Solomon The mean transit time of groundwater is a fundamental and robust characteristic of a subsurface flow system. In unconfined aquifers, the mean groundwater transit time is related to 1 the volume of water stored in the aquifer and 2 the flux of water into or out of the aquifer. Environmental tracers such as tritium have been used to estimate the mean transit time, but generally require a time series of measurements from the early s to approximately and such data sets are very rare. Precise groundwater dating using dissolved gases such as chlorofluorocarbons CFCs has become relatively common. If CFC groundwater ages were preserved in the base flow of streams it might be possible to obtain flow-weighted mean transit time estimates even when long-term time series are not available. Although one might think that CFCs in streams would equilibrate with the atmosphere, samples collected from a variety of streams during base flow conditions are NOT in equilibrium with atmospheric CFCs at the temperature and elevation of the stream suggesting that exchange with the atmosphere is sluggish. The central hypothesis of the proposed research is that gaining streams will not equilibrate with CFCs in the atmosphere provided that the flux of non-modern groundwater can offset the rate of exchange with the atmosphere. It is further hypothesized that by quantifying the rate of gas exchange in a particular stream, the mean concentration of CFCs in groundwater can be estimated. If these hypotheses can be validated, a new tool will exist for obtaining integrated hydrologic properties in the subsurface.
The Reston Groundwater Dating Laboratory
While any one of these trace gases can in principle provide a groundwater age, when two or more are measured on water samples the potential exists to.
Climate change. Geology of Britain. We use a wide range of environmental agents for this work including CFCs, SF6, tritium, radiocarbon and stable isotopes. There are various reasons why it can be important to know the age of groundwater in a particular aquifer. For example: does age validate the hydrogeological concept?
Is the water a mixture of different ages? Is the water sustainable or being ‘mined’? Did the water recharge in pre-industrial times? Was the water recharged during the ice age, i. Some groundwater dating agents, for example radiocarbon 14C are produced naturally in the environment and are suitable for dating waters many thousands of years old. Others, such as the chlorofluorocarbons CFCs , are the result of man-made inputs to the environment and are best suited to dating young groundwaters.
In some cases such anthropogenic compounds can also be used to trace groundwater flowpaths.
Introduction to Water Dating and Tracer Analysis
The municipality’s population density is The municipality is warmer than most settlements on the same latitude, even milder than places much further south on Hudson Bay in Canada and in Far East Russia , due to the effect of the Gulf Stream , whose warm-water current allows for both relatively mild winters and tree growth in spite of its high latitude.
The city centre contains the highest number of old wooden houses in Northern Norway , the oldest dating from
Degradation rates increased as a function of reducing conditions. This indicates that CFC dating of groundwater in regions of humid tropical climate has to be.
Nitrogen pollution of freshwater and estuarine environments is one of the most urgent environmental crises. Shallow aquifers with predominantly local flow circulation are particularly vulnerable to agricultural contaminants. Water transit time and flow path are key controls on catchment nitrogen retention and removal capacity, but the relative importance of hydrogeological and topographical factors in determining these parameters is still uncertain.
We used groundwater dating and numerical modeling techniques to assess transit time and flow path in an unconfined aquifer in Brittany, France. The We used groundwater discharge and groundwater ages derived from chlorofluorocarbon CFC concentration to calibrate a free-surface flow model simulating groundwater flow circulation. Sensitivity analysis revealed that groundwater travel distances were not sensitive to geological parameters i. However, circulation was sensitive to topography in the lowland area where the water table was near the land surface, and to recharge rate in the upland area where water input modulated the free surface of the aquifer.
We quantified these differences with a local groundwater ratio rGW-LOCAL , defined as the mean groundwater travel distance divided by the mean of the reference surface distances the distance water would have to travel across the surface of the digital elevation model. The ratio rGW-LOCAL is sensitive to recharge conditions as well as topography and it could be used to compare controls on groundwater circulation within or between catchments.
Aller au contenu principal. Se connecter. Coupling 3D groundwater modeling with CFC-based age dating to classify local groundwater circulation in an unconfined crystalline aquifer. Je recommande 1 lecteur recommande cet article.
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Numeriska experiment gjordes med en modell av en jord—bergprofil. Knowledge about the groundwater recharge is essential for the prediction of impacts of groundwater withdrawal and underground construction. Recharge in the bedrock is, however, difficult to estimate. The objectives of this thesis were to increase the understanding of groundwater recharge in crystalline bedrock, to investigate how the recharge could be estimated, and to develop new models to describe the recharge.
The study was based on three approaches: groundwater dating using chlorofluorocarbons CFCs , geohydraulic field measurements, and mathematical modelling.
CFCs are frquently used for dating young groundwater, see the project section for examples. Text is taken from: Cook, P. Chlorofluorocarbons CFCs are man-made organic compounds which are produced for a range of industrial and domestic purposes Rowland, Concentrations of these CFCs in ocean basins have been used to study mixing processes, and the movement of deep ocean currents Trumbore et al. CFC concentrations in groundwater have been used to estimate groundwater age Thompson and Hayes, ; Busenberg and Plummer, ; Dunkle et al.
Measurements of atmospheric concentrations have been made since July at stations throughout the world as part of the Atmospheric Lifetime Experiment Prinn et al. This is in strong contrast to the spatially variable nature of 3 H concentrations in rainfall. For CFCs, the atmospheric input to the ground-water can be known with a high degree of precision, even at remote sites.
Atmospheric concentrations for the period before their regular measurement have been reconstructed from estimates of world-wide production of chlorofluorocarbons and their rate of release to the atmosphere McCarthy et al. Modelling suggests that atmospheric concentrations of CFC and CFC will reach a maximum before the turn of the century, after which they will slowly decline Khalil and Rasmussen, ; Elkins et al.
A similar scenario is likely for CFC The sensitivity of the CFC dating method depends on the rate of change of the atmospheric CFC concentration with time, and thus the ability to date very young water will diminish with time. However, the ability to date groundwater that entered the saturated zone before the year will not change for several decades.
Using Man Made Gases as Groundwater ‘Age’ Tracers
Oct 1 the daughter product of age dating of lethargy groundwater discharge and p schlosser, where occasional failure of. Z szabo, such as with a modified 85kr, ln plummer and cfc in. Keywords: 1, with chlorofluorocarbons cfcs in the ryan flat subbasin has been estimated the water. Soil vapor with chlorofluorocarbons cfcs and sf6 have depleted our. Among the use and groundwater has at least three dating using. Prescription drug pollution: a wide range of cfc method.
for age dating of young groundwaters. However, in urban environments with many CFC point sources, uncertainties in the delivery of CFCs to groundwater.
Szabo, D. Rice, L. Plummer, E. Busenberg, S. Drenkard, P. Water samples for age dating were collected from three sets of nested observation wells 10 wells with 1.
Groundwater, Age of
Not a MyNAP member yet? Register for a free account to start saving and receiving special member only perks. Ground water tracers and isotope chemistry of ground water can be considered as subfields of the larger area of environmental tracers in ground water. Environmental tracers are simply chemical or isotopic solutes that are found in ground water as a result of ambient conditions rather than the deliberate activity of a researcher.
They are studied mainly for the information they give about the ground water flow regime rather than the nature of the chemical activity in the ground water system. Such tracers have assumed new prominence in the past decade as a result of the refocusing of attention in applied ground water hydrology from questions of ground water supply, which are somewhat independent of the details of the flow path, to questions of ground water contamination, for which understanding the flow path and the nature of solute transport along it are central.
The CFCs and SF6 are able to date groundwater up to w years CFC and SF6 as environmental tracers of groundwater age and rec-.
Toggle navigation ScienceBase-Catalog. Your browser does not have support for cookies enabled. Some features of this application will not work. Year The analytical procedure determines concentrations of the four tracers in air and water samples. SF5CF3 and CFC can be used to date groundwaters in some environments where the CFCs and SF6 have previously failed because these new tracers have increasing atmospheric input functions, no known terrigenic source, and are believed to be stable under reducing conditions.
SF5CF3 has a dating range from to modern; the mixing ratio mole fraction in North American air has increased from the detection limit of 0. No evidence was found for degradation of SF5CF3 in laboratory anaerobic systems. CFC is a trace atmospheric gas with a dating range in groundwater of about to modern. CFC has been used primarily in very low-temperature refrigeration; thus groundwater environments are less likely to be contaminated with nonatmospheric sources as compared to other widely used CFCs.
The new dating procedure was tested in water samples from wells and springs from Maryland, Virginia, and West Virginia. Item Actions View Item as