永凍土融化可能是環境品質退化的原因
by Emily C.
Dooley
發表於《自然》集團的期刊《通訊—地球與環境》(Communications: Earth and Environment)的新研究發現，在阿拉斯加極為偏遠的地區，有數十條河川溪流從澄清的藍色變成混濁的橘色，這些色彩的來源可能是因為永凍土融化而與外界接觸的礦物。
阿拉斯加北極門國家公園Kutuk河的空照圖，顯示河水變成了鐵鏽一般的顏色。永凍土融化造成礦物受到風化作用，使河水變酸，進而讓鐵、鋅、銅釋放出來。照片來源：Ken Hill，美國國家公園管理處
由美國國家公園管理處、美國地質調查局、加州大學戴維斯分校與其他研究機構組成的研究團隊，首次在阿拉斯加北部的布魯克斯山脈，面積為德州大小(約19個台灣)的區域中定位出75個河水品質退化的地點，並且對其中一部份進行採樣與建檔。
研究人員表示隨著氣候變遷加劇，這些退化的河川溪流可能會為北極流域的飲用水和漁業造成顯著影響。
「隨著我們空中巡察的次數增加，我們也開始注意到越來越多變成橘色的河川溪流，」主要作者Jon
O’Donnell表示。他是美國國家公園管理處的北極清查與監測網絡的生態學家。「有些地方的河水看起來幾乎就跟混濁的柳橙汁一模一樣。」
「這些橘色的溪水會帶來問題，不只是因為具有毒性，還有可能會阻礙魚類遷徙到牠們的產卵地點。」
在這張阿拉斯加北極門國家公園Kutuk河的空照圖裡，彷彿有一大桶橘色的顏料倒入澄清的藍色河水之中。照片來源：Ken
Hill，美國國家公園管理處
從太空也能看到
O’Donnell首次在2018年注意到這項問題，當時他去視查一條顏色變得跟鐵鏽一般，但是前一年還很乾淨的溪流。之後他開始到處詢問是否還有類似的地點並加以彙整。由於此區相當偏遠，通常只能靠直升機前往當地的河川，因此他也盡可能把握每次能採集河水樣品的機會。
「這些變色的河川面積大到從太空都能看見，」研究計畫主持人，加州大學戴維斯分校的環境毒理學助理教授Brett
Poulin表示。「它們的變色程度必須十分嚴重，我們才能從太空把它們挑出來。」
Poulin的專長為水化學。他認為這些變色的情形跟流域中開採酸性礦物時出現的變化很像，差別在於這些受損的河川——包括著名的鮭魚河和其他受聯邦保護的河流——周圍完全沒有礦場。
有個假說認為原因為永凍土(基本上就是凍結的地面)裡儲存的礦物。隨著氣候逐漸暖化，這些過去與外界隔絕的金屬礦石會和氧氣與水分接觸，造成酸性物質和金屬釋放出來。
「化學告訴我們礦物會受到風化作用，」Poulin表示。「而河水的組成就像指紋一樣，能讓我們瞭解究竟發生了什麼事情。」
這些受影響的河川位在公有土地，隸屬於美國內政部土地管理局、美國魚類及野生動物管理局與美國國家公園管理處，其中包括了北極門國家公園和科伯克河谷國家公園。
Poulin與博士候選人Taylor
Evinger分析了最初採到的樣品，並在去年八月親自去野外採樣，而其他研究人員也在去年六月與七月前往採樣。今年他們計畫在夏季進行三次野外考察以採集更多樣品。
釋出金屬的酸性河水
這些水質退化的河水樣品中有部分的pH值低到只有2.3，相較起來採自同一群河川的樣品pH值平均為8。這代表有硫化物礦物受到了風化作用，造成周遭環境變得非常酸且帶有腐蝕性，使得更多金屬釋放出來。樣品的鐵、鋅、鎳、銅、鎘濃度的測量值也有偏高或高的現象。
「我們在這些河水中看到了許多種不同的金屬，」Evinger表示。「最主要的金屬類型之一為鐵，也就是造成河水變色的原因。」
雖然O’Donnell第一次注意到河川出現變化的時間為2018年，但是從衛星影像卻發現河川變色的情形可以追溯至2008年。
「隨著時間經過，這道問題就像是從一道微小的源泉，緩慢擴散成較大的溪流一般，」他說。「當緊急問題或威脅出現時，首要之務是去理解它們。」
瞭解風險
這項三年期計畫目前來到了第二年，研究人員的目標是要瞭解河水當中究竟發生了什麼事，模擬還有哪些地方可能面臨同樣的風險，並且評估飲用水和魚群會受到何種衝擊。
日益成長的這道問題會影響生物棲地、水質以及其他生態系，使得健康的區域轉變成退化的棲地，而讓其中的魚類和無脊椎動物減少。如果農村聚落仰賴這些河川為飲用水，長期下來他們可能需要接受治療，而餵養當地居民的魚群也有可能遭受影響。
「這會帶來許多後果，」O’Donnell表示。「隨著氣候持續暖化，我們預期永凍土也會繼續融化，因此只要有這類礦物存在的地區，當地的溪流就有可能變成橘色，水質也因此惡化。」
他們還需要進行更多研究以深入瞭解這項問題，並且探討變色的河川溪流是否可以復原，也許天氣變冷使得永凍土重新形成之後就能變回去。
「我認為之後還得進行許多更加詳細的研究，才能解決一部分現在仍然無法定論的地方，」O’Donnell表示。
阿拉斯加太平洋大學、科羅拉多州立大學、阿拉斯加大學安克拉治分校、加州大學河濱分校的科學家也參與了此研究。
研究經費來自美國地質調查局—美國國家公園管理處的水質聯合計畫、美國地質調查局的北極生態系變遷倡議、美國國家公園管理處的北極清查與監測網絡。
Alaska's rusting
waters: Pristine rivers and streams turning orange
Thawing
permafrost may be driving degradation
Dozens of Alaska’s most remote streams
and rivers are turning from a crystal clear blue into a cloudy orange, and the
staining could be the result of minerals exposed by thawing permafrost, new
research in the Nature journal Communications:
Earth and Environment finds.
For the first time, a team of researchers from the
National Park Service, U.S. Geological Survey, the University of California,
Davis, and other institutions have documented and sampled some of the impaired
waters, pinpointing 75 locations across a Texas-sized area of northern Alaska’s
Brooks Range.
These degraded rivers and streams could have
significant implications for drinking water and fisheries in Arctic watersheds
as the climate changes, the researchers said.
“The more we flew around, we started noticing more
and more orange rivers and streams,” said lead author Jon O’Donnell, an
ecologist for the NPS’ Arctic Inventory and Monitoring Network. “There are
certain sites that look almost like a milky orange juice.
Those orange streams can be problematic both in terms
of being toxic but might also prevent migration of fish to spawning areas.”
Visible from
space
O’Donnell first noticed an issue when he visited a
river in 2018 that appeared rusty despite having been clear the year prior. He
began asking around and compiling locations while grabbing water samples when
possible in the remote region, where helicopters are generally the only way to
access the rivers and streams.
“The stained rivers are so big we can see them from
space,” said Brett Poulin, an assistant professor of environmental toxicology
at UC Davis who was a principal investigator in the research. “These have to be
stained a lot to pick them up from space.”
Poulin, whose expertise is in water chemistry,
thought the staining looked similar to what happens with acid mine drainage,
except no mines are near any of the impaired rivers, including along the famed
Salmon River and other federally protected waters.
One hypothesis is that the permafrost, which is
essentially frozen ground, stores minerals and as the climate warmed, the metal
ores that were once locked up were exposed to water and oxygen, resulting in
the release of acid and metals.
“Chemistry tells us minerals are weathering,” Poulin
said. “Understanding what’s in the water is a fingerprint as to what occurred.”
The impacted rivers are on federal lands managed by
Bureau of Land Management, Fish and Wildlife Service and NPS, including Gates
of the Arctic and Kobuk Valley national parks.
Poulin and Ph.D. candidate Taylor Evinger analyzed
initial samples, then collected their own on a trip last August, while others
took samples in June and July. This year, they will take three trips during the
summer to collect additional samples.
Acidic water
releasing metals
Some samples from the impaired waters have a pH of
2.3 compared to the average pH of 8 for these rivers. This means the sulfide
minerals are weathering, resulting in highly acidic and corrosive conditions
that release additional metals. Elevated or high levels of iron, zinc, nickel,
copper and cadmium have been measured.
“We see a lot of different types of metals in these
waters,” Evinger said. “One of the most dominant metals is iron. That’s what is
causing the color change.”
While O’Donnell first noticed a change in 2018,
satellite images have turned up stained waters dating back to 2008.
“The issue is slowly propagating from small headwaters
into bigger rivers over time,” he said. “When emergent issues or threats come
about, we need to be able to understand them.”
Understanding
risk
The researchers are in the second year of a
three-year grant aimed at understanding what is happening in the water,
modeling what other areas may be at risk and assessing implications for
drinking water and fishing stocks.
The problem is growing and affecting habitat, water
quality and other ecological systems, turning healthy areas into degraded
habitats with fewer fish and invertebrates. If rural communities rely on these
rivers for drinking water, they could require treatment eventually, and the
fishing stocks that feed local residents could be affected.
“There’s a lot of implications,” O’Donnell said. “As
the climate continues to warm, we would expect permafrost to continue to thaw
and so wherever there are these types of minerals, there’s potential for
streams to be turning orange and becoming degraded in terms of water quality.”
More work is needed to better understand the problem
and whether rivers and streams can rebound, perhaps after cold weather promotes
permafrost recovery.
“I think there will be a lot more detailed work to
follow up to address some of the uncertainties that we currently have,”
O’Donnell said.
Scientists from Alaska Pacific University, Colorado
State University, University of Alaska Anchorage and UC Riverside also
contributed to the research.
The research was funded by U.S. Geological Survey–NPS
Water Quality Partnership program, the U.S. Geological Survey Changing Arctic
Ecosystem Initiative and the NPS Arctic Inventory and Monitoring Program.
原始論文：Jonathan A.
O’Donnell, Michael P. Carey, Joshua C. Koch, Carson Baughman, Kenneth Hill,
Christian E. Zimmerman, Patrick F. Sullivan, Roman Dial, Timothy Lyons, David
J. Cooper, Brett A. Poulin. Metal mobilization from thawing permafrost
to aquatic ecosystems is driving rusting of Arctic streams. Communications
Earth & Environment, 2024; 5 (1) DOI: 10.1038/s43247-024-01446-z
引用自：University of California - Davis.
"Alaska's rusting waters: Pristine rivers and streams turning
orange."
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