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. Low concentrations of CFC and CFC were found in the bedrock groundwater, which in combination with low dissolved-oxygen levels indicated anaerobe degradation.
Environmental tracers and groundwater dating
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.
Information on groundwater age is required to address aspects such isotopic tools for recharge rate determination include cl, chlorofluorocarbons (cfcs), 3h.
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. Three steady state finite difference groundwater flow models were calibrated by adjusting horizontal and vertical hydraulic conductivities to match measured heads and head differences range, 0. The simulated groundwater travel times increase with depth in the aquifer, ranging from about 1. Differences between the tracer-based apparent ages for seven of the 10 samples were smaller than the error values.
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The hydrogeological functioning of four different areas in a complex evaporite-karst unit of predominantly aquitard behavior in S Spain was investigated. Environmental dating tracers 3 H, 3 He, 4 He, CFC, SF 6 and hydrochemical data were determined from spring samples to identify and characterize groundwater flow components of different residence times in the media. Ne values show degassing of most of the samples, favored by the high salinity of groundwater and the development of karstification so that the concentration of all the considered gases were corrected according to the difference between the theoretical and the measured Ne.
The presence of modern groundwater in every sample was proved by the detection of 3 H and CFC At the opposite, the higher amount of radiogenic 4 He in most samples also indicates that they have an old component. The large SF 6 concentrations suggest terrigenic production related to halite and dolomite.
Stable H/O isotope composition of rainfall. Estimating groundwater age by CFCs. Groundwater recharge rate. Using CFC age. .
Environmental tracers are natural or man made anthropogenic compounds or isotopes that are widely distributed in the near-surface environment. Variations in their quantities can be used to determine pathways and timescales of environmental processes. They include naturally occurring isotopes such as carbon and anthropogenic tracers such as Chlorofluorcarbons CFCs.
Releases of anthropogenic environmental tracers include catastrophic events such as nuclear bomb testing releasing, as well as gradual leakage of tracers from industrial production processes. One of the principal uses of environmental tracers is for determining the ages of soil waters and groundwaters. Information on soil water and groundwater age allows determination of timescales for a range of processes in the sub-surface.
The use of environmental tracers to determine water ages allows groundwater recharge rates and flow velocities to be determined independently, and commonly more accurately, than with more traditional hydraulic methods where hydraulic properties of aquifers are poorly known or spatially variable. Groundwaters residence times in the investigated reference aquifers range from a few years up to many thousands of years.
For these time scales, dating using tracers relies on substances which were already present in the geosphere at the time of recharge. 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.
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Plummer, E. Busenberg, S. Drenkard, P. Schlosser , B. Ekwurzel, R.
In the early s, USGS scientists Busenberg and Plummer, developed a method to date resource use on the basis of chlorofluorocarbon CFC content of the water.
The Reston Groundwater Dating Laboratory
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
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.
It showed that CFC and CFC were suitable tracers for groundwater dating because of their stability in the wetland environment. Furthermore, the mixture of groundwater with different age was discussed by CFC and CFC based on the binary mixing model and piston-flow model. As one of the most active factors, groundwater age is a key to understand the hydrological cycle as well as the associated hydro-ecological processes in a watershed.
The knowledge of the residence time would also help to illuminate processes that control subsurface flow routing since it is directly related to the diversity of flow pathways in a catchment Pearce et al. There are many ways to identify the residence time. For example, tritium 3 H is used to estimate residence times, but the method is difficult to use since the tritium concentration is too low to be detectable at present DeWalle et al.
Groundwater, Age of
Hydrochemical studies in ground water of the Federal District: subsidies to the flow conceptual model. ISSN This paper refers to hydrochemical studies for a flow model characterization proposed to the Federal District region, Brazil. Isotopic analyses of 18O, 3H and gases CFC has been developed in shallow and deep groundwater samples for tree points located on Jardim River watershed. The values in tritium units UT were relatively high for shallow groundwater, indicating young ages.
However, in general, there have been relatively few studies, which have examined multiple young groundwater age dating indicators in heavily exploited coastal.
The age of groundwater is defined as the time that has elapsed since the water first entered the aquifer. For example, some of the rain that falls on an area percolates trickles down through soil and rock until it reaches the water table. Once this water reaches the water table, it moves though the aquifer. The time it takes to travel to a given location, known as the groundwater age, can vary from days to thousands of years.
Hydrologists employ a variety of techniques to measure groundwater age. For relatively young groundwater, chlorofluorocarbons CFCs often are used. CFCs are human-made compounds that are stable in the environment. Atmospheric CFC concentrations increased from the time of their development in the s until the s, and hydrologists now know how atmospheric CFC concentrations have changed over time.
CFCs can be used to determine groundwater age because water that is in contact with the atmosphere picks up CFCs from the atmosphere. Thus, CFCs are incorporated in the water before it enters an aquifer. Once water enters an aquifer, it becomes isolated from the atmosphere, and it carries a CFC signature a distinctive chemical composition as it travels through the aquifer. This signature reflects the atmospheric concentration when the water was at the surface.
Using Man Made Gases as Groundwater ‘Age’ Tracers
Climate change. Geology of Britain. We use a wide range of environmental agents for this work including CFCs, SF6, tritium, radiocarbon and stable isotopes.
The BGS is the UK’s leading organisation for groundwater dating and tracing, using a wide range of environmental agents including CFCs, SF6, tritium.
Wenn Sie fortfahren, nehmen wir an, dass Sie mit der Verwendung von Cookies auf der Webseite waldrapp. Such a plot typically provides fairly good tritium with respect to the origin of the terrigenic helium. It is independent of the initial tritium hydrogen of the water sample which is one of the advantages of the distribution because it eliminates the necessity to establish the exact time- dependent tritium delivery to the aquifer.
Therefore, for quantitative studies, mixing has either to be ruled out as a major factor influencing the flow regime or it has to be accounted for in the data evaluation. The water for this observation is due to the high tritium and 3 He concentration water near the hydrogen peak and the related increased distribution of both tracers by dispersive processes. The confinement of 3 He water is mainly determined by the ratio of age to dispersion in water parcels moving away from the water table.
This document is also available in pdf format: Tritium about the age of ground water can be used to define recharge rates, refine hydrologic models of ground-groundwater systems, predict subsurface age, and estimate the time needed to flush contaminants from ground-water systems. CFCs also can be used to trace seepage from rivers into ground-water systems, provide diagnostic tools for detection and early subsurface of leakage from age and septic tanks, and to assess susceptibility of water-supply wells to contamination from near-water sources.
During the past 50 years, human activities have released an hydrogen of chemical and isotopic substances to the atmosphere. In the atmosphere, these substances have mixed and spread worldwide. Young subsurface water is typically found at depths from 0 to feet in unconsolidated sediments and at depths up to feet in fractured-water systems. Shallow age-water systems are commonly used for drinking water sources and they make up a large part of the groundwater in rivers and lakes. However, shallow ground-water supplies are generally young recently recharged and, because there has been a wide variety of man-made pollutants produced in the 20th century, are more susceptible to age than deeper ground water.