Dating with the 3 He-Tritium method

Introduction

Little is know about the age of groundwater in the unconsolidated aquifer of the study area. The use of environmental or man made tracers for age and provenance of groundwater proved to be a powerful tool in hydrogeology (e.g. Cook and Herczeg 2000). Especially the helium-tritium method is an excellent tool for dating groundwater (Schlosser et al. 1988, 1989). Therefore, within the framework of a pilot study for the applicability of the ³Helium- Tritium method, samples from selected wells in the study area were taken. The samples were taken along the presumed Wadi Hisban alluvial fan flow path. In the following paragraphs the Helium-Tritium method is described, followed by the sampling and analyses procedure. Most of the information was taken from Sültenfuß and Massman 2004. The excursus commences with the description of the results and their implications for this study.

Method

Tritium method:

The use of Tritium for age determination is widely accepted. Apart from low analysis costs and easy sampling procedure, forty years of experience exists. Tritium is measured in tritium units (TU), which are defined as the ratio between tritium nuclei to ¹H hydrogen nuclei where 1 TU = 1/10^{18 3}He/¹H (or as activity per mass; 1 TU = 8.38 Bq/kg). As a results of surface nuclear bomb testing Tritium had its atmospheric peak concentration in the late 50ies and beginning of the 60ies. Surface atomic bomb testing stopped in the following period and the emission of large quantities of Tritium into the atmosphere seized. Tritium decays with a half-life of 12.32 years to ³He (Lucas and Unterweger 2000). Therefore the contemporary concentration of Tritium in coastal areas is back at its pre- bomb natural concentration (usually around 2-5 TU in coastal areas). Tritium concentrations in continental areas are slightly higher. The total decay of one TU enlarges the ³He concentration per kg water by 2.5

* 10^-12 Nml.

Age determination on the basis of Tritium concentrations

The comparison of the Tritium concentration in groundwater samples to Tritium concentrations in rainfall at the time of infiltration gives a time window. Fig. 5.2-17 shows the theoretical Tritium concentrations of samples taken throughout the period of 1962 to 1995 in Hof (Germany), if they would have been stored until the 01.01.2002 and analyzed at the same date (Sültenfuß and Massman 2004). Therefore the Tritium concentration of rainfall water in the infiltration area should be sufficiently known. In the case of a fast percolation to the saturated zone the Tritium concentration of rainfall water equals the one of groundwater. If infiltration happens over a larger time span, than a decay of the Tritium concentration of 5.5 %/year has to be assumed. Fig. 5.2-17 also illustrates, that groundwater that infiltrated between 1982 to 1995 and between 1967 to 1979, cannot be distinguished.

5. Groundwater quality and salinization

In this study only a rough idea about the age of groundwater was desired; either very young or old water, or young water mixed with old water. Old water is understood in the sense of older than 80 years. High variations in Tritium concentrations between summer and winter, as reported by Sültenfuß and Massman (2004), are not expected in the study area, since precipitation falls only in the winter months. However, even during the winter months Tritium concentration may vary. But mixing of groundwater, that have different ages, results in the loss of significance of the sample when the Tritium method is applied alone.

Fig. 5.2-17: Theoretical remaining Tritium concentration for rainwater samples taken and conserved between 1963 – 1995. The plot shows the theoretical results of an analysis of all samples on the 01.01.2002 (Sültenfuß and Massmann 2004).

3Helium Tritium method

The disadvantages of the above mentioned Tritium method can be avoided by using the ³Helium- Tritium method. The combined method of measuring Tritium and ³Helium for the age determination of groundwater was already proposed in 1969 (Tolstinkhin and Kamenskij 1969). However the method was not used before the late 80ies (i.e. Schlosser et. al. 1988, 1989). Here, in addition to the measurement of the Tritium concentration its decay product ³He is measured. ³He is dissolved in groundwater as it flows in the saturated zone as long as it has no exchange with the atmosphere. ³He is also chemically inert. From the relationship between the Tritium and ³Helium concentrations the groundwater age time window can be determined independent of seasonal or regional variations:

T = λ^-1 * ln(1+ ³He * ³H^-1) where

t = time parameter

λ = decay constant = 0.05626 y^-1

Since the decay product ³He is a gas, it results in the fact, that it fumigates when it gets in contact with the atmosphere. Therefore it can only accumulate once it is in and stays under a closed system condition. The ³Helium- Tritium method clock starts ticking once the infiltrated rainfall becomes under a closed system condition (no exchange with the atmosphere) and that is under saturated conditions. Once the infiltrated rainwater is under closed conditions, the decayed Tritium product ³He accumulates until carefully sampled and analyzed. In addition, a determination of mixing with older Tritium free water is possible. ³H and ³He concentrations are summed up and are compared to the Tritium concentration of rainfall in the recharge area at the calculated time window.

However some prerequisite have to be fulfilled: first, all other helium sources must be quantified;

second, once rainfall infiltrates into the saturated zone no exchange between groundwater and atmosphere must happen; and third, the dispersive transport must be << than the advective transport.

Regarding the first prerequisite, apart from ³He generated by the decay of tritium, four other sources for helium exist: first, helium that was dissolved in the precipitation water according to the solution equilibrium (He_equi); second, surplus share accumulating from ³He stored in the pores of the unsaturated material, that would dissolve into the infiltrating water on the way to the saturated zone (He_excess); third, Helium generated by the decay of Uranium and Thorium (which are generated in the subsurface), so called radiogene Helium (He_rad); and forth, Helium exhalated from the earth deep zones through faults, the so called premodiale Helium (He_prim).

Therefore ³He measured in the sample could have the following different ³He components:

3He_sample = ³He_trit + ³He_equi + ³He_excess + ³He_rad + ³He_prim

Since ³He_equi is a function of temperature, water salinity, and atmospheric pressure ³He_equi can be calculated. ³Heexcess cannot be calculated directly, but estimated indirectly with the help of Ne Isotopes.

The only source for Neon dissolved in water is of atmospheric nature.

Nesample = Neequi (which can be calculated) + Neexcess

This makes the calculation of Heexcess possible. A separation of the non-atmospheric generated ³Herad

and ³Heprim is difficult. However, ³Heprim only occurs in geological active fault zones. In waters where no anthropogenic generated Tritium is present an upper level for natural generated tritium can be estimated (usually around 5 TU for the continental Middle Europe; Roether 1967). That means, if the analyzed sample exceeds the estimated ³Hetrit plus the calculated ³Heequi and ³Heexcess the presence of

3He_prim must be assumed. ³He_rad can be estimated with the help of ⁴He_rad (⁴He_rad = ⁴He_sample – ⁴He_equi –

4He_excess).

Sampling and Analyzing

The noble gas samples were taken in a copper pipe (volume around 40ml) provided by the Institute of Environmental Physics of the University of Bremen (Fig. 5.2-18). Since exchange of the groundwater with the atmosphere would result in the outgassing of ³He, samples have to be taken carefully.

Therefore a lucent plastic pipe was added between the outlet of the well water and the copper pipe on one side and a second one with a valve that will be partly closed during the sampling procedure. The flow of water in the pipe should be down to up. This enhances the pressure in the copper pipe and should prevent outgassing. Afterwards the pipe is closed, first at the outlet and than at the inlet. For each well two samples were taken (Fig. 5.2-19). The sampling procedure however proved to be a difficult task. Most, if not all wells in the study area were built in a basic way (chapter 4.3). No proper sampling valves at the well outlets are available. On the outlet of the well heads plastic pipes are plugged over the well head and smelted to the metal well head which makes sampling a difficult task.

Moreover, the pumps in the irrigation well cannot be regulated. Either the pump is turned on or off.

Since hydraulic conductivity in most wells is low compared to the pump capacity, strong variations in hydraulic head change are common (chapter 4.3.) and turbulent flow conditions within the well pipe might take place. This in turn leads to an exchange between the atmosphere and groundwater. High

5. Groundwater quality and salinization

contamination with atmospheric air is the consequence. Unfortunately this happened in three out of the four selected wells. Only one sample was suitable for analyzing. The samples were transferred to Germany and analyzed in the Helium Isotope Laboratory at the Institute of Environmental Physics (University of Bremen). Detailed analytical procedures are described at Sültenfuß and Massman 2004.

Fig. 5.2-18: Helium isotope sampling procedure.

Fig. 5.2-19: Location of isotope sampling sites.

Results

For all sampling locations two Tritium and two He-Isotope analyses were taken. The Tritium concentrations are listed in Tab. 5.2-15 and represent average values. Only one He-Isotope Analysis, the sample from W024 was suitable for analyzing. In all other samples the atmospheric gas content was too high, which affirms that sampling from wells in the study area is difficult.

For the samples taken from wells W013 and W024 a clear contribution of water that infiltrated after 1950 can be stated. But the low Tritium concentrations can only be explained by a high fraction of water that is older than 1950. Therefore a mixture of older with younger water can be concluded, whereby the highest fraction is older than 60 years. If the sample of well W001 is not a mixture between old and newer water an age of around 75 years can be assigned (when a natural concentration

of 5 TU is assumed). In the case of mixing, the sample can also consists of a small portion (1%) containing Tritium (infiltrated after 1955) and a large portion (99%) of old tritium free water. The sample of well W020 is free of Tritium. Therefore an age of >100 years can be assumed.

Tab. 5.2-15: Results of Tritium analysis. For each sample two analyses were conducted in the Helium Isotope Laboratory at the Institute of Environmental Physics (University of Bremen). The Tritium concentrations in the table are average values.

Sample ID Tritium

[TU] Tritium error [TU]

Fraction of young

components Remarks

W 001 0.08 0.02 <= 1%

W 013 1.62 0.02 <10 %

W 020 -0.01 0.02 0 %

W 024 1.08 0.02 < 10% Contains Mantle Helium

The Neon concentration of sample W024 was 2.5 times above the Neon equilibrium. This indicates a high content of atmospheric air in the sample. The ⁴He/Ne ratio shows, that apart from the high concentrations of atmospheric air, the sample contains another source of Helium (Fig. 5.2-20). Since the Tritium concentration is 1.08 TU, the contingent of tritiogene ³He cannot exceed 16 TU (antropoghenic generated Tritium with sufficient entry since 1955). Therefore the high ³He/⁴He- ratio cannot be explained by the accumulation of decayed Tritium but rather by the inflow of mantle components (³He_prim). The composition of sample W024 is shown in Fig. 5.2-21.

Fig. 5.2-20: ⁴He- Ne concentration of sample W024.

5. Groundwater quality and salinization

Fig. 5.2-21: ⁴He- sample from well W024 split into its different components.

5.3 S