5日發表在《自然》期刊的一項最新研究發現，大量低鹽份的地下水源，竟藏身在人們最料想不到的地方──全球大陸棚海床下。
過去認為，海底地下水只有在某些罕見的特殊條件下才會產生。但越來越多證據顯示，全球各地有外海的地下淡水和鹹水蘊藏量。「我們的研究顯示，海床下的淡水和鹹水地下水層其實是很常見的。」研究的主要作者、國家地下水研究培訓中心及澳洲弗林德斯大學環境學院地下水文學家Vincent Post博士說。
「全世界許多地方都有這樣的現象。2萬年前冰帽開始融化、海平面上升，這些地區就被海水覆蓋。許多地下水層因層層粘土和沈積物的覆蓋，得以與海水隔離。」這些海底地下水層數百萬年前開始形成，當時平均海平面比今日要低得多，海岸線也較靠外。「下雨時雨水滲入地下，在過去是陸地而今日是海洋的地區，形成地下水層。」Post說。
海底地下水層和陸地地下水層類似，鹽份也夠低，能夠被淨化成飲用水。估計約有50萬立方公里的低鹽份地下水藏在澳洲、中國、北美和南非附近海域。「這樣的水量是1900年以來人類從地表抽取的地下水的好幾百倍，足以供給部分地區數十年的用水。」Post說。
取得海底地下水有兩種方式：興建海上平台，從海上平台鑽入海床，或是從海底地下水層附近的大陸或島嶼開始鑽。不過從外海鑽海底地下水的費用相當高昂。
雖然許多國家因此有了新的淡水水源，但Post博士表示，取用這些海底地下水，需從成本、永續性和對其他水源的影響等多方面進行考量。Post特別強調，管理海床必須謹慎小心，「可能有海底地下水的海域必須小心不被污染。鑽油或開採天然氣時，有時會鑽到含水層，含水層也可能被用以處置二氧化碳，這些行動都可能威脅到海底地下水的品質。」
Post博士也警告，海底地下水層不是可再生資源，「必須謹慎使用。海底地下水層被用完之後是無法填補回來的，除非等到下一次海平面下降──那不知是多久以後了。」
Huge reservoirs of low-salinity water have been discovered where they are least expected – buried under the seabed on continental shelves around the world.
A study published today in the scientific journal “Nature,” reveals that an estimated half a million cubic kilometres of low-salinity water has been located off Australia, China, North America and South Africa.
“The volume of this water resource is a hundred times greater than the amount we’ve extracted from the Earth’s sub-surface in the past century since 1900,” says lead author Dr. Vincent Post of the National Centre for Groundwater Research and Training and the School of the Environment at Australia’s Flinders University.
“This volume of water could sustain some regions for decades,” said the groundwater hydrologist.
Groundwater scientists have known of freshwater under the seafloor, but thought it occurred only under rare and special conditions.
“Our research shows that fresh and brackish aquifers below the seabed are actually quite a common phenomenon,” Post said.
These reserves were formed over the past hundreds of thousands of years when on average the sea level was much lower than it is today, and when the coastline was further out, he explained.
“So when it rained, the water would infiltrate into the ground and fill up the water table in areas that are nowadays under the sea,” the scientist said.
“It happened all around the world, and when the sea level rose when ice caps started melting some 20,000 years ago, these areas were covered by the ocean,” he said. “Many aquifers were, and are still, protected from seawater by layers of clay and sediment that sit on top of them.”
The aquifers are similar to the ones below land and their salinity is low enough for them to be turned into potable water, says Dr. Post.
There are two ways to access this water – build a platform out at sea and drill into the seabed, or drill from the mainland or islands close to the aquifers.
While offshore drilling can be costly, Dr. Post says this source of freshwater should be assessed and considered in terms of cost, sustainability and environmental impact against other water sources.
There is mounting evidence for the global occurrence of offshore fresh and brackish groundwater reserves. But while many nations may now have new reserves of freshwater offshore, Dr. Post says they must manage the seabed with care.
“Where low-salinity groundwater below the sea is likely to exist, we should take care to not contaminate it,” he warns.
“Sometimes boreholes are drilled into the aquifers for oil and gas exploration or production, or aquifers are targeted for carbon dioxide disposal. These activities can threaten the quality of the water,” cautions Post.
Post also warns that these water reserves are not renewable. “We should use them carefully,” he said. “Once gone, they won’t be replenished until the sea level drops again, which is not likely to happen for a very long time.”
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