Using man made gases as groundwater 'age' tracers
The age dating results at Waingaehe Stream indicate relatively young water Lake Rotorua catchment, Waingaehe Stream, groundwater dating, tritium, CFCs, phased out of industrial use because of their destructive effects on the ozone . adsorbing tracers, such as 85Kr or SF6, CFC can therefore serve as a reactive retardation tracer exploring mean tracer and the age dating of a groundwater sample. omethanes: Chlorine atom catalyzed destruction of ozone. Estimated ground-water-recharge rates based on apparent CFC age. . Variations in apparent chlorofluorocarbon recharge date of ground water grouped by sample characteristics. destruction of ozone: Nature, no. , p.
Freon is a trade name for a group of CFCs which are used primarily as refrigerantsbut also have uses in fire-fighting and as propellants in aerosol cans. Bromomethane is widely used as a fumigant. Dichloromethane is a versatile industrial solvent. In the late s, Thomas Midgley, Jr. In searching for a new refrigerant, requirements for the compound were: In a demonstration for the American Chemical SocietyMidgley flamboyantly demonstrated all these properties by inhaling a breath of the gas and using it to blow out a candle  in Nevertheless, after the war they slowly became more common in civil aviation as well.
In the s, fluoroalkanes and bromofluoroalkanes became available and were quickly recognized as being highly effective fire-fighting materials.
- Production and mean life time:
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Much early research with Halon was conducted under the auspices of the US Armed Forces, while Halon was, initially, mainly developed in the UK.
By the late s they were standard in many applications where water and dry-powder extinguishers posed a threat of damage to the protected property, including computer rooms, telecommunications switches, laboratories, museums and art collections.
Beginning with warshipsin the s, bromofluoroalkanes also progressively came to be associated with rapid knockdown of severe fires in confined spaces with minimal risk to personnel. By the early s, bromofluoroalkanes were in common use on aircraft, ships, and large vehicles as well as in computer facilities and galleries. However, concern was beginning to be expressed about the impact of chloroalkanes and bromoalkanes on the ozone layer. The Vienna Convention for the Protection of the Ozone Layer did not cover bromofluoroalkanes as it was thought, at the time, that emergency discharge of extinguishing systems was too small in volume to produce a significant impact, and too important to human safety for restriction.
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Groundwater age dating with chlorofluorocarbons
Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. February Learn how and when to remove this template message Since the late s, the use of CFCs has been heavily regulated because of their destructive effects on the ozone layer. After the development of his electron capture detectorJames Lovelock was the first to detect the widespread presence of CFCs in the air, finding a mole fraction of 60 ppt of CFC over Ireland.
In a self-funded research expedition ending inLovelock went on to measure CFC in both the Arctic and Antarctic, finding the presence of the gas in each of 50 air samples collected, and concluding that CFCs are not hazardous to the environment. The experiment did however provide the first useful data on the presence of CFCs in the atmosphere.
Using Man Made Gases as Groundwater 'Age' Tracers
The damage caused by CFCs was discovered by Sherry Rowland and Mario Molina who, after hearing a lecture on the subject of Lovelock's work, embarked on research resulting in the first publication suggesting the connection in It turns out that one of CFCs' most attractive features—their low reactivity— is key to their most destructive effects.
CFCs' lack of reactivity gives them a lifespan that can exceed years, giving them time to diffuse into the upper stratosphere. NASA projection of stratospheric ozone, in Dobson unitsif chlorofluorocarbons had not been banned. Byin response to a dramatic seasonal depletion of the ozone layer over Antarcticadiplomats in Montreal forged a treaty, the Montreal Protocolwhich called for drastic reductions in the production of CFCs.
Indiplomats met in London and voted to significantly strengthen the Montreal Protocol by calling for a complete elimination of CFCs by the year Such "environmental tracers" are of natural origin for old groundwaters and, in general, a result of human impact for waters infiltrated within the last 50 years.
The dating range of a method is given by the characteristic time scale on which the tracer concentration varies over time. Half-live, input function or accumulation rate are the main characteristics of a tracer which determine the residence times on which a tracer is most sensitive.
A whole set of tracers is therefore required to cover the entire age range of an aquifer or a set of different aquifers. The most important applied tracer methods with the corresponding timescales are presented graphically below. CFCs and sulphur hexafluoride SF6 are particularly useful for dating groundwaters less than years old. Water extracted from a borehole or flowing from a spring is generally considered to be a mixture of waters from all the flow lines reaching the discharge point.
The age frequency distribution and mean age of the mixture affect the interpretation of the groundwater dating methods as well as the interpretation of other groundwater solute fluxes. There are four hypothetical mixing models that can be typically used to describe some of the variation seen in groundwater mixtures: In some cases, water reaching the open interval of a borehole or discharging from a spring is nearly uniform and can be approximated with a piston-flow model, analogous to water flowing through a pipe from the point of recharge to the point of discharge without mixing during transit.
The exponential-piston model corresponds to a situation in which an aquifer receives distributed recharge in an up-gradient unconfined area, then continues beneath a down-gradient confined area.
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Exponential mixing can be used to describe discharge from an unconfined aquifer receiving uniform areal recharge. Binary mixing of young water with old pre-tracer water is one of the simplest models to consider and is perhaps the most important in many fractured-rock environments although not all. In binary mixing, simple dilution occurs because the old fraction is assumed to be free of the tracer and, consequently, the age of the young fraction can be calculated from the ratio of the two tracers.
Understanding the groundwater flow properties under the Chalk in Portsmouth, Hampshire through the combined use of CFC and SF6 as groundwater tracers.