Global groundwater volumes in the Earth’s continental crust are critical for water security.
Estimates of global groundwater as a potable water resource have been based on studies that have approximated a one- or- two-kilometer lower boundary of the Earth’s continental crust.
Recent research (published in Geophysical Research Letters, August 2021) led by
Dr. Grant Ferguson (University of Saskatchewan) and Dr. Jennifer McIntosh (University of Arizona) have revised those estimates to a depth of up to ten kilometers. In fact, the researchers have also revised estimates of the Earth’s total volume of groundwater (including deep groundwater) to be a volume larger than the ice sheets in Greenland and Antarctica – previously thought to be the largest reservoir of water on Earth.
WATERTODAY connected with both Dr. Ferguson and Dr. McIntosh.
Working at the interface of hydrology, geochemistry, and microbiology to understand processes throughout the Earth’s crust, Dr. Jennifer C. McIntosh is a Distinguished Scholar and Professor, Hydrology and Atmospheric Sciences; and Joint Professor Geosciences at the University of Arizona. In a telephone interview told WT, “We looked at the depth of meteoric water.”
Meteoric water is the water derived from snow and rain. This includes lakes, rivers, and ice melts which all originate from precipitation indirectly.
Deep circulation of waters, coming from precipitation, connects the Earth’s surface with deeper subsurface environments, transferring water, energy, and life critical for key processes, such as deep mineral weathering, release of nutrients, and geothermal energy systems.
McIntosh adds that we learn from this how microbial life gets to the surface.
The depth of meteoric water circulation varies considerably across North America as a function of topography and fluid density, in addition to permeability.
“The depth of the water depends on location,” McIntosh says. “The deepest circulation of meteoric groundwater is found in thermal springs in the mountainous areas of Western North America, whereas in the Great Lakes area, it is shallow.”
The shallowest circulation depths are associated with oil/gas-produced waters in sedimentary basins and mines in crystalline bedrock.
McIntosh says that the most recent research that she and Ferguson led built on previous studies. “We looked at those studies to make our predictions and general models.”
The researchers used water stable isotopes (non-radioactive forms of atoms that can be measured in water samples). The stable isotopes revealed the origins and history of the water which they compared to predictions made on the depth of circulation based on topography and the geometry of the subsurface.
“The primary evidence came from the isotopic composition of the water,” Dr. Grant Ferguson told WT. “The hydrogen and oxygen of the water molecule can have different masses depending on their origin. For example, seawater will be distinguishable from precipitation waters from the Earth’s mantle. Using these differences, we are able to establish water that originated as rain or snowmelt versus deeper groundwater, which tends to have other origins. These waters appear to be weakly connected to the rest of the hydrological cycle.”
The water in the subsurface can be thousands to millions of years old and in some places travelled kilometers deep before reaching its way back to the surface.
According to Dr. Grant Ferguson, University of Saskatchewan, School of Environment and Sustainability, and member of the Global Institute for Water Security,
“The response times of these deeper systems are often on the order of millennia, and they can contain water from the last ice age,” Ferguson says.
Along the flow path the water has reacted with rocks and released elements that it has come into contact with as the water circulates.
While some of these deeper waters are of sufficient quality to be used for agriculture or drinking water supplies, they would be a stop gap to address climate change caused water shortages because they would be slow to be replenished if at all.
McIntosh and Ferguson tell us that the presence of meteoric waters at depths of a few kilometers may provide clues into the long-term fate of any waste disposed of in these deep groundwater systems.
Ferguson adds, “We are also examining the potential impact of the oil and gas industry on deep groundwater systems in the Canadian Prairies.”
McIntosh, a nuclear waste expert, adds that perhaps contaminants could be placed beneath the newly discovered groundwater depth boundary. “We could put nuclear waste where it is not going to return to the flow path.”
However, Ferguson says, “Rates of contaminant transport are likely to be very low in these systems but this raises some questions about legacy impacts well into the future – potentially over timeframes of thousands to millions of years for some contaminants.”
The researchers add this is a new frontier of hydrology that will require further study to provide important insights into how the legacy of the Anthropocene might persevere over deep time in the subsurface.