The largest supply of melted fresh water in the world is stored in aquifers beneath our feet. This groundwater accounts for half the world’s drinking water supply and half the water used to irrigate crops globally. Temperature influences aspects of groundwater quality like arsenic and manganese levels, impacting both human use and the ecosystems that depend on it. Eighteen countries currently have safe drinking water temperature guidelines, with an average recommended temperature of 25°C or 77°F. However, researchers don’t know how groundwater will respond to increasing ground surface temperatures resulting from future warming.
Susanne A. Benz and colleagues hypothesized that increased surface temperatures would negatively impact the chemistry of groundwater and threaten the communities and environments that rely on it. To test this hypothesis, the research team created a computer model to project groundwater temperature trends from the years 2000 to 2100.
They began by compiling climate data from around the world. They relied mainly on past data collected since 1981, and a more recent database used by international climate scientists, called ERA5. ERA5 provides estimates of climate variables like temperature, humidity, wind patterns, and precipitation at locations forming a grid across the Earth. They input this data into their model to predict global groundwater temperatures down to a depth of around 100 meters or about 330 feet.
First, the researchers calibrated their model by projecting groundwater temperatures back in time using a method called hindcasting. They compared their backward projection to actual measured average groundwater temperatures at the water table from 2000 to 2020 and made adjustments until their model accurately predicted past temperatures. They found that local seasonal temperature variations most affected shallow groundwater in the past.
Next, Benz and colleagues used their model to estimate future groundwater temperatures based on 2 different climate change scenarios, one with no climate change mitigation and the other with “modest” levels of mitigation. They explained that these scenarios were determined based on metrics used by the Intergovernmental Panel on Climate Change to determine how different socioeconomic variables will influence climate change.
Based on this model, they estimated that global groundwater temperature would increase by 2.1°C to 3.5°C by the year 2100. The team also estimated that future climate change could penetrate much more deeply than past seasonal variability did, potentially more than 30 meters or 100 feet into the Earth, albeit with slightly more lag time. They also predicted that groundwater would not warm uniformly around the globe, since areas with higher rates of warming in the atmosphere or shallow groundwater will have higher rates of deeper groundwater warming.
Benz and colleagues pointed out that warmer groundwater holds thermal energy that can be utilized as a low-carbon alternative for heating buildings. However, it can also cause a multitude of negative effects. Water stored in aquifers generally has low oxygen levels, and higher temperatures reduce gas solubility further, creating an environment deprived of oxygen. Oxygen-deprived environments can release toxic metals like manganese and arsenic. They also release phosphorus into the groundwater, which can cause algal blooms in groundwater-fed streams and ponds or drinking water supplies.
The team suggested that future climate change could heat groundwater above healthy thresholds. They estimated that as of 2020, over 29 million people already inhabit regions where the average groundwater temperature is above the highest guideline of 34°C or 93°F. They indicated that changes in groundwater temperature can also restrict surface-water ecosystems that harbor temperature-sensitive species. Ecosystems that depend on groundwater, such as spring-fed streams, are often home to specialized species adapted to narrow windows of acceptable temperatures.
The researchers concluded that future groundwater warming may threaten the safety of drinking water and endanger groundwater-dependent ecosystems. However, they pointed out that their groundwater temperature model could be improved by incorporating other aspects of global climate models, like river temperatures, to further test how global water resources will respond to our changing climate.
