Ancient Groundwater – Our Time Press

Why the water you drink can be thousands of years old

As surface water recedes in the western United States, people are drilling deeper wells and tapping into older groundwater that can take thousands of years to replenish naturally.

Marissa grune
Alain seltzer
Kevin M. Befus

Some of North America’s groundwater is so old that it fell as rain before humans arrived here thousands of years ago. Maria Fuchs via Getty Images
Communities that depend on the Colorado River are facing a water crisis. Lake Mead, the river’s largest reservoir, has fallen to levels not seen since it was created by the construction of the Hoover Dam about a century ago. Arizona and Nevada face their very first mandatory water cuts, as water is released from other reservoirs to keep Colorado River hydroelectric plants running.

If even mighty Colorado and its reservoirs are not immune to the heat and drought made worse by climate change, where will the West get its water?
There is a hidden answer: underground.
As rising temperatures and drought dry up rivers and melt mountain glaciers, people increasingly rely on the water beneath their feet. Groundwater resources currently provide drinking water to nearly half of the world’s population and about 40% of the water used for irrigation worldwide.

What many people don’t realize is the age – and vulnerability – of much of this water.
Most of the water stored underground has been there for decades, and much of it has been in place for hundreds, thousands, and even millions of years. Older groundwater tends to reside deep underground, where it is less easily affected by surface conditions such as drought and pollution.
As shallower wells dry up under the pressure of urban development, population growth and climate change, old groundwater becomes increasingly important.

Drinking old groundwater
If you bit into a 1,000 year old piece of bread, you would probably notice it.
Water that has been underground for a thousand years can also taste different. It leaches the natural chemicals from the surrounding rock, altering its mineral content. Some natural contaminants linked to the age of groundwater – like mood-enhancing lithium – can have positive effects. Other contaminants, such as iron and manganese, can be troublesome.
Older groundwater is also sometimes too salty to be consumed without expensive treatment. This problem can be worse near the coast: excessive pumping creates space that can draw seawater into aquifers and contaminate water supplies.

Ancient groundwater can take thousands of years to replenish naturally. And, as California saw during its 2011-2017 drought, natural underground storage spaces compress as they empty, so they can’t fill to their previous capacity. This compaction in turn causes the soil above to crack, warp and sag.
Yet today people are drilling deeper wells in the West as droughts deplete surface water and farms rely more on groundwater.

What does it mean for water to be “old”?
Imagine a rainstorm over central California 15,000 years ago. As the storm hits what is now San Francisco, most of the rain falls in the Pacific Ocean, where it will eventually evaporate into the atmosphere. However, rain also falls in rivers and lakes and on dry land. As this rain seeps through the layers of soil, it slowly enters the “flow paths” of the groundwater.
Some of these paths lead deeper and deeper, where water collects in crevices in bedrock hundreds of meters underground. The water collected in these underground reserves is somehow cut off from the active water cycle – at least on time scales relevant to human life.

In California’s arid Central Valley, much of the ancient accessible water was pumped out of the ground, primarily for agriculture. Where the time scale for natural reconstitution would be of the order of several millennia, agricultural infiltration has partially recharged certain aquifers with more recent water – too often polluted. In fact, places like Fresno are now actively filling aquifers with clean water (such as treated wastewater or stormwater) in a process known as “managed aquifer recharge”.
In 2014, midway through their worst drought in modern memory, California became the last western state to pass legislation requiring local groundwater sustainability plans. Groundwater can withstand heat waves and climate change, but if you use it all, you’re in danger.

A response to the demand for water? Drill deeper. Yet this response is not sustainable.
First, it’s expensive: Large agricultural companies and lithium mining companies tend to be the kind of investors who can afford to drill deep enough, while small rural communities cannot.
Second, once you have pumped out old groundwater, the aquifers need time to fill up. Flow pathways can be disrupted, choking the natural water supply from springs, wetlands and rivers. Meanwhile, the change in pressure underground can destabilize the earth, sink the earth, and even lead to earthquakes.

Third, contamination: While deep, mineral-rich groundwater is often cleaner and safer to drink than younger, shallower groundwater, over-pumping can change that. As water-scarce regions rely more on deep groundwater, over-pumping lowers the water table and attracts polluted modern water which can mix with older water. This mixing causes a deterioration in water quality, leading to a demand for ever deeper wells.

Read the story of the climate in ancient groundwater
There are other reasons to be concerned about ancient groundwater. Like real fossils, the extremely ancient “fossil groundwater” can tell us about the past.
Imagine our prehistoric rainstorm again: 15,000 years ago, the climate was very different today. The chemicals that dissolved in ancient groundwater are detectable today, opening windows to a past world. Some dissolved chemicals act like clocks, telling scientists how old groundwater is. For example, we know how quickly dissolved carbon-14 and krypton-18 decay, so we can measure them to calculate when water last interacted with air.
The younger groundwater that disappeared underground after the 1950s has a unique, man-made chemical signature: high levels of tritium from atomic bomb testing.
Other dissolved chemicals behave like tiny thermometers. Noble gases like argon and xenon, for example, dissolve more in cold water than in hot water, along a precisely known temperature curve. Once the groundwater is isolated from the air, the dissolved noble gases do little. As a result, they preserve information about the environmental conditions when water first entered the basement.
Noble gas concentrations in fossil groundwater provided some of our most reliable estimates of Earth’s temperature during the last ice age. Such findings provide insight into modern climates, including the sensitivity of the Earth’s average temperature to carbon dioxide in the atmosphere. These methods support a recent study that found a warming of 3.4 degrees Celsius with every doubling of carbon dioxide.

Past and future of groundwater
People in some areas, such as New England, have been drinking ancient groundwater for years with little risk of depleting usable supplies. Regular rainfall and various water sources – including surface water in lakes, rivers and the snowpack – provide alternatives to groundwater and also fill aquifers with new water. If aquifers can meet demand, water can be used sustainably.
In the West, however, more than a century of unmanaged and exorbitant water use means that some of the most groundwater dependent places – arid regions vulnerable to drought – have squandered ancient water resources. that once existed underground.
A famous precedent for this problem is in the Great Plains. There, ancient water from the Ogallala Aquifer provides drinking water and irrigation for millions of people and farms from South Dakota to Texas. If people were to pump this aquifer dry, it would take thousands of years to fill naturally. It is a vital buffer against drought, but irrigation and water intensive agriculture are lowering its water levels at unsustainable rates.

As the planet warms, ancient groundwater becomes more and more important – whether it drains from your kitchen faucet, irrigates food crops, or offers warnings about Earth’s past that can help us. prepare for an uncertain future.

Marissa Grunes is a postdoctoral environmental researcher at the Harvard Environmental Center, where she is working on a narrative non-fiction book on Antarctica for the general public.
Alan Seltzer is a scientific assistant in marine chemistry and geochemistry at the Woods Hole Oceanographic Institution, where tracer measurements and modeling are combined to track the physical and biogeochemical interactions between the atmosphere, ocean, hydrosphere, cryosphere and the solid Earth.

Kevin M. Befus is Assistant Professor of Hydrogeology at the University of Arkansas. He studies hydrological systems with an emphasis on groundwater.

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