SASKATOON — Two University of Saskatchewan (U of S) water researchers are part of an international team that has discovered that global reserves of underground water once assumed to be pure are vulnerable to contamination.
The findings, published today in the prestigious journal Nature Geosciences, have implications for assessing water quality risks and sustainably managing groundwater reserves which supply about 40 per cent of the water for global irrigated agriculture and provide drinking water to billions of people around the world.
“It has long been thought that groundwater from deep wells was mostly protected from surface water contamination,” said Jeffrey McDonnell, U of S hydrology professor and associate director of the Global Institute for Water Security.
“We had expected to find the ancient water pumped from deep wells would not have any evidence of modern-era water, but we found that many of the wells tested had evidence of water that had recharged in the last 50 years, exposing the wells to potential contamination.”
The project was led by University of Calgary assistant professor Scott Jasechko, McDonnell’s former post-doctoral student, and 10 other researchers including McDonnell and Grant Ferguson, U of S associate professor of geological engineering.
The team found evidence that much of the water deep underground, previously considered almost entirely free from human-made contaminants, is susceptible to contamination from water on the surface.
Data taken from more than 6,000 wells around the globe showed that between 42 and 85 per cent of all fresh unfrozen water on Earth is “fossil” water—that is, water that entered the ground more than 12,000 years ago. The fossil water samples were taken mostly from waters pumped from wells deeper than 250 metres.
About half of those wells also had detectable “modern” water from the last 50 years, based on the presence of tritium, an isotope of hydrogen. Tritium is present in all precipitation on the planet due to hydrogen bomb testing in the atmosphere during the 1950s and 1960s.
While deep wells pump predominantly fossil groundwater, the team concluded that when modern water does get into the underground aquifers, there is potential for this water to act as a conduit for other modern-era, human-created pollutants.
“If modern water can get in there, than all the contaminants that go along with modern water could potentially be present,” said Ferguson, co-author and current chair of the Commission on Groundwater and Energy section of the International Association of Hydrogeologists, adding that further study will be needed to determine how this happens.
“Finding any amount of modern water in these aquifers has serious implications for how we understand and manage our water supplies,” he said. “Often only a small amount of contaminants would be needed to render a water supply useless.”
The team suspects the modern water they detected in aquifers is likely caused by leakage from poorly constructed wells or changes in groundwater flow patterns due to pumping, rather than from the natural flow of the water cycle.
Ferguson said the potential for mixing of young and old waters has implications for construction, maintenance and decommissioning practices for the more than 80,000 groundwater wells in Saskatchewan, even though they were not part of the study.
“We suspect outdated or poor practices in the areas studied led to some of this mixing and could be important in contaminant transport to deep aquifers,” he said.
The authors note that dependence upon fossil groundwater to meet water demands is rising due to increasing groundwater withdrawals and deeper drilling in some regions of the world.
“The findings of this study indicate that we need to consider land use practices, particularly those that could lead to groundwater contamination, in the area above these groundwater supplies that were previously thought to be isolated,” Ferguson said.
The authors note that because aquifers containing fossil groundwater require millions of years to be flushed, any contamination may also persist for millennia.
The research was funded by an NSERC Discovery Grant and supported by an UPGro (Unlocking the potential of Groundwater for the Poor) grant jointly from the U.K.’s Department for International Development, the Natural Environmental Research Council, and the Economic and Social Research Council. The full text of the article is available here: http://dx.doi.org/10.1038/ngeo2943
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