by Sanket Jain, Yale Climate Connections
September 7, 2026
Extreme heat and widespread drought related to climate change have been in the headlines this summer everywhere from Europe to the Colorado River Basin to Puerto Rico and Bangladesh. But drought is only the beginning of the problem for drinking water. Scientists say the depleted groundwater supplies that remain will warm by 2.1 degrees C globally by the end of this century – changing their chemistry and leading to contamination.
And heat is just one concern. Droughts and fluctuating rainfall patterns are also altering groundwater quality. The good news is that around the world, local and regional authorities are experimenting with ways to replenish groundwater, including methods for collecting excess rainfall or surface runoff and channeling it to replenish aquifers. The work is in early stages, but some methods already are showing promise.
Warming water, changing chemistry
As groundwater warms, dissolved gases become less soluble, while organisms underground can consume oxygen more rapidly. Because many aquifers already contain little oxygen, modest warming could make them even more oxygen-poor.
In some aquifers, this shift can mobilize naturally occurring contaminants such as arsenic, manganese, and phosphorus from surrounding rocks into groundwater, making it less safe to drink. The extra phosphorus can fuel harmful algal blooms when groundwater eventually flows into rivers and lakes. Warmer groundwater may also favor pathogens linked to waterborne diseases.
Under a stable climate, Earth gets warmer deeper underground because heat from the Earth’s core moves toward the cooler land surface, explained Barret Kurylyk, a professor of civil and resource engineering at Dalhousie University and Canada Research Chair in Coastal Water Resources.
“In the past few decades, the land surface has warmed due to climate change,” he said.
As the surface becomes warmer than shallow aquifers, the usual temperature difference reverses, causing heat to move downward toward groundwater. Movement of rain or snow downward can also transfer heat into groundwater, exacerbating the change in groundwater chemistry.
This heat transfer matters because groundwater accounts for 99% of the Earth’s liquid freshwater, provides about half the world’s drinking water, and supplies nearly 40% of the water used for irrigation. By 2100, an estimated 77-188 million people are expected to live in areas where groundwater exceeds the highest drinking-water temperature acceptable threshold adopted by any country.
What happens after a drought
In the remote Kasanal village in Southern India’s Karnataka state, Santosh Naik says the annual monsoon is no longer enough to fill his two wells. The 38-year-old farmer said that by August 2026, his 60-foot wells were 90% empty, even though they would normally fill within a month of the rains.
Naik has spent over a decade drilling deeper in search of water. Since 2011, he has dug 14 borewells – narrow, machine-drilled shafts – across his five-acre farm, each about 300 feet underground. Now only three are functioning. But he is not just worried about declining levels. The water quality is changing too, he says.
“Earlier, we used to drink water from borewells and wells, but if you drink that water now, you will definitely fall ill,” he said.
He added that the entire village has stopped drinking water from its wells.
Naik grows sugarcane, soybean, peanuts and sorghum, but his harvests have been falling as his water supply has gotten saltier. Since 2020, his annual sugarcane production has been about 80,000 kilograms lower, costing him about 280,000 Indian rupees ($2,935).
His experience illustrates a broader problem across India. An analysis of 20,994 groundwater observations estimated that about 29% of the study area, including 25% of cropland, had elevated groundwater salinity.
Changing contaminants
Scientists confirm that climate extremes can change how water moves through the soil and what it carries. One team monitored 13 groundwater wells across three aquifer systems in Germany for up to eight years. As groundwater levels declined, its molecular composition increasingly resembled that of water draining through the soil.
Senior study author Gerd Gleixner, a biogeochemist and professor at the Max Planck Institute for Biogeochemistry, said that during drought, larger pores open up in soils. When heavy rain follows, these pores can carry water rapidly toward groundwater instead of allowing it to move slowly through the upper soil.
“The problem is that this water that comes down is not supposed to go down directly. It is supposed to go more slowly, so that there is more time for progressive processing in the upper part,” Gleixner said.
The upper soil normally acts as a kind of “main bioreactor,” processing substances before they reach the groundwater. But rapid flow through these pores can bypass some of this processing, allowing substances from the surface to reach groundwater more directly.
Gleixner said this rapid downward movement can carry unwanted substances from the surface into groundwater, including antibiotics and, potentially, microplastics.
Scientists are also investigating what happens to groundwater when it is pumped more heavily during droughts. Researchers in California’s San Joaquin Valley analyzed water-quality records from over 3,000 public drinking-water wells over the 2012-2016 drought and the recovery that followed. During the drought, nitrate and total dissolved solids increased in many wells. As pumping intensified, wells drew a larger share of modern-aged groundwater, which was more likely to contain contaminants linked to human activities, such as nitrate. As wetter conditions returned and groundwater levels recovered, many of these changes reversed.
The water people once trusted
Farmer Shivaji Tasgave, 83, no longer trusts the water from a 200-year-old well that served his family for generations after it turned blackish and foul-smelling.
But when the rains failed to arrive as expected, he was forced to rely on this water to irrigate his crops. Half his peanut and sugarcane failed to grow.
In his remote Jambhali village in Western India, Tasgave said that almost 150 wells in his neighborhood have become unusable because of deteriorating groundwater quality.
Along coastlines, groundwater quality can change when the balance between freshwater flowing toward the sea and seawater moving inland is disturbed. Under natural conditions, groundwater flows from land toward the sea, creating a pressure that helps keep seawater from moving inland. When that freshwater pressure weakens, seawater can move farther inland into freshwater aquifers.
A team of scientists in Germany found many coastal groundwater systems are vulnerable to this shift. Between 1990 and 2024, they analyzed about 480,000 coastal groundwater monitoring locations worldwide. They found statistically significant groundwater-level changes in 28% of observations over nine-year windows, and 21% over 19-year windows. Declines became more frequent in the last nine years.
“These declines may reflect a combination of causes, including lower rainfall, drought and reduced groundwater recharge, but also potentially unsustainable groundwater abstraction that exceeds groundwater recharge,” said Annika Nolte, an environmental and data scientist at the University of Bremen, Germany, who led the study.
“Seawater intrusion can make groundwater less suitable or unsuitable for human consumption and may ultimately threaten reliable drinking-water supplies from wells,” she added.
When saline groundwater is used for irrigation, it can also reduce soil fertility and crop yields. A global meta-analysis found that irrigation with salty water reduced crop yields by 17.3% compared with freshwater irrigation.
Giving groundwater a chance to recover
In many parts of the world, researchers are experimenting with different ways to help aquifers recover. Monitoring groundwater levels and salinity, and improving estimates of present and future groundwater recharge, are essential to determine how much water can be withdrawn without weakening freshwater heads, Nolte said.
In the Netherlands, a solution named “Water Battery” has been operating near the village of Epe since 2015. Water from natural springs is collected in a human-made reservoir and then pumped to infiltration ponds, where it seeps into the groundwater. The water is stored underground, while natural biogeochemical processes improve its quality before the groundwater is extracted.
California has been adopting a similar strategy. Excess surface water can be diverted onto selected farm fields to replenish depleted aquifers, increasing groundwater storage and improving water availability during dry periods. Separate field studies in California almond orchards have also shown that flooding fields during the trees’ dormant winter period, in moderately drained to well-drained soils, has little effect on root production or crop yields.
Cities are also finding ways to use stormwater effectively. In the Argentine coastal city of Pinamar, researchers evaluated rain gardens, which are shallow, planted areas designed to collect stormwater and let it soak into the ground. Gardens improved groundwater recharge, particularly during low-intensity rains, where recharge efficiency increased more than sixfold.
Although the approaches differ, the idea remains the same: Instead of moving excess water away, let the ground hold on to it.
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