Posted by: shipbright | November 9, 2009

Climate change and freshwater part 4…freshwater challenges of Islands

About 12 miles off the coast of Maine near the mouth of Penobscot Bay an island rises out of the Atlantic like a great whale.  Monhegan Island is about a 1.7 miles long and .7 miles wide…a small fishing and artist community that today supports around 70 people year-round and too many “summer complaints” in the summer.  I grew up there as a “summer complaint” in our family’s home, built by my great-grandmother back in the 1920’s and it is here that I learned what island life has to teach us all. monhegan labor day and beyond 2009 003

Living on an island makes one aware of your own “footprints on the beach”.  You pay attention to the water you need, the food you eat, the sewage you cause, the energy you use, the trash you generate, the noise you make and your own behavior because anonymity is not an option.  These are all valuable lessons that more of us should heed not only for our environment but for our society as well.  Living on an Island in Maine is a juxtaposition of American Yankee fiscal conservatism with moderate social allowances and a time-tested and honored wisdom of the intertwining of the environment and the economy.  It’s a higher evolved balance of human social attitudes which translates into behavior that while never perfect and always evolving is still a higher standard than I have found on most mainland communities.  In all my travels whatever self-description and uniqueness island people put on themselves–it’s still a familiar worldview that Islanders around the world share.  Which is why I want bring the whole concept of Climate Change back to islands and freshwater. 

Long before the sea levels rise to overtake and flood low-lying islands, such as those in Oceania or in the Caribbean basin, the rising sea will intrude upon the freshwater resources of these islands–all islands low or tall.  It begins with the physics of water:

  • Ocean saltwater weighs about 8.55 lbs per gallon [metric conversion:  3.878 kilograms per 3.785 liters];
  • Freshwater weighs about 8.34 lbs per gallon [metric conversion:  3.783 kilograms per 3.785 liters].

So freshwater weighs less than saltwater…common sense since the freshwater doesn’t have all those salts nor as many minerals.  On an island, rainwater percolates through the bedrock and forms a “freshwater lens” that literally floats on top of saltwater.

freshwater lens1

This freshwater is also influenced by tidal changes and by storm surges.  As the ocean rises it pushes up the freshwater lens.  As long as your well is dug very carefully you will not pierce the lens and extract saltwater.  Here’s video of a simulation of a freshwater lens and in the author’s words: 

“This movie shows the development of a fresh water lens over a 2-year period beneath a small island surrounded by tidal flats subject to a semi-diurnal tide. Red colors indicate saline water, blue colors indicate fresh groundwater. The blue prism surrounding island represent the surface water in the lagoon. Groundwater recharge only occurs in the center of the island.” [no audio in the simulation video]

As you can see the freshwater lens on an island is a dynamic phenomenon.  Pulling water out of the lens requires great sensitivity to precipitation and recharge, sea levels, storm surges and pollution.  Once the lens is polluted you don’t have other choices like people on the mainland have.

Freshwater is an issue of national security for island nations.  In the Bahamas, the residents and tourists of the capital city Nassau need approximately 8.1 million gallons of freshwater everyday.  The government must barge over approximately 4.33 million gallons everyday from the neighboring island of Andros because Nassau [island of New Providence] doesn’t have enough water to satisfy the demand.  If the freshwater lens of Andros becomes compromised the inhabitants and tourists, upon which the economy of the Bahamas is so dependent upon, will find themselves with half their freshwater cut off.  That’s not enough to sustain the population.


Here’s some information from the Bahamian Water and Sewage Corporation at:

“All freshwater in the Bahamas is only available as groundwater, which comes about as a result of rainfall. The freshwater resources occur as concave lens-shaped bodies:   

bahamas lens90% of all freshwater lenses are within five feet of the surface.  Freshwater resources occur as three-dimensional lens-shaped bodies, which overlies brackish and saline waters at depth.  The size, shape and orientation of the island, the subsurface, geology and the amount of rainfall control the shape, size and thickness of freshwater bodies.  The only source of drinking and irrigation water is from groundwater lenses, which float on brackish water due to differing densities.  Due to the shallow depth of the freshwater lenses, the resources are vulnerable to several environmental risks.  Additionally, over-extraction of groundwater lenses is an ongoing concern. 

In order to meet the water requirements of a growing population on the island of New Providence, the water lenses have been used beyond their sustainable or safe yields.  This has caused a mixing of fresh and brackish lenses resulting in a steady rise in the salinity of the water supplied.”

On the other side of the globe the islands of the South Pacific face water security issues as well.  Here’s an excellent primer from Islands First:

“An important problem already affecting numerous coastal areas including small islands is salt water infiltration in soils or salinisation, especially on atoll countries such as Tuvalu, Maldives, or Kiribati.  Atolls get their freshwater supplies from rainfall or groundwater (rainfall filtered in the ground). Freshwater is lighter than salt water; a lens is formed under the atoll with freshwater on top. This reserve of freshwater is vulnerable to decrease in rainfall (as the lens cannot replenish) and over drilling which can contaminate the lens water with brackish water.

As projections from the IPCC have shown, there is strong evidence that water resources and distribution of rainfall on small islands will be compromised with climate change. In Kiribati, for instance, a 10% reduction in average rainfall by 2050 would lead to a 20% reduction in the size of the freshwater lens. In addition, increase frequency of extreme weather events, sea level rise and resulting land loss, are likely to increase the stress on freshwater lens on atolls. For example, studies in Tarawa, Kiribati, demonstrated that a 50 cm rise in sea level accompanied by a reduction in rainfall of 25% would reduce the freshwater lens by 65%.

These negative impacts of climate change cumulated with population increase put the availability of fresh water resources at risk. Water quality is likely to be degraded by salt water infiltration. This could lead to health problems related to the scarcity of freshwater, and to the spread of water born diseases. As freshwater runs scarce, life on islands will be more difficult to sustain. The inhabitants of the Carteret islands in Papua New Guinea are currently suffering increase water shortage and rely on coconut water since average precipitations have decreased and their freshwater supplies have been contaminated by saltwater infiltration.

Salt water infiltration has also severe adverse impacts on agricultural practices. As salt water infiltrates the aquifers and soils, many salt intolerant traditional crops, such as taro or pulaka, die from salt contamination, which affects the traditional diet of islanders. For example, across the Pacific, taro crop is a fundamental element of islanders’ diet. Because of salt contamination in soils, many island communities have been forced to relocate their plantations further inland or in higher grounds. For atolls, which culminate at less than 5 meters above sea level, the situation is even more precarious: some island communities, in Tuvalu for instance, have started growing traditional crops in tin cans since they are unable to relocate to higher grounds…

Fresh water is lighter than salt water. Therefore, fresh water “floats” on top of salt water. This principle becomes extremely important when considering the drilling of a well in order to tap into the ground water of any island. The weight of the rain water that percolates into the ground depresses the salt water beneath it forming a profile that has the appearance of a lens. This is called the Ghyben-Herzberg lens. The principle of this relationship was discovered independently by a Dutch scientist named Baden-Ghyben and a German scientist named Herzberg.

The underground boundary that separates the fresh water layer from the salt water is not a sharp boundary line. In reality, this boundary is a transition zone of brackish water (fresh/salt mixture). This is caused by seasonal fluctuations in rainfall, tidal action, and the amount of water being withdrawn either by humans or by natural discharge.”

Islands teach us about resource management in an “in your face” style.  Managing an islands resources in a sustainable manner is critical for survival and for economic development…and nothing is more critical than freshwater.  Pressures of irrigating crops for food along with freshwater needs for hydration and hygiene are all wrapped up in a world where you are dependent upon a fragile freshwater lens sitting on top of saltwater.  

Changes to the hydrologic cycle, like cyclonic events, are predicted to get stronger as a result of climate change and sea levels are already rising.  The Bahamas experienced this with Hurricane Katrina…Andros [Arawak Cay] pumped salt water for a few days until the sea level subsided and the freshwater lens settled back down to the well points. These are all real challenges to islands and island nations. They hold lessons for those of us on the mainland as these issues, while magnified by the nature of island living, are our issues as well.

When the wells run salt there is nowhere to go.  It’s like being on a ship at sea and the ship catches fire…there’s nowhere to go…you have to deal with it and do it quickly.  Otherwise you abandon ship or in this case de-populate in a planned manner, die from dehydration, or succumb to chaos in a desperate bid for survival when the veneer of civility is stripped off us:  no water…no civility…no civilization 

It is our responsibility to plan ahead, conserve our resources and come up with some fresh[water] ideas for a thirsty planet or islands in this case. 

Pay attention to the islands.  They are the Miner’s Canary. 

But for now, as winter sets into the northern hemisphere, for all my friends in the Commonwealth of the Bahamas we all still know “…it’s better in the Bahamas…”  😎


[P.S. for those in the South Pacific and feel it is better there just send a plane ticket and I’ll be happy to come verify that–preferably around January or February up here in Maine…I know, shameless appeal for a free trip to the tropics]



  1. Fantastic article. Well written, informative and interesting. I’ve had a fascination with islands for a few years now (obviously entertaining the idea of eventually getting to visit and/or retire to one) and take time out regularly to educate myself on all facets of life on these fragile gems.

  2. I thank you for all your research and information! We have a mission on LaGonave, an island off the coast of Haiti by Port – a – Prince. We are creating self-sustainable communities wher the people can support themselves and one of the problems we are now encountering is the upward movment of the salt water intrusion of the lens. We knew about the lens research in the Caribbean. over the last few decades NGO’s have drilled too many wells and have given it for free to the people. One NGO even punched through the lens and before it could be contained polluted a fair amount of area. The people have never been taught water conservation and even though there cisterns on the island there ar not even near enough to stave off the intrusion. We now have the “ears” of the island leaders and we are going to do an educational approach about conserving water and cisterns even down to the smallest community. The island has a population of 120,000 people, so this will take some time but as you stated, once the fresh water is gone it is hard to sustain that population as we too have witnessed in the Bahamas. Thanks again and God bless,

  3. HAHA free ticket—hey! semi tropical isn’t so bad either—best time of the year.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s


%d bloggers like this: