14c dating groundwater
In many carbonate-poor catchments, waters along shallow flowpaths in the soil zone have characteristically light C values of organic materials can provide extremely useful information about sources of contaminants.One recent technological advance, compound-specific stable isotope geochemistry, allows the isotopic analysis of individual carbon and nitrogen "peaks" in substances separated by gas chromatography, combusted, and then carried into the mass spectrometer in a flow of helium (Macko, 1994). It was discovered in 1934 by Grosse as an unknown activity in the mineral endialyte. This half life has later been re-determined by Godwin. Libby recognized that due to its occurrence in natural materials, either by simple mixing processes or by carbon exchange.In the same year, Kurie (Yale) exposed nitrogen to fast neutrons and observed long tracks in a bubble chamber. The mean life time of roughly 8000 years is ideal for dating of reservoirs that are a few decades to a few ten thousand yeas old.Consequently, without further information, DIC produced solely by the third reaction is identical to DIC produced in equal amounts from the first and second reactions.Under favorable conditions, carbon isotopes can be used to understand the biogeochemical reactions controlling alkalinity in watersheds (Mills, 1988; Kendall et al., 1992).With additional chemical or isotopic information, the C may provide evidence that such processes are insignificant for a particular study.Carbon isotopes can also be useful tracers of the seasonal and discharge-related contributions of different hydrologic flowpaths to streamflow (Kendall et al., 1992).
The primary reactions that produce DIC are: (1) weathering of carbonate minerals by acidic rain or other strong acids; (2) weathering of silicate minerals by carbonic acid produced by the dissolution of biogenic soil COC value exactly intermediate between the compositions of the carbonate and the carbonic acid.This method provides a much more distinctive signature of organic sources than the bulk isotopic composition usually measured.For example, the origins of hydrocarbons from oil spills in coastal regions or oil-field brines dumped into groundwater wells can potentially be identified by comparing the isotopic "chromatograms" of the pollutants and several likely sources for the hydrocarbons.Because of the long residence time for groundwater in the hydrologic cycle, the last few decades have seen expanding study of groundwater systems.It is therefore important to continually refine our interpretation of hydrogeologic, geochemical and isotopic data to better understand the spatial and temporal movement of water in the subsurface.
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In addition, the C is probably the most important radio-isotopic tool for dating deep groundwater, despite serious problems related to potential interactions of DIC species with the carbonate minerals and possible contributions of C data is used within the constraints of a geochemical reaction path model which accounts for the sources and sinks of carbon along the flowpath (Plummer et al., 1983; 1991).