The world’s increasing reliance on deep groundwater for agricultural, residential and industrial use is fuelling crop-damaging soil salinity and depleting the world’s supply of fresh water, according to a new study in the journal Nature Climate Change.
Low-lying crop lands here in B.C. could be threatened, as water brought to the surface for human needs compounds the rise in sea levels predicted by climate change models.
The most immediate impact of over-use of groundwater will be on our ability to feed ourselves, according to one of the study’s co-authors Diana Allen, a professor of earth sciences at Simon Fraser University.
About half of British Columbia’s food supply is imported, much of it from California, which has suffered from drought and is projected to become even more reliant on groundwater as precipitation declines due to climate change.
“Water security and food security are inextricably linked,” Allen said. “And the resources that are available in our most productive agricultural areas are already showing signs of water stress.”
Climate scientists expect regions close to the Equator to become progressively drier while northern and southern regions will experience increased precipitation and extreme weather.
In warmer regions that may mean less rain and extended drought not seen in North America since the 1930s. In a wet climate such as British Columbia that could mean that winter rains will come so hard and fast that much of the fresh water that might have replenished groundwater simply runs off into the ocean, Allen said.
“If the rain just runs off it won’t penetrate and it won’t recharge (our) aquifers,” Allen said.
Heavy rains are also associated with microbial contamination of shallow groundwater supplies, fuelling outbreaks of diarrheal disease in both high- and low-income nations, the paper says.
Climate change models generally agree that ocean levels are likely to rise over the next 50 to 100 years.
But as water is removed from deep groundwater systems that are not being replenished, it is added to the volume of water on the surface, something climate change models have failed to represent adequately, the paper’s authors say.
Even if the ocean levels don’t rise enough to top the dikes that protect Metro Vancouver’s most productive farmland in Richmond, Surrey and Delta, sea surges could push water over the top during violent storms, leading to saltwater flooding, Allen said.
In B.C. we may already be feeling the effects of a changing balance between groundwater and surface water supplies as salmon-bearing rivers and streams warm up and run low, said Allen.
Long after the snow has melted and the rains have stopped for the summer in B.C.’s Interior, groundwater continues to flow into the province’s streams and rivers at a temperature of about 10C, which keeps the water cool for spawning fish during the summer and moderates water temperatures essential to egg and fry survival during the winter.
In the Okanagan, surface water licenses for agriculture are fully allocated, which has fuelled an increase in the number of groundwater wells that have been drilled, Allen noted.
“If people keep drilling more wells, eventually the groundwater that discharges into the streams will be reduced,” she said. “Groundwater and surface water are totally linked.”
Last year’s dry summer and early fall saw the Cowichan River on southern Vancouver Island run dry after a rainless period lasting only a few months, she said.
In the Fraser Valley, too, agriculture is placing stress on groundwater systems. Chemicals used in agriculture are also swept into the groundwater system by over-application of fertilizer, irrigation and poor manure management, Allen said.
“The Abbotsford-Sumas Aquifer, which straddles the Canada-U.S. border, has nitrate concentrations that are well above Canadian drinking water guidelines, largely due to intensive agriculture,” she said.
Drinking water from the aquifer is not drawn from wells which register the high nitrate content.
Irrigation water drawn from deep aquifers often has a higher mineral content than water close to the surface.
“When that water is used for irrigation, some of it evaporates and causes salts to accumulate in the soil,” said Allen. “Rains leach those salts out and cause a deterioration of the quality of water closer to the surface.”
Deep groundwater systems, found at depths of 100 metres and more below the Earth’s surface, were recharged by precipitation and melting glaciers at the end of the last ice age, between 10,000 and 20,000 years ago.
“When we take groundwater out of the system and it is not being replenished, you are basically mining it,” Allen said.
The paper, Groundwater and Climate Change, examines the role of groundwater on the world’s eco-systems, water and food security and potential impacts on climate change models that were largely unexamined by the latest reports from the United Nations Intergovernmental Panel on Climate Change, Allen said.
“Most climate change models haven’t taken into account the effects of taking deep groundwater and adding it to the surface systems,” said Allen. “It’s really all one system.”