Isotopic dating of groundwater hydrology, recommended For You
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An ideal age-dating isotope should behave conservatively by not experiencing any losses or additions during the transit time of the water. If a major urban drinking water supply well from a Southwest basin pumps thousand-yearold water, for example, then it is mining the groundwater resource at a much faster rate than natural recharge. For long-term process studies within a groundwater system, cosmogenic, subsurface production, and uranium decay-series isotopes all may be appropriate. Sources of Isotopes Both anthropogenic and natural sources exist for many isotopes. Unstable isotopes periodically but predictably emit a particle or break into two smaller nuclei.
For example, hydrogen with two neutrons is known as tritium, an unstable element. For example, the most common element in the universe, hydrogen, by definition contains one proton in its nucleus, but it can contain none, one, or two neutrons. Anthropogenic sources are a result of the nuclear age of weapons testing, nuclear power generation, fuel rod reprocessing, and nuclear medical waste.
Uranium is referred to as a primordial isotope because it was incorporated into the Earth during its formation. Subsurface production occurs through the by-products of natural radioactive decay series. Cosmogenic isotopes arise from highenergy cosmic rays known as electrons and photons and lower-energy cosmic rays such as protons and other light nuclei. Due to their varying concentrations through time, if more than one anthropogenic isotope is measured, then more precise age determinations may be possible. Therefore, it is more direct to measure the parent atom remaining Nt and daughter produced Dt at the sample collection time.
Tritium decays by emitting a radioactive beta particle and converting into a stable helium element. Some isotopes are stable, meaning they do not decay to any other form over time, and others are unstable, or radioactive, meaning they spontaneously decay at a predictable rate to form a new element.
This is rarely the case, but we will discuss the ideal case to illustrate the straightforward age-dating technique. They can also be directly used for agedating groundwater. The time period between emissions is known as the halflife for radioactive decay, and forms the basis for most age-dating methods. Age Dating Fundamentals Before we delve into practical aspects of age-dating isotopes, shanghai matchmaking association it is worth mentioning a common misconception.
If the focus is on recharge or vadose zone processes, then cosmogenic isotopes are a good choice because they are incorporated into rain, snow, or dry deposition. This approach is appropriate if the radioactive parent isotope decays to a stable daughter product that remains with the water parcel containing the parent isotope. For practical reasons, we might have to make assumptions regarding the initial parent atom concentration, which creates larger uncertainties. Another method to determine residence time is to compare measured concentrations with the time-varying concentrations known as input source functions. These activities have elevated many isotopes to concentrations well above their natural state.
These isotopes often must be accounted for when interpreting cosmogenic isotope transit times. Isotopes of a particular element have the same number of protons but a different number of neutrons in the nucleus, resulting in a different atomic mass.
Thus, many larger volume samples are collected to be measured in low-level counting labs in order to lower the cost. Isotopes are forms of the same element that have variable numbers of neutrons in their nuclei. The cost of analyses is quite reasonable for many isotopes, and a variety of commercial and research labs are available to perform the analyses.