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Recharge

Recharge

FIGURE Conceptual diagram for determining groundwater age using isotopes. Water moving in the aquifer is assumed to move as "packets" or "plugs" in pipeflow fashion from an area of recharge to an area of discharge. However, few aquifers are so simple, and a combination of hydraulic and geochemical processes can lead to incorrect estimates of ages for the water. Therefore, order-of-magnitude dating is considered realistic for most cases in which groundwater is "dated" using isotopes. SOURCE: Bethke and Johnson (2008). © 2008 by Annual Reviews, Inc. Reproduced by permission of Annual Reviews.

to become systematically "lighter" from low to high latitudes. In other words, groundwater at high latitudes will have greater proportions of 16O relative to 18O than does groundwater at lower latitudes.

Other isotopes, known as radiogenic isotopes, radioactively decay at known rates. Carbon-14 and tritium (3H) are the most widely used radiogenic isotopes in hydrogeology. Carbon-14 forms from bombardment of atmospheric nitrogen by cosmic radiation. Large amounts of tritium were injected into the hydrological system by thermonuclear tests in the late 1950s through the 1960s. Other radioactive isotopes that may be used to "date" the age of water include krypton, argon, chlorine, and iodine. Because the rates of radioactive decay are known, these isotopes behave as natural clocks, allowing hydrogeologists to measure directly how long the water has been in aquifers, at least to the order-of-magnitude scale, when these isotopes are present in the water. Details on the methodology of the approaches can be found in Clark and Fritz (1997) and Kazemi et al. (2006).

further documenting the hydrogeological compartmentalization of the San Juan Basin (Riese et al., 2005).

Oldaker and Fehn (2005) report that surface waters and shallow groundwater are less than 60 years old in the Raton Basin but that produced water from CBM wells more than 1,800 feet deep could be at least 1.2 million years old, based on tritium (3H), carbon-14, and chlorine isotope analyses. Although comprehensive isotopic studies similar to those in the San Juan Basin have not yet been conducted in the Raton Basin, these results suggest a conceptual model for CBM water in the Raton Basin similar to the San Juan Basin, particularly given the common depositional environments for the coals in the two basins.

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