Original Articles
Dating very old pore waters in impermeable rocks by noble gas isotopes

https://doi.org/10.1016/S0016-7037(98)00198-7Get rights and content

Abstract

The 4He, 40Ar, and 136Xe content dissolved in the pore water of sedimentary rock samples was measured on samples from borehole cores near the repository for nuclear waste in Morsleben, Germany. Due to the very low permeabilities of the rock formations, conventional groundwater sampling was almost impossible. Hence, we developed a new sampling method for noble gases in the pore water of freshly drilled rock cores. This method provides vertical noble gas profiles in high depth resolution, even in impermeable rocks. By application of the new technique quantitative age informations of groundwater and pore water have been derived. We find palaeowaters from the last glaciation depleted in δD and δ18O with a 4He age of about 55 kyr. The high saline pore solutions below are at least 6 Mio years old. This has been concluded from the profiles of radiogenic 4He and 40Ar close to diffusion in steady-state and from xenon isotopes produced by spontaneous fission of 238U in the rocks. A 4He flux of 2 · 10−7 cc STP cm−2 yr−1 is derived from the profile, which is due to local 4He production within the investigated sediments.

Introduction

Dating old and very old groundwater is a key factor for the safety assessment of nuclear waste disposals in deep geological formations. Radiogenic noble gas isotopes can in some circumstances provide such age information due to their inertness, their well-known sources and their increasing concentrations within time Marine 1979, Andrews 1985, Torgersen and Ivey 1985, Stute et al 1992, Solomon et al 1996, Zuber et al 1997. The radiogenic noble gases are produced by the subsurface decay of naturally occurring radionuclides and accumulate in pore waters and groundwaters as a function of the residence time in the aquifer, the geochemical environment, and the transfer rate between rock matrix and pore water. In context with the underground storage of nuclear waste, 4He and 40Ar have previously been used to estimate groundwater ages in several hard rock formations Marine 1979, Bottomley et al 1984, Zaikowski et al 1987, Andrews et al 1989, Pearson et al 1991, Sonntag 1992, Suckow and Sonntag 1993, Tolstikhin et al 1996.

At Morsleben, northern Germany, a former salt mine inside a Zechstein diapir has been used as a repository for low and medium level nuclear waste since 1978. The salt dome is covered by a 250 m thick sequence of caprock and sedimentary rock formations, which are characterized by very low hydraulic conductivities of 10−9 m s−1 and even less (Balzer et al., 1993). Due to these poor hydraulic properties, conventional groundwater sampling was restricted to very few boreholes only, where water inflow from cracks and fissures occured. To overcome this problem, we developed a new sampling method for noble gases as well as for environmental isotopes and dissolved salts in the pore water of freshly drilled rock cores (Osenbrück, 1996). In 1994/1995 an extensive hydrogeological investigation was performed at Morsleben in order to check the barrier effect of the sedimentary rock cover above the salt dome. Part of this study was the application of the new methods to analyse the dissolved noble gases in the pore water of sediment cores drilled during the project. This enabled us to derive vertical tracer profiles in high depth resolution from all boreholes. In this paper, we give a brief description of the promising new noble gas technique and present depth profiles of 4He, 40Ar, and 136Xe in the pore water of core samples from the deepest borehole in the vicinity of the Morsleben diapir. The results are presented in Table 1.

The investigated core consists of a 735 m series of Triassic sedimentary rocks. Keuper claystones with dolomite at the base form the upper 300 m of the core. Between 300 m and about 550 m we find claystones with remnants of gypsum and anhydrite (and partly halite) divided by a 40 m thick compacted sandstone layer in about 370 m. The deepest part of the core (590 m down to 735 m) consists of limestones and dolomite from the Muschelkalk. In about 700 m a salt layer of 10 m in extend was present, which is also of Muschelkalk age.

Section snippets

Experimental methods

Borehole core samples for the analysis of noble gases dissolved in the pore water have been taken immediately after lifting the core to ground surface in order to minimize degassing losses. Sections of about 8 cm are separated from the 4-inch core and the outer parts, which are likely to be contaminated by drilling fluid are cut off. The remaining sample of about 300 g and about 5 cm in diameter is quickly put into a vacuum tight sampling container. The air inside the container is removed by

Results and discussion

The depth profile of the 4He concentration (Fig.1 ) shows an increase over 5 orders of magnitude. The radiogenic origin of the analysed helium is proven by the helium isotopic ratio 3He/4He of about 5·10−8, which is typical for crustal produced helium (Mamyrin and Tolstikhin, 1984). 4He contents close to the equilibrium concentration of 4.4 · 10−8 cc STP g−1 indicate the presence of Holocene groundwater in the upper 120 m below ground surface. In a depth of 200–280 m pore waters depleted in

Acknowledgements

—We wish to thank Ms. K. Pitz and Ms. H. Zimmermann at the noble gas lab of the University of Heidelberg for their assistance with a large number of measurements, and especially Mr. M. Gröning, now IAEA Vienna, for his assistance and support in the development and successful application of the new sampling method.

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