
Some of these passages go deeper and deeper, where water accumulates in crevices within hundreds of meters underground. The water collected in these underground reserves is in a sense cut off from the active water cycle – at least at times relevant to human life.
In California’s arid Central Valley, most of the accessible pristine water has been flushed out of the earth, mostly for agriculture. Where natural replenishment times will be on the order of millennia, agricultural seepage has partially filled some aquifers with new – often polluted – water. In fact, places like Fresno now actively refill aquifers with clean water (such as treated wastewater or stormwater) in a process called “managed aquifer recharge.”
In 2014, in the midst of its worst drought in modern memory, California became the last western state to pass a law requiring local groundwater sustainability plans. Groundwater may be resilient to heat waves and climate change, but if you use it up, you’re in trouble.
A reaction to the demand for water? Drill deeply. Yet that answer is not sustainable.
First, it is expensive: large agricultural companies and lithium mining firms tend to have investors who can drill to sufficient depths, while smaller rural communities cannot.
Second, once you’ve pumped out pristine groundwater, the aquifers need time to refill. Flow paths can be obstructed, disrupting the natural water supply to springs, wetlands and rivers. Meanwhile, changes in underground pressure can destabilize the Earth, causing land to sink and even cause earthquakes.
Third is pollution: While deeper, mineral-rich pristine groundwater is often cleaner and safer to drink than smaller, shallower groundwater, overpumping can change that. Since water-scarce areas rely more heavily on deep groundwater, overpumping lowers water levels and brings down polluted modern water that can mix with older water. This mixing leads to deterioration of water quality, which always increases the demand for deep wells.
Reading Climate History in Ancient Groundwater
There are other reasons to care for pristine groundwater. Like real fossils, extremely old “fossilized groundwater” can teach us about the past.
Imagine our prehistoric hurricanes again: 15,000 years ago, the climate was vastly different from today. Chemicals dissolved in ancient groundwater can be discovered today, opening windows into the world of the past. Some dissolved chemicals act like clocks, telling scientists the age of groundwater. For example, we know how fast carbon-14 and krypton-18 decompose, so we can measure them to calculate when the water last interacted with the air.
The tiny groundwater that disappeared underground after the 1950s has a unique, man-made chemical signature: high levels of tritium from nuclear bomb testing.
Other dissolved chemicals behave like tiny thermometers. For example, noble gases such as argon and xenon, with a precisely known temperature curve, dissolve more in cold water than in hot water. Once the groundwater is separated from the air, the dissolved noble gases don’t do much. As a result, they preserve information about the state of the environment at the time water first seeped into the subsurface.
The concentrations of noble gases in fossil groundwater have provided some of our most reliable estimates of temperature on land during the last ice age. Such findings provide insight into modern climate, including how sensitive Earth’s average temperature is to carbon dioxide in the atmosphere. These methods support a recent study that found a warming of 3.4 °C with each doubling of carbon dioxide.
The past and future of groundwater
In some areas, such as New England, people have been drinking pristine groundwater for years, with no danger of running out of usable supplies. Regular rainfall and various water sources – including surface water in lakes, rivers and snowpack – provide alternatives to groundwater and also fill the aquifer with new water. If the aquifer can meet the demand, the water can be used sustainably.
Outside the West, however, more than a century of unmanaged and excessive water use has meant that some places dependent on groundwater – arid regions vulnerable to drought – have ruined ancient water resources that once existed underground. .
A well-known example of this problem is in the Great Plains. There, the pristine waters of the Oglala Aquifer supply drinking water and irrigation for millions of people and farms from South Dakota to Texas. If people dried and pumped this aquifer, it would take thousands of years to refill it naturally. It is an important buffer against drought, yet irrigation and water-intensive farming are reducing their water levels at unsustainable rates.
As the planet warms, ancient groundwater is becoming increasingly important—whether flowing through your kitchen tap, irrigating food crops, or warning about Earth’s past that leaves us uncertain. Can help prepare for the future.
[Understand new developments in science, health and technology, each week. Subscribe to The Conversation’s science newsletter.]