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Invisible Rivers
Science News Online ^ | 10-15-2005 | Sid Perkins

Posted on 10/16/2005 4:47:06 PM PDT by blam

Invisible Rivers

Fresh water also flows to sea through the ground

Sid Perkins

About 2,000 years ago, the Roman geographer Strabo wrote about the residents of Latakia, Syria, who rowed their boats 4 kilometers out into the salty Mediterranean, dove a few meters to the ocean floor, and collected fresh drinking water in goatskin containers for their city. No miracle, this—marine boaters could do the same today at a spot about 10 km east of Jacksonville, Fla. In fact, similar freshwater springs erupt on the seafloor near many shores. These flows of water originate on land and end up in the sea, just as rivers do—only they take a subterranean route to their destinations.

TWO EXITS, NO WAITING. Fresh water from land can reach the sea through porous material near the shoreline or via deeper strata that discharge far from shore. Swarzenski

Such underground rivers form only under certain geologic conditions. At some sites, the water flows from onshore aquifers to the sea through porous rocks and then seeps up through the seafloor. At many more locales, groundwater drains at low tide through sand or other porous shoreline sediments into the ocean.

Collectively called submarine groundwater discharge, such flows to the sea are gaining increasing attention in scientific circles. Their flow rates are often low—sometimes just a few liters per day for each square meter of seafloor—but those trickles become significant when tallied over large areas. That influx of fresh water alters the ocean's salinity near the seafloor, a factor that influences the makeup of the ecosystems in those places.

Many of these ecosystems are threatened by increasing amounts of nutrients or pollutants in the water arriving from the shore. Those substances can have widespread effects, fueling algal blooms and microbial growth in sediments or smothering coral reefs.

Few, if any, people now take advantage of submarine springs—it's far too easy today to drill a hole on land to reach fresh water. But as scientists become more aware of the large volume of submarine groundwater discharge in some locations, they're figuring out how to detect and measure the phenomenon. They're also constructing theoretical models of how it works and developing devices that can accurately measure gases and nutrients dissolved in the water.

No-man's-land

Although submarine groundwater discharges have been known for centuries, they've often been considered just local curiosities. Scientific scrutiny of such flows has been slow in coming.

For a long time, many of the scientists who studied the movement of fresh water over and through the ground felt that their purview ended at the shoreline. Meanwhile, many oceanographers knew that fresh water seeped from land into the oceans, but, suspecting that volumes were small, they considered that influx to be of little consequence. Agencies that sponsor research were often mystified about how to classify proposed projects to study the phenomenon. That confusion tended to stifle the flow of research funds, says William C. Burnett of Florida State University in Tallahassee.

Calculating the amount of water that rivers and other surface runoff carry to the sea is relatively easy. But the low flow rates characteristic of submarine groundwater discharge make it tricky to assess, and its underwater milieu makes detailed study difficult. Also, variations in porosity in the rocks carrying the discharge can cause rates of seepage to vary significantly from one spot to another, requiring scientists to take measurements over broad areas to get a good estimate of the overall flow rate. Few of the world's coastlines have had wide-ranging surveys of their submarine groundwater discharge. Nevertheless, individual research projects, such as a study recently conducted in Florida's Biscayne Bay, suggest that such seepage can be significant.

Water samples taken from several research wells on the Biscayne Bay floor between August 2002 and March 2004 were much less salty than water in the bay itself. This indicates that fresh water was making its way into the bay from land, says chemical oceanographer Peter W. Swarzenski of the U.S. Geological Survey in St. Petersburg, Fla.

Using direct measurements of seepage during March 2004, Swarzenski and his colleagues estimated that each day about 230 liters of fresh water enter the bay through each square meter of the discharge zone. Therefore, the volume of fresh water supplied to the bay by submarine groundwater discharge is about 6 percent of the volume entering the bay through rivers or surface runoff. The researchers reported their findings in May at a meeting of the American Geophysical Union in New Orleans.

While many stretches of the East Coast receive comparable percentages of their fresh water from submarine groundwater discharge, locales along the West Coast may receive only 1 percent, if that, says Swarzenski. But in other areas, such as along the shores of Mexico's Yucatán, as much as 15 percent of the ocean's freshwater input comes from submarine groundwater discharge.

Overall, data from the smattering of studies conducted over the past decade or so suggest that between 5 and 6 percent of the fresh water that makes its way to the world's oceans does so along subterranean routes, says Burnett.

Wherever submarine groundwater discharge is occurring, the seeping water carries a variety of dissolved substances, some of which can have profound effects on local ecosystems. For example, the groundwater coming from land may contain concentrations of dissolved nitrate at least 10 times the concentrations typically found in coastal waters, says Ivan Valiela, a marine biologist at Boston University. Nitrate, which comes from septic systems, fertilizer, and decomposing marsh plants, is an important nutrient for algae and other phytoplankton at the base of the sea's food chain.

Nutrient-stimulated algal blooms are a double-edged sword. In the short term, they provide an increased food supply for fish, but over the long haul, their decomposition can rob the water of dissolved oxygen and threaten marine life such as coral colonies.

Change of season

Just as a region's volume of precipitation varies from season to season and from year to year, so, too, does the seepage rate of submarine groundwater. For example, ocean tides influence the rate at which groundwater seeps upward through the seafloor. During high tide, discharge sites lie beneath a tall column of water, whose pressure counteracts the fresh water's seepage. The shorter column of overlying seawater during low tide permits more groundwater discharge.

Another factor that controls submarine groundwater discharge is the elevation of the water table on land. The higher that water table, the more pressure on the water in aquifers and the more readily water is driven offshore. Although the water table typically doesn't fluctuate much day to day, over the course of a year, it can move up and down several meters, says Charles F. Harvey of the Massachusetts Institute of Technology.

The effects of these seasonal changes showed up during a study that Harvey and his colleagues conducted at Waquoit Bay, Mass. Data taken between 1999 and 2003 during summer months, when the water table on land was high, showed large volumes of groundwater seeping into the bay—on average, about 200 liters per hour for each square meter of discharge zone. However, data gathered in February 2004—a time of year when the water table is typically low—indicated that salt water was actually being drawn into the bay floor.

Several factors contribute to this seepage seesaw, the researchers propose in the Aug. 25 Nature. First, the aquifer recharges between late autumn and early spring. Because it takes a few months for water to filter down through the uppermost layers of unsaturated soil, the peak seepage of water into the bay doesn't occur until summer.

From late spring until early autumn, the water table drops because growing plants are extracting more water from the soil than is returning as precipitation. So, after a few months, bay water is drawn into the submerged sections of the aquifer.

Searching for water

Increasingly, scientists are employing a variety of high-tech dowsing devices to identify undersea sites where fresh water seeps into coastal shallows.

One such instrument is a radon detector. Radon is produced by the decay of radium, a radioactive element found in nearly all soil and rock. Therefore, groundwater contains much higher concentrations of the dissolved gas than seawater does.

Because radon itself is radioactive, it makes an ideal marker for seafloor seepage, says John F. Bratton of the U.S. Geological Survey in Woods Hole, Mass. He and his colleagues developed a system that, in a matter of minutes, pumps seawater into a chamber, extracts the dissolved radon, and measures the water's temperature and salinity. Instruments record those data along with navigational information that enables scientists to map where groundwater is seeping into coastal waters.

Another technology to identify seepage sites works somewhat as ground-penetrating radar does. In this method, scientists tow behind a boat a device that repeatedly emits and receives electrical charges to measure the electrical resistance—and hence the salinity—of the water along the ocean bottom.

The discharge sites that Burnett and his colleagues identified during research cruises in Florida's Sarasota Bay matched those identified by radon and other geochemical tracers, he noted last December at a meeting of the American Geophysical Union in San Francisco. This electrical method is far quicker than laboratory analysis using tracers, which may take weeks to complete.

Burnett's team also measured water temperatures and found that the waters near discharge sites in summer months were cooler than they were in the rest of the bay. Such temperature differences can easily be exploited to find discharge sites, Tomochika Tokunaga and his colleagues from the University of Tokyo said at the San Francisco meeting.

BETRAYED BY HEAT. These aerial infrared images taken at night near Shiranui, Japan, in December 2004 clearly show the warm, fresh water from land (light blue) seeping into coastal waters at high tide (left) and being shaped by an incoming tide (right). Dotted circles denote seepage zones. Tokunaga et al.

In a nighttime aerial survey of a shallow coastal site near Shiranui, Japan, these scientists used thermal infrared-imaging equipment to scan the sea's surface over a known discharge zone. In August, the ocean temperature was about 14°C warmer than that of the groundwater seeping from the seafloor. When the seafloor spring lay 1.1 m below the water's surface, the surface temperature was about 0.4°C cooler than that of nearby waters not situated over discharge zones.

Once seepage sites are located, scientists deploy instruments to measure flow rates. The seepage meters used most often by researchers today are 55-gallon drums that have been sawed in half and had their open ends shoved into the seafloor. Water flowing out of the sediment forces water trapped in the enclosure through a nozzle on the upper end of the drum and into a plastic bag, which can be recovered and weighed to gauge the rate of discharge—an effective but decidedly low-tech method.

Increasingly, researchers are turning to fancier gadgets to measure flow rates. In one such instrument, a computer-controlled heating coil zaps water as it enters one end of a tube placed in the seep. Sensors then measure the time it takes for the heated water to reach the other end. The speed of water flow through the tube enables scientists to calculate the rate of discharge.

Other researchers are developing seafloor instruments that can identify substances dissolved in seafloor seepage. For example, the prototype equipment designed by Arnaud Bossyut and Gary M. McMurtry of the University of Hawaii at Honolulu measures dissolved gases such as methane, carbon dioxide, and hydrogen sulfide. The device is battery powered and designed to operate at depths of up to 1 km for 6 months at a time. Future versions of the apparatus may identify traces of dissolved nitrates or other nutrients, McMurtry notes.

The recent flurry of scientific interest in submarine groundwater discharge heartens Burnett, who, with a few crusading colleagues, has apparently convinced other researchers that seafloor seepage is important. "It's about time," he notes.


TOPICS: News/Current Events
KEYWORDS: archaeology; godsgravesglyphs; history; invisible; rivers
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1 posted on 10/16/2005 4:47:09 PM PDT by blam
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To: Carry_Okie; RightWhale

May be a source of fresh water for southern California farmers?


2 posted on 10/16/2005 4:48:16 PM PDT by blam
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To: blam

I love it-- absolutely no mention of HM. Ahhhhhhh. Interesting, too.


3 posted on 10/16/2005 4:49:14 PM PDT by Clara Lou
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To: blam

Quite interesting.


4 posted on 10/16/2005 4:51:01 PM PDT by Bigg Red (Do not trust Democrats with national security!)
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To: blam

There are underground rivers. The flow rate is low, although the volume may be large. Any such rivers in California would supplement surface runoff from the Sierra Nevada. That might suffice for a few small towns or farms, but California will have to look to desalinization as their usage gets heavier.


5 posted on 10/16/2005 4:58:08 PM PDT by RightWhale (Repeal the law of the excluded middle)
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To: SunkenCiv

Possibly Pingworthy? (considering the first paragraph)


6 posted on 10/16/2005 5:00:36 PM PDT by solitas (So what if I support an OS that has fewer flaws than yours? 'Mystic' dual 500 G4's, OSX.4.2)
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To: blam

They need it, for ever gallon of strawberries, it takes two gallons to quench the thirst of the manual laborers and their families, since California farmers have no incentive to innovatively mechanize .


7 posted on 10/16/2005 5:05:47 PM PDT by seastay
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To: blam

ping for ancient trade, technology and general getalong


8 posted on 10/16/2005 5:10:00 PM PDT by Graymatter
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To: blam

Oh, nevermind.
9 posted on 10/16/2005 5:11:23 PM PDT by struggle ((The struggle continues))
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To: solitas
"Possibly Pingworthy? (considering the first paragraph)

That's what first got my attention, lol. Not GGG Ping worthy though.

10 posted on 10/16/2005 5:15:03 PM PDT by blam
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To: blam

There are a couple of almost bottomless holes of fresh water near Santa Rosa, New Mexico and there is rumors that the bodies of some who drowned in those holes were a long time later recovered from the Gulf of Mexico.

No refrences to post and I no longer remember who said it, but I did hear it once upon a time.


11 posted on 10/16/2005 5:15:32 PM PDT by F.J. Mitchell (Don't quag Miers!!!!)
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To: solitas; SunkenCiv
Here's an article from the Sunday Herald I wanted to post to the GGG gang but, we can't post from this newspaper. We can link it though:

What Did The Romans Empire Do For Us?

12 posted on 10/16/2005 5:20:04 PM PDT by blam
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To: blam
This is new knowledge...?

We have been taping aquifers for generations, it is a new revelation that some coastal aquifers exit to the sea.

There are rivers that run totally underground from Michigan to Kentucky, I used to have eyeless fish from one of those rivers in my tank. They were boring, they used to bump into my guppies and the rocks all the time.

White goldfish with no eyes... I fed them to my oscars.
13 posted on 10/16/2005 5:20:51 PM PDT by mmercier
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To: blam
.... seafloor seepage is important. "It's about time," he notes.

No it's not. It's also not been sucking cash out of our pockets to feed the brain of a lonely scientists' quest for glory either, thank you very much.

The reason Florida is the hot spot for this reasearch is that the entire state sits on a fresh water aquifer and the flows and directions of these have been charted, tested and controlled for the past 65 years. The quest for tax dollars to fund research always kicks off with a pointless argument, adds nice pictures and points to a mystery. But this isn't reasearch to find or create fresh drinking water or suck water from where it doesn't exist. It looks like it's just reasearch for the sake of research.

14 posted on 10/16/2005 5:21:12 PM PDT by JoeSixPack1 (The Price of Freedom is Written on the Wall.)
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To: F.J. Mitchell
"No refrences to post and I no longer remember who said it, but I did hear it once upon a time."

There is a fresh water lake near Pascagoula, Miss. with similar rumors. It rises and falls with the tides too.

15 posted on 10/16/2005 5:22:32 PM PDT by blam
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To: blam
May be a source of fresh water for southern California farmers?

No way.

The federill government is more likely to consider such streams critical to habitat and breeding. These streams would thus be a way to regulate the use of private wells that might draw off the water source. Can't harm the ocean ya know, that belongs to the whole earth. ;-)

16 posted on 10/16/2005 5:30:37 PM PDT by Carry_Okie (The environment is too complex and too important to manage by central planning.)
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To: blam
It might be GGG pingworthy if any cultures have legends of freshwater outflows distant from the shore, and this provides the proof...

_I_ haven't heard of any; but who knows? <shrug>

17 posted on 10/16/2005 5:34:20 PM PDT by solitas (So what if I support an OS that has fewer flaws than yours? 'Mystic' dual 500 G4's, OSX.4.2)
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To: F.J. Mitchell

Bottomless Lakes State Park, NM ?

http://www.lasr.net/pages/lake.php?Lake_ID=NM05lk001


18 posted on 10/16/2005 5:37:36 PM PDT by seastay
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To: blam; FairOpinion; StayAt HomeMother; Ernest_at_the_Beach
Thanks Blam. Not really pingworthy, but will add it to the catalog and the Digest (which I'm about to start, a day late).

To all -- please ping me to other topics which are appropriate for the GGG list. Thanks.
Please FREEPMAIL me if you want on or off the
"Gods, Graves, Glyphs" PING list or GGG weekly digest
-- Archaeology/Anthropology/Ancient Cultures/Artifacts/Antiquities, etc.
Gods, Graves, Glyphs (alpha order)

19 posted on 10/16/2005 6:43:05 PM PDT by SunkenCiv (Down with Dhimmicrats! I last updated by FR profile on Sunday, August 14, 2005.)
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To: blam

Keith Brackpool, British CEO of Cadiz Inc., tried to sell a deal in which he would store water from the Colorado River, during wet years, in a fresh water aquifer in the Mojave Desert which would then be pumped to the Metropolitan Water District of Southern California in dry years. They already owned the desert land.

Opponents claimed there was no way to determine how much pumping from the aquifer would be too much, drying up springs and wells.

He was smart enough to give a quarter million dollars to Gray Davis but the deal soured when he couldn't win over Dianne Feinstein who has made desert protection a hallmark of her career in the senate. The Water District narrowly voted down the proposal.

You know what they say out west - "Whiskey's for drinking, water's for fight'n."

Cadiz Groundwater Storage and Dry-Year Supply Program

Mojave Water Grab

20 posted on 10/16/2005 6:44:50 PM PDT by concentric circles
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