What is groundwater dating?

How we calculate age of Groundwater?

by

Dr. Nitish Priyadarshi

photo credit: http://water.usgs.gov/ogw/karst/img/features/arbuckle/vendomeWell.jpg

Groundwater age, based on measurement of the concentrations of chemical and isotopic substances in water, refers to the time elapsed since the introduction of the chemical or isotopic substance into the water, or to be more precise, the time elapsed since the chemical or isotopic substance was recharged and isolated from the modern reservoir. For example, some of the rain that falls on an area percolates (trickles) down through soil and rock until it reaches the water table. Once this water reaches the water table, it moves though the aquifer. The time it takes to travel to a given location, known as the groundwater age, can vary from days to thousands of years.
Although we often refer to dating of groundwater, we are actually dating a chemical substance that is dissolved in the groundwater, not the water itself. Rather than referring to groundwater dating, it would probably be more correct to say we are interpreting temporal aspects of chemical and isotopic substances in groundwater. Isotopes can be separated into environmental isotopes, which are found in the groundwater, and isotopes that are introduced into the ground as a part of a groundwater study.

Environmental isotopes can be either radioactive or stable. They can be used to determine the locations of groundwater recharge areas, circulation patterns in aquifers, sources of dissolved solids in groundwater, and the age of groundwater- the length of time it has been out of contact with the atmosphere.

Various environmental isotopes and tracers are used to determine the age of groundwater. Carbon-14 is used to date groundwater older than 1000 years. Chlorofluorocarbons (Freon) and tritium techniques are used to date groundwater that is less than 50 years old.

Theoretically, ages can be estimated by (1) the travel time of groundwater from the point of recharge to the subsurface point of interest as calculated by Darcy's law combined with an equation of continuity, (2) the decay of radionuclides which have entered the water from contact with the atmosphere, (3) the accumulation of products of radioactive reactions in the subsurface, (4) the degree of disequilibrium between radionuclides and their radioactive daughter products, (5) the time-dependent changes in the molecular structure of compounds dissolved in water, (6) the presence of man-made materials in groundwater, (7) the correlation of paleoclimatic indicators in the water with the known chronology of past climates, and (8) the presence or absence of ions which can be related to past geologic events that have been previously dated. Owing to uncertainties in each of the methods, as many methods as possible should be used in every field situation.

The residence time of water underground has always been a topic of considerable speculation. But with the advent of radioisotopes, determination of the age of groundwater has become possible.

Chlorofluorocarbons-

Hydrologists employ a variety of techniques to measure groundwater age. For relatively young groundwater, chlorofluorocarbons (CFCs) often are used. CFCs are human-made compounds that are stable in the environment. Atmospheric CFC concentrations increased from the time of their development in the 1930s until the 1990s, and hydrologists now know how atmospheric CFC concentrations have changed over time.
CFCs can be used to determine groundwater age because water that is in contact with the atmosphere picks up CFCs from the atmosphere. Thus, CFCs are incorporated in the water before it enters an aquifer. Once water enters an aquifer, it becomes isolated from the atmosphere, and it carries a CFC signature (a distinctive chemical composition) as it travels through the aquifer. This signature reflects the atmospheric concentration when the water was at the surface. By measuring the CFC concentration in groundwater, hydrologists know how long ago the water entered the aquifer.

In the United States and other developed countries, CFCs are being phased out of use because they contribute to atmospheric ozone depletion. As a consequence, atmospheric CFC concentrations have begun to decrease. Atmospheric concentrations of CFCs are not expected to decrease quickly, so CFC dating will continue to work for most young groundwater for many years to come. However, for very young groundwater (groundwater entering aquifers after the late 1990s), CFC dating soon will yield ambiguous results.

Sulfur hexafluoride (SF 6 )-

Hydrologists recently have developed another dating technique that may ultimately replace CFC dating. The new technique uses sulfur hexafluoride (SF 6 ) concentrations in groundwater to determine groundwater age. SF 6 is another stable, human-made compound that has exhibited increasing concentrations in the atmosphere. Unlike CFC concentrations, atmospheric SF 6 concentrations are expected to increase for the foreseeable future. The method, although relatively new, shows promise.

Tritium-

Tritium, 3H, is an unstable isotope of hydrogen with a half life of 12.4 years. It is produced in the upper atmosphere by cosmic radiation; carried to earth by rainfall and hence underground, this natural level of tritium begins to decay as a function of time, such that

A= A0 e-λt

Where A is the observed radioactivity, A0 is the activity at the time the water entered the aquifer, λ is the decay constant, and “t” is the age of the water.

Tritium in the atmosphere is typically in the form of the molecule H3 HO and enters the groundwater as recharging precipitation. Prior to 1953, rainwater had less than 10 tritium units (TU). Starting in 1953, the manufacturing and testing of nuclear weapons have increased the amount of tritium in the atmosphere, with a resulting increasing in tritium in the groundwater. As a result 3H can be used in a qualitative manner to date groundwater in the sense that groundwater with less than 2 to 4 TU is dated prior to 1953 and if the amount is significantly greater than 10 to 20 TU it has been in the contact with the atmosphere since 1953.

Tritium has been used to trace the seepage of contaminated groundwater from low-level nuclear waste disposal areas.

Radiocarbon dating of groundwater (Carbon-14)-

Radiocarbon dating methods can be applied to obtain the age of groundwater. Carbon exists in several naturally occurring isotopes, 12 C, 13 C and 14 C.

Like tritium, carbon 14 is produced in the atmosphere by interaction of cosmic rays with nitrogen, and was introduced in large amounts by nuclear weapons testing. Unlike tritium 14 C has a half life of 5730 years, making it a useful tool for dating water as old as 50,000 years. The 14 C generated in the atmosphere is carried down to the earth’s surface by precipitation, and becomes incorporated into the biomass or transported into water bodies such as lakes, the ocean and groundwater. 14 C undergoes radioactive decay (to 14 N), so that once isolated from the atmosphere, the amount of 14 C decreases with time according to the equation

(14 C)t = (14C)0e –Kt

Where (14 C)t is the amount present at time “t”, (14C)0 is the amount present at t = 0, and K is the decay constant, which is related to the half-life T1/2 by the equation

T1/2 = In2/k
To determine the time since a water was last in contact with atmosphere, it is necessary to know (14 C)0. This is determined by tree rings for the most recent 7000 years; there is no accurate way to determine it prior to 7000 years, so it is generally assumed arbitrarily to have been constant. This gives rise to a time scale in “14 C years”, which may be different from astronomical years.


There are some complications in the behavior of 14 C during recharge, so that the “absolute” age of a groundwater cannot be determined reliably. However, if the 14 C concentration is measured at several points along a flow line within an aquifer, the differences in age between the points and hence the flow velocity can be determined. One complication is that dissolution of carbonate minerals or oxidation of organic matter within an aquifer may add “old” or “dead” (no detectable 14 C) carbon to the water and give an erroneously old age. The contribution of carbon from these sources can sometimes be estimated from 13 C/ 12 C measurements and chemical arguments, so that corrections can be made. Another complication is mixing. A low 14 C concentration may mean that we are looking at relatively “old” water, or it may mean that we are looking at a mixture of relatively “young” water and “dead” water. 14 C measurements can be interpreted as ages only when mixing is insignificant.

Measurements of water samples taken from deep wells in deserts of the United Arab Republic and Saudi Arabia indicate ages of 20,000 to 30,000 years. This period is compatible with the Wisconsin Ice Age, when these desert areas last had high rainfall capable of recharging the underlying major aquifers.

Reference:
Drever, J.I.(1982). The Geochemistry of Natural Waters. Prentice-Hall, Englewood cliffs, NJ.

Thatcher, L. et.al. (1961). Dating desert groundwater. Science 134(3472): 105-106.

Todd, D.K. (1995). Groundwater Hydrology. Wiley, Toronto, pp. 24-25.

Wigley, T.M.L. (1975). Carbon-14 dating of groundwater from closed and open systems. Water Resour. Res. 11: 324-328.

Wigley, T.M.L. (1976). Effect of mineral precipitation on isotopic composition and 14C dating of groundwater. Nature 263: 219-221.
http://www.freedrinkingwater.com/water-education2/6-age-groundwater.htmhttp://www.waterencyclopedia.com/Ge-Hy/Groundwater-Age-of.html

Distribution of Uranium in world coals.

Jharkhand coal contains trace amount of Uranium in North Karanpura coal field.

by

Dr. Nitish Priyadarshi

Coal is largely composed of organic matter, but it is the inorganic matter in coal—minerals and trace elements— that have been cited as possible causes of health, environmental, and technological problems associated with the use of coal. Some trace elements in coal are naturally radioactive. These radioactive elements include uranium (U), thorium (Th), and their numerous decay products, including radium (Ra) and radon (Rn). Although these elements are less chemically toxic than other coal constituents such as arsenic, selenium, or mercury, questions have been raised concerning possible risk from radiation.


Uranium association with coal has a long history. There is a continuing interest in uranium in coal, because it is a source of radioactivity and because it may be an economic source of uranium. It is just 200 years since the discovery of uranium by M.H. Klaproth. The first detection in coal was by Berthoud (1875) who found up to 2% uranium in coal from near Denver, USA. The samples were collected from a mineralized section of the coal-bed. This mine was soon abandoned.

Subsequent field studies have proven several areas with high uranium coals, especially in the United States, mainly in the Dakotas, Wyoming, Montana, Colorado and New Mexico (Vine,1956). It seems that uranium is carried into the coal swamp in solution as carbonate complexes (Breger, et. al. 1955), which then release uranyl ions to form uranyl-organic complexes. In many coals, especially low-U coals, Uranium is predominantly organically bound.

After World War II, a very intensive uranium search was initiated. The measurement of coal radioactivity were performed in many countries; however only a few are documented. For example, in year 1967 scientists have measured uranium concentration in lignites from Spain (Huesca, Lerida, Ternel, Galicia, Murcia) and reported concentration values 20 to 1200 parts per million (ppm).

Gott (1952) has determined uranium distribution in lignites, shales, and limestones from throughout the US, and a possible mechanism for uranium accumulation in lignites was suggested. Highest uranium concentrations were prevalent in lignites from the Dakotas, Wyoming, and Montana (0.01%), and from high ash Nevada lignite which contained up to 0.05 % uranium. It was postulated that uranium was possibly concentrated in lignite by the action of percolating surface waters after having been leached from volcanic ash.

Uranium bearing coal in the Red Desert area in Wyoming has been studied by Masursky; his findings are documented in several reports. In the first report in year 1952, core and channel samples taken from the Red Desert area in Wyoming were used to investigate the origin of uranium in the coal of the region. Specific uraniferous zones examined included the Sourdough, Monument, Battle, and Luman zones. Areas which were topographically higher and in which coal was overlain by conglomerate showed the highest uranium concentration. Studies of core samples revealed that uranium concentration in the coal beds correlates well with the degree of permeability of adjacent rocks. Where coal beds are overlain or underlain by sandstone, the greatest concentrations of uranium occur at the top and / or bottom of the bed.

J.R. Gill and others in the year 1959 have studied uranium bearing lignite in South Dakota and Montana. They have reported some lignite deposits containing as much as 0.1% uranium.

Coal samples were analyzed for uranium concentration in the coals from the Western United States and approximately 300 coals from the Illinois Basin. In the majority of samples, concentrations of uranium fall in the range from slightly below 1 to 4 parts per million (ppm). Coals with more than 20 ppm uranium are rare in the United States (http://energy.er.usgs.gov/products/databases/ CoalQual/intro.htm).



Results for the uranium in world coals are as follows (Swaine,1990):

Australia- 0.01-4.5 ppm
Brazil- 2.7-19 ppm
Canada- 0.2-7.2 ppm
China- 0.16-21 ppm
Germany West- less than 1 – 13 ppm
India- 1.1-3.6 ppm.
New Zealand- 0.015-0.46 ppm
South Africa- 1.2- 7.3 ppm
Turkey- 1.4-6.4 ppm
UK- 1.1- 3.0 ppm.

Traces of uranium have been also found in the Permian coals of Jharkhand State of India. Areas are KDH, Dakra, Rohini, and Rai Bachra in the North Karanpura coalfield. Channeled samples were analyzed with the help of XRF instrument.

Occurrence of uranium in coals:

Three hypotheses advanced to explain the occurrence of uranium in some coals were described by Denson (1959) as follows.

1. Syngenetic: Uranium was deposited from surface waters by living plants or in dead organic matter in swamps prior to coalification.
2. Diagenetic: Uranium was introduced into the coal during coalification by waters bringing the uranium from areas marginal to the coal deposits or from the consolidating enclosing sediments.
3. Epigenetic: Uranium was introduced in the coal after coalification and after consolidation of the enclosing sediments by groundwater deriving uranium from hydrothermal sources or from unconformably overlying volcanic rocks.

Uranium is associated with clays, zircon and phosphates and may also be organically bound in coal. The accumulation of uranium in coal may vary markedly from place to place, and the occurrence of uranium in each deposit should be interpreted in relation to the geologic history of the region. Field evidence favors the epigenetic hypothesis of the origin of uranium in U.S. western coals. Secondary concentration of uranium in coal may occur when solution of small quantities of uranium by groundwater from overlying volcanic rocks is followed by downward percolation of these waters through previous strata until the uranium is taken up and retained by the highest of the underlying lignite beds. Application of this theory led to the discovery of uranium-bearing coal in Wyoming, Montana, Idaho and New Mexico.

Most thorium in coal is contained in common phosphate minerals such as monazite or apatite. In contrast, uranium is found in both the mineral and organic fractions of coal. Some uranium may be added slowly over geologic time because organic matter can extract dissolved uranium from ground water. In fly ash, the uranium is more concentrated in the finer sized particles. If during coal combustion some uranium is concentrated on ash surfaces as a condensate, then this surface-bound uranium is potentially more susceptible to leaching. However, no obvious evidence of surface enrichment of uranium has been found in the hundreds of fly ash particles examined by USGS researchers.

Most coal also contains potassium-40, lead-210, and radium-226. The total levels are generally about the same as in other rocks of the Earth's crust. Most emerge from a power station in the light flyash, which is fused and chemically stable, or the bottom ash. Some 99% of flyash is typically retained in a modern power station (90% is some older ones), and this is buried in an ash dam.

The amounts of radionuclides involved are noteworthy. In Victoria, 65 million tonnes of brown coal is burned annually for electricity production. This contains about 1.6 ppm uranium and 3.0-3.5 ppm thorium, hence about 100 tonnes of uranium and 200 tonnes of thorium is buried in landfill each year in the Latrobe Valley. Australia exports 235 Mt/yr of coal with 1 to 2 ppm uranium and about 3.5 ppm thorium in it, hence up to 400 tonnes of uranium and about 800 tonnes of thorium could conceivably be added to published export figures (http://www.world-nuclear.org/info/inf30.html).

Other coals are quoted as ranging up to 25 ppm U and 80 ppm Th. In the USA, ash from coal-fired power plants contains on average 1.3 ppm of uranium and 3.2 ppm of thorium, giving rise to 1200 tonnes of uranium and 3000 tonnes of thorium in ash each year (for 955 million tonnes of coal used for power generation). Applying these concentration figures to world coal consumption for power generation (7800 Mt/yr) gives 10,000 tonnes of uranium and 25,000 tonnes of thorium per year(http://www.world-nuclear.org/info/inf30.html).

Reference:
Berthoud, E.L. 1875. on the occurrence of uranium, silver, iron etc., in the Tertiary Formation of Colorado Territory. Proc. Nat. Acad. Sci., Philadelphia, 27, 363-365.
Breger, I.A., Deul, M. and Meyrowitz, R. 1955. Geochemistry and mineralogy of a uraniferous subbituminous coal. Econ. Geol., 50, 610-624.
Bouska, V. 1981. Geochemistry of Coal. Elsevier Scientific Publishing Company, New York.
Denson, N.M., 1959. Uranium in coal in the Western United States, U.S. Geological Survey Bull. 1055.
Gill, J.R. 1959. Reconnaissance for uranium in the Ekalaka Lignite field, Carter County, Montana. US Geological Survey, Bull. 1055.
Gott, G.B. 1952. Uranium in black shales, lignites and limestones in the United States. Selected papers on uranium deposits in the United States. U.S. Geological Survey, Circ.220,Washington. 31-35.
Swaine, D.J. 1990. Trace elements in coal. Butterworths, London.
Valkovic, V. 1983. Trace elements in coal. CRC Press, Inc. Florida.
Vine, J.D. 1956. Uranium-bearing coal in the United States. US Geol. Surv. Prof. Pap., No 300, 405-41.

Noah's Ark Found in Turkey?

The expedition team is "99.9 percent" sure. Others, well, aren't.

compiled by

Dr. Nitish Priyadarshi

Near the top of Mount Ararat (seen from Armenia in a file photo) in Turkey, explorers claim to have found Noah's ark.
Photograph by Martin Gray, National Geographic

National Geographic News
Published April 28, 2010

A team of evangelical Christian explorers claim they've found the remains of Noah's ark beneath snow and volcanic debris on Turkey's Mount Ararat (map).
But some archaeologists and historians are taking the latest claim that Noah's ark has been found about as seriously as they have past ones—which is to say not very.

Turkish and Chinese explorers from a group called Noah's Ark Ministries International made the latest discovery claim Monday in Hong Kong, where the group is based.
"It's not 100 percent that it is Noah's ark, but we think it is 99.9 percent that this is it," Yeung Wing-cheung, a filmmaker accompanying the explorers, told The Daily Mail.

The team claims to have found in 2007 and 2008 seven large wooden compartments buried at 13,000 feet (4,000 meters) above sea level, near the peak of Mount Ararat. They returned to the site with a film crew in October 2009.
Many Christians believe the mountain in Turkey is the final resting place of Noah's ark, which the Bible says protected Noah, his family, and pairs of every animal species on Earth during a divine deluge that wiped out most of humanity.
"The structure is partitioned into different spaces," said Noah's Ark Ministries International team member Man-fai Yuen in a statement. "We believe that the wooden structure we entered is the same structure recorded in historical accounts. ... "
The team says radiocarbon-dated wood taken from the discovery site—whose location they're keeping secret for now—shows the purported ark is about 4,800 years old, which coincides roughly with the time of Noah's flood implied by the Bible.

On its Web site, Noah's Ark Ministries International says the Turkish government plans to apply to the United Nations to put the Noah's ark discovery site on the UNESCO World Heritage list, a designation given to places of special cultural or physical significance.

Noah's Ark is the vessel which, according to the Book of Genesis, was built by Noah at God's command to save himself, his family, and the world's animals from a worldwide deluge. The Ark features in the traditions of a number of Abrahamic religions, including Judaism, Christianity, Islam, and others.

The Book of Genesis, chapters 6-9, tells how God sends a great flood to destroy the earth because of man's wickedness and because the earth is corrupt. God tells Noah, the righteous man in his generation, to build a large vessel to save his family and a representation of the world's animals. God gives detailed instructions for the Ark and, after its completion, sends the animals to Noah. God then sends the Flood, which rises until all the mountains are covered, and most living things died, except the fish. Then "God remembered Noah," the waters abate, and dry land reappears. Noah, his family, and the animals leave the Ark, and God vowed to never again send a flood to destroy the Earth.