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Invisible rivers: fresh water also flows to sea through the ground.

About 2,000 years ago, the Roman geographer Strabo wrote about the residents of Latakia Latakia or Lattakia (both: lătəkē`ə, lätə–), city (1995 est. pop. 320,100), capital of Latakia governorate, W Syria, on the Mediterranean Sea. , 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 drinking water

supply of water available to animals for drinking supplied via nipples, in troughs, dams, ponds and larger natural water sources; an insufficient supply leads to dehydration; it can be the source of infection, e.g. leptospirosis, salmonellosis, or of poisoning, e.g.
 in goatskin goat·skin  
1. The skin of a goat.

2. Leather made from a goatskin.

3. A container, as for wine, made from a 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.

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 SUBMARINE GROUNDWATER DISHARGE – YARRAGADEE AQUIFER

The discharge of groundwater into near shore marine waters, in terms of the function of the volume of discharge to the life cycle of any groundwater dependent marine biota, is largely unexplored.
, 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 microbial

pertaining to or emanating from a microbe.

microbial digestion
the breakdown of organic material, especially feedstuffs, by microbial organisms.
 growth in sediments or smothering smothering

death by asphyxiation. Occurs where poultry are carelessly herded into a corner where they cannot escape and where they are piled four or five birds deep; they will die of asphyxia very quickly. See also crowding.
 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 The part of a statute or a law that delineates its purpose and scope.

Purview refers to the enacting part of a statute. It generally begins with the words be it enacted and continues as far as the repealing clause.
 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 mys·ti·fy  
tr.v. mys·ti·fied, mys·ti·fy·ing, mys·ti·fies
1. To confuse or puzzle mentally. See Synonyms at puzzle.

2. To make obscure or mysterious.
 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 Florida State University, at Tallahassee; coeducational; chartered 1851, opened 1857. Present name was adopted in 1947. Special research facilities include those in nuclear science and oceanography.  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 The American Geophysical Union (or AGU) is a nonprofit organization of geophysicists, consisting of over 50,000 members from over 140 countries. AGU's activities are focused on the organization and dissemination of scientific information in the interdisciplinary and  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 Yucatan, 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 marine biologist

specialist in the biology of marine life.
 at Boston University. Nitrate, which comes from septic systems, fertilizer, and decomposing marsh plants, is an important nutrient for algae algae (ăl`jē) [plural of Lat. alga=seaweed], a large and diverse group of primarily aquatic plantlike organisms. These organisms were previously classified as a primitive subkingdom of the plant kingdom, the thallophytes (plants that  and other phytoplankton phytoplankton

Flora of freely floating, often minute organisms that drift with water currents. Like land vegetation, phytoplankton uses carbon dioxide, releases oxygen, and converts minerals to a form animals can use.
 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 overlying

suffocation of piglets by the sow. The piglets may be weak from illness or malnutrition, the sow may be clumsy or ill, the pen may be inadequate in size or poorly designed so that piglets cannot escape.
 seawater seawater

Water that makes up the oceans and seas. Seawater is a complex mixture of 96.5% water, 2.5% salts, and small amounts of other substances. Much of the world's magnesium is recovered from seawater, as are large quantities of bromine.
 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 Massachusetts Institute of Technology, at Cambridge; coeducational; chartered 1861, opened 1865 in Boston, moved 1916. It has long been recognized as an outstanding technological institute and its Sloan School of Management has notable programs in business, .

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 (language) SEESAW - An early system on the IBM 701.

[Listed in CACM 2(5):16 (May 1959)].
, 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 unsaturated /un·sat·u·rat·ed/ (un-sach´ur-at?ed)
1. not holding all of a solute which can be held in solution by the solvent.

2. denoting compounds in which two or more atoms are united by double or triple bonds.
 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 dowsing

Occult practice used for finding water, minerals, or other hidden substances. A dowser generally uses a Y-shaped piece of hazel, rowan, or willow wood (also called a dowser or a divining rod).
 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 radium (rā`dēəm) [Lat. radius=ray], radioactive metallic chemical element; symbol Ra; at. no. 88; at. wt. 226.0254; m.p. 700°C;; b.p. 1,140°C;; sp. gr. about 6.0; valence +2. Radium is a lustrous white radioactive metal. , 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 A Matter of Minutes is an episode from the television series The New Twilight Zone. Cast
  • Michael Wright: Adam Arkin
  • Maureen Wright:Karen Austin
  • Supervisor: Adolph Caesar
, 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 Burner and his colleagues identified during research cruises in Florida's Sarasota Bay matched those identified by radon and other geochemical tracers Tracers

Refers to investment trusts which are populated by corporate bonds. In October 2001, Morgan Stanley's Tradable Custodial Receipts (Tracers) was launched. Tracers contain a number of coporate bonds and credit default swaps which are selected for liquidity and diversity.
, 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 “Todai” redirects here. For the restaurant called Todai, see Todai (restaurant).

The University of Tokyo (東京大学
 said at the San Francisco meeting.

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[degrees]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[degrees]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. MeMurtry of the University of Hawaii (body, education) University of Hawaii - A University spread over 10 campuses on 4 islands throughout the state.

See also Aloha, Aloha Net.
 at Honolulu measures dissolved gases such as methane, carbon dioxide carbon dioxide, chemical compound, CO2, a colorless, odorless, tasteless gas that is about one and one-half times as dense as air under ordinary conditions of temperature and pressure. , and hydrogen sulfide hydrogen sulfide, chemical compound, H2S, a colorless, extremely poisonous gas that has a very disagreeable odor, much like that of rotten eggs. It is slightly soluble in water and is soluble in carbon disulfide. . 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 It's About Time may refer to:

  • It's About Time (TV series), a 1966 American television show.
  • It's About Time (musical), a 1951 Broadway production.
," he notes.
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Author:Perkins, Sid
Publication:Science News
Geographic Code:1USA
Date:Oct 15, 2005
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