- Isotope methods for dating old groundwater|INIS
- Dating of Old Groundwater — History, Potential, Limits and Future
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For several decades, one of the major tools for obtaining information about groundwater origin, and its properties and movement has been the use of isotopes, which has often provided insights not available using other techniques. Information on groundwater age is required to address aspects such as recharge rates and mechanisms, resource renewability, flow rate estimation in aquifers and vulnerability to pollution, especially when dealing with shared water resources.
Age information, mainly provided by radionuclides and modelling, is considered highly relevant for validating conceptual flow models of groundwater systems, calibrating numerical flow models and predicting the fate of pollutants in aquifers. Isotope tracers are now used to study groundwater age and movement, covering time spans from a few months up to a million years.
The understanding of groundwater occurrence and movement in large continental basins has been a matter of debate among experts. Despite the large number of studies which have been carried out in the past, many open questions remain, and ideas and concepts are often revised based on new conceptual models, isotope and tracer analyses or water flow models.
The book's 14 chapters explain what is currently understood about the use and application of radionuclides and related geochemical tracers and tools to assess groundwater age and movement over time spans beyond a few thousand years. Publication Year Publication Year. Reference Number Reference Number. Get permanent URL for this record. Please provide a name for this query: Figures - uploaded by Mebus A Geyh. Author content All content in this area was uploaded by Mebus A Geyh.
Estimation of A0 DIC by means of the increase of the water age along the flow path Vogel and Ehhalt, demonstrated with 14 C dates by Phillips et al. The reservoir correction is about — years. Groundwater recharge in New Mexico was interrupted during the first part of the Holocene.
Scheme of hydrodynamic non-steady state conditions: Groundwater recharge might have occurred before 26, yr, between 23, and 18, yr, as well as and yr.
The tracer velocities calculated from the 14 C dates of the wells A-B and C-E are valid for the corresponding pluvial periods but vtracer of the wells B-C and A-D are hydraulically meaningless as groundwater recharge was interrupted. Content uploaded by Mebus A Geyh. After the discovery of the 14 C method as a chronometer for old groundwater in. In the eighties the quantitative interpretation of 14 C dates has become the.
The difficulty to transfer 14 C dates into a bsolute water ages has also. Uranium isotope ratios, krypton, chlorine, helium-4 and 14 C in.
Isotope methods for dating old groundwater|INIS
History of 14 C dating of old groundwater. In the history of 14 C dating of old gr oundwater se veral phases can be. The elaboration of the principles of 14 C dating of old groundwater. Designing models to convert 14 C dates of DIC into absolute groundwater ages. Designing conceptual models for quantitative interpretation of numerical dates. The 14 C dating method developed by Libby and coworkers Anderson et al.
The initial 14 C value Ao of atmospheric. Then, the conventional 14 C age t. In groundwater dating the. It appeared to be an abstruse idea of the physics student Franke that 14 C. The princ iple idea behind Munich's concept was that rain water.
C O 2 dissolution. As a result of the dissolution of young 14 C rich CO 2 b y decomposition of soil. Carbon dates calculated from the results of 14 C measurements of DIC are.
Dating of Old Groundwater — History, Potential, Limits and Future
They yield, however, absolute age. Such cases are identifiable by e. These conditions are often met in confined fresh water. They enable the estimation of the tracer velocity, the delineation of the. The hydrochemical process described by Eq. Because the carbonate in the unsaturated zone and the aquifer as one of the carbon-. This means that 14 C age calculated with Eq. One reason is that bicarbonate is only kept in dissolution if CO 2 is dissolved in. As function of pH this process results. There are many other hydrochemical and isotope-fractiona tion.
The mentioned models suppose closed-system conditions where only initially. In the unsaturated zone, open- system conditions. The dissolved bicarbonate further exchanges with the gaseous. CO 2 in the pores Wendt et al. This process results in an. It adjusts iteratively the simulated. Congruent a nd i ncongruent dissolution of carbonate, gy psum. An even more realistic mixing-exchange model Fontes and. Garnier, uses the molar alkaline-earth content corrected for sulphate. The initial 14 C values determined with these models usually deviate from each.
As a result many hydrogeologists have become.
Considering specific cases the actual. The few applications of his approach are promising. A serious problem for the application of these models is that several basic. In contrary to the. The reason is the formation of 14 C bearing secondary lime. A similar effect to A 0 DIC arises. Aggarwal and Dillon, In order to overcome these difficulties empirical approaches were developed to.
They have the advantage that all of the. A statistical approach from Vogel and Ehhalt advises the general.
Geyh recommends the use of individual A 0 DIC. Initial 14 C values and corresponding reservoir corrections. The decrease of the 14 C value in DIC by radioactive decay along the flow path. If the recharge area is. A 0 DIC is given where 3 H. The concept behind is that bomb 14 C in DIC should be absent if.
Chemical and hydrodynamic modification of 14 C of DIC during. A 0 DIC is the 14 C va lue immediately after the termination of groundwater. It allows a correction of the bias between the conventional 14 C time. The advanced models Reardon and Fritz,. Estimation of A 0 DIC by means of the increase of the water age along the.
Groundwater recharge in New.
Mexico was interrupted during the first part of the Holocene. Tritium below the detection limit hatched area verifies absence of bomb 14 C. The initial 14 C value is about 87 pMC Verhagen et al. The hydrochemical processes during the groundwater recharge as well as the. It is based on an integrated mass-balance. There are still particular hydrochemical reactions which are not yet. One of them is silicate weathering, which is. It produces HCO 3 — resulting in 14 C ages overestimated by up.
Corrections can be made by means, for instance, of the. Physical and hydraulic modification of groundwater ages. Separately from the mentioned pure hydrochemical processes which determine. DIC and the carbonate of the aquifer rock as well as congruent dissolution and. Quantifying the effects of these processes Gonfiantini and Zuppi introduce. Finally palaeohydrologically reasonable results. Similar effects are observed in dual-porosity structured aquifers. Carbon ages of DIC considerably increase due to tracer retardation, sorption. Traditional geochemical models do not correct 14 C for these.
In practice, it remains a. In shallow aquifers containing very old groundwater, diffusion of 14 CO 2 of the. The palaeohydrologic information might become considerably truncated. Advection and admixture also may control the 14 C value of DIC. Geyh and Backhaus and Geyh et al. The seepage of young.
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Lumped-parameter and conceptual m odels are usable for complex. Application of the 14 C chronometer i n applied hydrology. Increasing experience in the interpretation of 14 C data of DIC has continuously. Meanwhile the number of related publications is so large that. In the Sahara desert, groundwater ages were found to be more than tens. In humid regions or under pluvial palaeoclimatic conditions in present arid. In piston-flow systems it is proportional to the Darcy. The factor of proportionality is the total porosity n total Geyh and Backhaus,.
Due to the usually long residence time of. Equation 5 opens the possibility of determining the regionally valid total. In addition, 14 C dates permit delineation of the flow path. In open aquifers 14 C profiling of DIC delivers present or past recharge rates. Vogel, , Geyh and Ploethner, , Fig.
After the indispensable check.
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The hydraulic conductivity of aquifers and sometimes also aquitards is. The results are, however, only representative. In leaky aquifer systems, 14 C values of DIC. Estimation of the recharge rate by 14 C profiling in the Thar desert. Hydrodynamic non-steady-state conditions exist when the recharge rate is. They are widespread in semi-. In such areas the pumping rate often exceeds the infiltration rate of. Burdon discovered already in the seventies that the present. It may be just. Substantial information has already. In regions with present temperate climate groundwater recharge was interrupted.
Precipitation was considerably reduced. Wendt, , Andres and Geyh, , Bath et al. Archaeological, historical, palynological and other. Important time markers are related to the beginning and the end of periods of. Hydrogeologists and numerical modellers seldom consider that the gradient of.
Then, the present gradient reflects non-steady state conditions: It has become evident for such cases that 14 C dating of. If discontinuous hydraulic conditions are not taken into a ccount, numerical. Conceptual modelling of isotope dates adapted to the palae ohydrologic. On the other hand, isotope hydrologists seldom realize that tracer velocities of. Figure 5 elucidates the background for a present arid region. According to this, only. Along the flow path of the groundwater four wells A, B, C and D. The two apparent tracer velocities derived. The gradient of the water table of this fossil water resource decays as well as.
Therefore the 14 C dates do not allow an estimate. The tracer velocity derived from 14 C ages of t he adjacent wells B-C or all wells. A-D is mean ingless as the groundwater recharge was interrupted during the period. In urban, industrial and intensively used ag ricultural areas groundwater. Then, the groundwater t able drops and. Young or old groundwater resources become mobilized and mixed. Before the anthropogenic impact, 14 C ages of DIC reflect the undisturbed.
Reliable estimates of the actual tracer velocity and. As soon as the groundwater is excessively pumped,. If young groundwater from e. If old ground water. Monitoring the 14 C age. The resulting deterioration of the water. Under suitable hydrologic conditions conceptual models applied to 14 C ages and. The difficulty of detecting secondary hydrochemical changes of 14 C in DIC and.
Additional reasons for these. Therefore, the effects of advective and dispersive flow dynamics to. Groundwater mining study in the Azraq spring area, Jordan: Uranium isotopes appear to have a great potential for absolute groundwater. The uranium concentration in groundwater is.
In the oxygenated zone of the aquifer near the. There are several theoretical and empirical studies on uranium isotopes as a. However, the complex interaction of. In an y case reasonable agreement. They find a reasonable promising relationship. Large basins are usually filled with groundwater of ages exceeding the dating. Moreover, these groundwaters are often highly. Chlorine as a cosmogenic isotope with a ha lf life of ka. The first successful attempt confirme d the. Other studies demonstrated the. Limits of 36 Cl. Andrews and Fontes, In order to overcome the resulting effects, Mazor.
Another promising cosmogenic isotope a s a chronometer for brine and. The theoretical dating range is million years. But, I dates may successfully complement 36 Cl dating attempts. Problems encountered with the dating of highly mineralized groundwater and. Interferences by hydrochemical interactions and underground. However, only up to 81 Kr atoms are present per l itre of.
They are detected with accelerator mass spectromete rs AMS after an. By introduction of even more advanced measurement techniques. Old brines with ages of up. Alpha decay of radionuclides of both the uranium and thorium series produces. The production rate is controlled by. The relative time scale or the corresponding accumulation rates are best. Many fields in applied hydrology and geosciences have benefited from the. How ever, a direct and. Basic research is required.
Figure 7 elucidates t he problem. For numerical modelling the aquifer to be. Geohydraulic parameters are assigned to each of them, based on available. The hydraulic pressure field is obtained by iterative. Solute-transport modelling of natural or.