Our Need For Water Is Tapping Ancient Underground Wells. How Long Can They Last?

The Colorado River is facing water shortages in communities that depend on it. Lake Mead is the largest reservoir in the river and has seen levels not seen since the Hoover Dam was built around a century ago.
Arizona and Nevada face their first ever mandatory water cuts. Meanwhile, water is being released to other reservoirs in order to maintain the hydropower plants at the Colorado River.

The West will not have water if even the Colorado River and its reservoirs aren't immune to heat and drought caused by climate change.

Underground is the secret to finding the answer.

People are becoming more dependent on the water beneath their feet due to rising temperatures and drought. Nearly half of the world's water supply comes from groundwater, with roughly 40% being used for irrigation.

Many people don't realize how old and vulnerable that water is.

The vast majority of underground water has been stored for decades. Many of this water has sat for hundreds of thousands or even millions of year. Groundwater that is older tends to be deeper underground where it is less affected by pollution and drought.

Old groundwater becomes increasingly important as the shallower wells become less viable due to climate change, urban growth, and population growth.

Drinking groundwater from ancient times

You'd likely notice if you ate a slice of bread that was 1000 years old.

Water that has been submerged for more than a thousand years may taste different. It can also leach natural chemicals from surrounding rocks, altering its mineral content.

Groundwater age can also have beneficial effects from natural contaminants like mood-boosting lithium. Other contaminants like iron or manganese can also be problematic.

Sometimes, older groundwater can be too salty to drink without costly treatment. This problem is more severe near the coasts. Overpumping can lead to seawater entering aquifers, which can contaminate drinking water supplies.

It can take thousands of year for ancient groundwater to recover naturally.

California discovered during the 2011-2017 drought that underground storage spaces can compress as they are empty. This means they don't have enough capacity to restock. Compaction causes the above-ground land to crack, buckle, and sink.

People are still drilling deeper wells in West because droughts have depleted surface water, and farmers rely more on groundwater.

What does it mean to water to be "old"?

Let's picture a rainstorm in central California one thousand years ago. The storm passes over San Francisco and most of the rain is absorbed into the Pacific Ocean. It will eventually evaporate into the atmosphere.

Some rain can also fall into rivers, lakes and on dry land. Rainwater seeps through soil layers and enters underground water "flowpaths".

Some of these paths go deeper than others, and water can collect in crevices hundreds of meters below the bedrock.

These underground water reserves are in a sense disconnected from the active water cycle, at least for human life-relevant timescales.

California's Central Valley is dry and much of the water available has been extracted from the earth. This is mostly done for agriculture. Some aquifers have been partially replenished with polluted water by agricultural seepage, where the natural replenishment period would take several millennia.

Fresno, for example, is now actively replenishing aquifers using clean water (such treated wastewater or stormwater), in what's called "managed aquifer refill".

California was the westernmost state to pass legislation requiring groundwater sustainability plans in 2014. This occurred mid-way through 2014's worst drought. Although groundwater is resilient to climate change and heat waves, if you use it all you are in trouble.

Water demand: One solution? Drill deeper. But that is not sustainable.

It's also expensive. Large agricultural and lithium mining companies are more likely to have the capital to drill deep enough. Small rural communities cannot afford this.

Aquifers require time to replenish their water supplies after you have pumped old groundwater. The flowpaths could be blocked, which can cut off natural water supply to rivers, springs, and wetlands. The earth can be destabilized by underground pressure changes, which can cause land to sink or even lead to earthquakes.

Third, contamination: Although deep, mineral-rich, ancient groundwater is generally safer than shallower, younger groundwater, overpumping could cause it to become contaminated.

Overpumping reduces water availability and brings down polluted water. This causes water quality to decline, which leads to increased demand for deeper wells.

Reading climate history from ancient groundwater

There are many other reasons to take care of ancient groundwater. As fossils, very old "fossil water" can tell us about the past.

Imagine the prehistoric rainstorm: 15,000 years ago, it was very different from today. Today, chemical compounds found in ancient groundwater can be detected. This opens up a window into the past. Some dissolved chemicals can be used as clocks to tell scientists the age of groundwater.

We know, for example, how fast dissolved carbon-14 or krypton-18 decomposes, and can therefore measure these to determine when water last interacted.

Younger groundwater, which has disappeared underground in the 1950s, now bears a unique chemical signature. It contains high levels tritium from atomic bomb tests.

Other dissolved chemicals behave in the same way as tiny thermometers.

Noble gases such as argon or xenon, like for example, will dissolve more in cold than in warm water. This is according to a temperature curve that is well-known. Groundwater can be isolated from the air and dissolved noble gases won't do much. They preserve information about the environment at the time water seeped into the subsurface.

We have used the concentrations of noble gas in fossil groundwater to provide some of the most reliable estimates of land temperature during the last Ice Age.

These findings give insight into the modern climate, and how sensitive Earth's average temperature to carbon dioxide in its atmosphere. These methods back a recent study which found that carbon dioxide doubling causes a 3.4 degree Celsius increase in temperature.

Groundwater's future and past

Some regions, such as New England, have had ancient groundwater supplies for many years without running out. Regular rainfall, varied water sources, including lakes, rivers and snowpack, provide alternative groundwater sources and replenish aquifers with fresh water.

Water can be used sustainably if the aquifers are able to keep up with demand.

However, the West has seen over 100 years of unmanaged, exorbitant water consumption. This means that many of the most vulnerable areas to drought, and groundwater arid, have lost the ancient water resources that once existed underground.

The Great Plains is a famous example of this problem. The Ogallala Aquifer, an ancient water source, supplies water for irrigation and drinking water to millions of people across the country from South Dakota to Texas. It would take thousands of year for the aquifer to replenish itself naturally if it was pumped dry.

It acts as a buffer against droughts, but irrigation and water intensive farming are decreasing its water levels at an unsustainable rate.

As the planet heats, ancient groundwater becomes more important. It can be used to water your kitchen faucet, irrigate food crops, and provide warnings about Earth’s past that may help us plan for an uncertain future.

Marissa Grunes is an Environmental Fellow at Harvard University. Alan Seltzer is Assistant Scientist in Marine Chemistry, Geochemistry, Woods Hole Oceanographic Institution. Kevin M. Befus is Assistant Professor of Hydrogeology at the University of Arkansas.

This article was republished by The Conversation under Creative Commons. You can read the original article.

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