The age of that water is therefore a good measure of the mean transit time HELIUM‐3/TRITIUM DATING WW 4of Table 1. Tritium (3H or T) is the radioactive isotope of hydrogen that decays with a half life of calculate an age from the radioactive mother/daughter ratio (tritium/3He age) . If no terrigenic helium is contained in the groundwater sample, 3Hetrit can be of advection to dispersion in water parcels moving away from the water table. Abstract. Groundwater age dating through the combination of transient tracer methods below the water table), from about 10 to 25 years for the medium- depth wells (screen transient tracers tritium and helium 3 (3H/3He age) [ Schlosser.
Because of its heterogeneity, the efficiency of the ditch varies strongly, resulting in even more complicated flow patterns . The groundwater is confined up to 2 - 3 km inland and, depending on the river water level, artesian for the first m.
The hydraulic situation in the central polder strongly deviates from the situation near the river. The groundwater is largely unconfined. However, human interference due to the levee construction, agriculture ploughing and lowering of groundwater levels have reduced the clay content of the soil.
Drainage activities have reportedly caused volume changes in the soils due to shrinking, leading to morphological texture changes which influence the recharge rates since precipitation might percolate quickly along shrinking fissures, circumventing the primary pore system . Sample Collection and Measurements Seventeen samples of surface water and groundwater were collected for analysis of helium and neon isotopes in the study area.
The samples were collected in special pinch-off copper tubes containing about 40 cc of water, and fitted with stainless steel pinch-off clamps at each end.
The copper tube was fixed in an aluminum channel holding the stainless steel pinch-off clamps. By closing the clamps, the copper tube was shut absolutely gas tight and it was stored till the time of measurement.
Water samples were analyzed for helium He and neon Ne isotopes at the noble gas laboratory of the Institute of Environmental Physics, University of Bremen. In the laboratory, the copper tubes were connected to a high vacuum system and permanent gases were separated from water and stored in glass ampoules. Leak checks and residual gas checks assured transfer efficiency greater than The glass ampoules were opened in a high vacuum inlet system.
Water vapor transferred gases to a cryo system kept at 25 K to separate Ne from other gases. Overall calibration with air aliquots and internal water standards assured accuracies for Ne concentrations in water higher than 1.
Basics of 3H/3He dating
For most samples double measurements were conducted. The extraction efficiency of the system is at least The degassed water was flame sealed in glass bulbs, which were stored in a freezer for nearly four weeks until a sufficient amount of tritiogenic 3He had accumulated to allow determination of tritium content by the 3He-ingrowth method.
Results and Discussion 4. Figure 4 shows that some samples have considerable excesses of both noble gases relative to the solubility equilibrium with the atmosphere under conditions typical for the aquifer. This strongly indicates that excess air component is not fractionated . The nearest wells from the river 4He terrigenic and sometimes zero 4He terrigenic concentration Table 1. The analysis of 4He components for the study area.
In the recent study only He and Ne data are available so that the assumption of zero concentration of 4He terrigenic is considered to facilitate the calculation of 3He tritiogenic by Equation 4. For these wells Equation 4 was used to calculate 3He tritiogenic.
In general, some of the young water samples 4He terrigenic and some others have very small quantities of 4He terrigenic, when compared to the concentration of solubility equilibrium. The calculated 3H-3He ages against distance from the river Oder. Initial stable tritium in TU against the calculated 3H-3He ages in years. The calculated 3H-He ages against 3He tritiogenic.
The data points from the sampling location in Oderbruch area are superimposed on the input function Figure 9 in the following ways: This means that these samples were not influenced by mixing.
These fissures are formed as a result of the hydraulic situation and the transition from confined to unconfined conditions, which cause the loam to dry out and form fissures . The initial tritium concentration of this sample is nearly 5 Figure 9. Comparison of the initial tritium concentration and the input function.
TU, which fulfilled the result deduced by Kaufman and Libby in . The sample from well T falls below the input curves but one can consider that the sample from this well was also formed in a pre-bomb era and not affected by mixing this because it is in the range of its calculated error.
The samples from wells T and F are characterized by a relatively high component of 4He terrigenic, which is a good indication of the presence of a significant component of old water. But the initial tritium points for these samples fall on the input curves, which gives a bad indication of whether these samples were mixed or not.
It also provides valuable information for the management of the water resources. The assumption of excess air formed by complete dissolution of small air bubbles trapped in the soil during infiltration, and therefore the composition of excess air is identical to air, sometimes leads to negative concentrations of 4He terrigenic.
In the recent study only He and Ne data are available so that the assumption of zero concentration of 4He terrigenic is considered to make calculations. These samples have a part which mixed with young water from different sources. The last set of samples is water with ages greater than 40 years. These samples only occurred in the large distances from the Oder River and one can expect these waters were formed before the hydrogen bomb test and have initial tritium of approximately 5 TU.
The occurrence of a high 4He terrigenic in the groundwater samples gives a good indication to the presence of a significant component of old water.
Tritium/Helium-3 Dating of River Infiltration:An Example from the Oderbruch Area, Berlin, Germany
If the infiltration temperature is unknown, additional noble gases measurements other than He and Ne are required to determine the infiltration temperature.
In this equation, only 3Hetot and 3Heeq are determined through measurements. The total 4He concentration measured in a groundwater sample can be written as: If no terrigenic helium is contained in the groundwater sample, 3Hetrit can be calculated by using equation 4: For separation of terrigenic helium, we have to use neon measurements.
This component can be radiogenic helium, mantle helium, or a mixture of these two components. The most practical approach to determining Rterr is to measure it in groundwater samples from the same aquifer that are free of tritium. If there is no tritium-free groundwater in the studied aquifer, an estimate of Rterr can be obtained in most cases by plotting 3He versus 4He.
Such a plot typically provides fairly good clues with respect to the origin of the terrigenic helium. The atmospheric 3He component 3Heatm can be written as: It is independent of the initial tritium concentration of the water sample which is one of the advantages of the method 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.