Atmospheric and radiogenic gases in groundwaters from the Stripa granite

doi:10.1016/0016-7037(89)90304-9

Geochimica et Cosmochimica Acta

Volume 53, Issue 8, August 1989, Pages 1831-1841

https://doi.org/10.1016/0016-7037(89)90304-9 Get rights and content

Abstract

Groundwaters from depths of 350 m to 1250 m in the Stripa granite contain dissolved radiogenic He in amounts up to 50,000 times that due to air-saturation. The groundwater He-contents increase with depth and lie close to the expected profile for He loss by aqueous diffusion (D = 0.032 m² a⁻¹). Measurements on core samples show that the rock has retained about 10% of the possible cumulative radiogenic He and that this component is lost by matrix diffusion (D = 5 × 10⁻⁷ m² a⁻¹). Diffusive equilibrium between He in fracture fluids and in the adjacent rock matrix is rapidly established for the narrow fracture widths of the flow system. A major loss of stored He by both diffusion and advection along fluid-filled fractures is attributed to the proximity of a major fraction of uranium to the aqueous flow system because of its deposition within an interconnective microfracture system.

The crustal flux of He is limited by its diffusion coefficient in the matrix of a granitic crust but may be supplemented by transport due to fluid circulation.

The $^{3} He^{4} He$ ratio of the excess He present in the Stripa groundwaters, corresponds to that expected for radiogenic He production within the granite. The $^{40} Ar^{36} Ar$ ratio of dissolved Ar shows that radiogenic ⁴⁰Ar has been released from the rock matrix, especially for groundwaters from greater than 450 m depth. Slow alteration reactions are the most probable cause of this radiogenic ⁴⁰Ar release which has occurred in the more saline groundwaters. Groundwater recharge temperatures, estimated from their noble gas contents, are about 3°C lower than those for modern shallow groundwaters in the locality and are related to the stable isotope composition of the groundwater. Most groundwater age measures at Stripa are the result of mixing between recent recharge waters (<100 a) and a much older fracture-stored brine.

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The relationship between He and Cl<sup>−</sup> as a criterion for the formation of groundwater composition
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The groundwater samples considered in this article are derived from reservoirs of varying geological ages. Among them, 285 parallel determinations of chloride and He concentrations belong to the groundwaters of crystalline pre-Paleozoic rocks of Scandinavian (Ahonen et al., 2011; Andrews et al., 1982, Andrews and Hussain, 1989; Gascoyne, 2000, 2014; Mahara et al., 2008; Pitkanen et al., 1998, Pitkanen and Partamies, 2007; Sherwood Lollar et al., 1993) and Canadian (Bottomley et al., 1984, 1990; Greene et al., 2008; Pinti et al., 2011) shields, Karatwaal craton in southern Africa (Lippmann et al., 2003), as well as Paleozoic granites in California (Torgersen and Clarke, 1992) and Europe (Lehmann et al., 1996; Pearson Jr. et al., 1991). The groundwaters of the Paleozoic sediments have 178 paired determinations of chloride and He concentrations.
The analysis of the relationship between 1611 parallel determinations of helium and chloride concentrations in the groundwater of crystalline shields and sedimentary basins was performed. The comparison showed that groundwaters have helium and chlorides concentrations in the range of 1.8 × 10⁻⁸ to 6.8 mol/m³ and of 5.6 × 10⁻³ to 6488 mol/m³, respectively, and relatively stable He/rCl ratios equal to 5.5 × 10⁻⁵ in sedimentary basins and 7.5 × 10⁻⁴ in crystalline shields. Such behavior of helium and chlorides shows that the formation of the groundwater composition is determined primarily by the processes of mixing fresh meteoric and saline sea waters with ancient brines from great depths. During sedimentation, sea water penetrates the subsurface and mixes with ancient brines. On land, fresh meteoric waters dilute the mixtures of seawater and ancient brines. Most of the radiogenic ⁴He and chlorides comes from the subsurface ancient brine. The radiogenic ⁴He in the host rocks plays a secondary role. Therefore, the concentration of ⁴He in saline groundwater reflects the size of the impurities of the ancient brine with a very high content of ⁴He, rather than the residence time within the subsurface.
Noble gas residence times of saline waters within crystalline bedrock, Outokumpu Deep Drill Hole, Finland
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Noble gas isotopes (only He and Ar) have been previously measured in deep groundwaters of the Fennoscandian Shield by Andrews et al. (1982) from the Stripa mine, by Hilton and Craig (1989) from the Siljan Deep Well (∼7 km deep), and by Sherwood Lollar et al. (1993) from several sites in Finland, but these studies did either not include absolute noble gas concentration determinations or suffered from disturbed sampling conditions, thus hampering the determination of residence times. Samples from the Stripa mine were later studied for their residence time by Andrews et al. (1989), but without success as the He concentrations were concluded to be controlled by steady state processes. Delos et al. (2010) reported the distribution of helium in groundwaters down to a depth of 1 km in crystalline rock in Sweden (Forsmark and Laxemar) and Finland (Olkiluoto).
Noble gas residence times of saline groundwaters from the 2516 m deep Outokumpu Deep Drill Hole, located within the Precambrian crystalline bedrock of the Fennoscandian Shield in Finland, are presented. The accumulation of radiogenic (⁴He, ⁴⁰Ar) and nucleogenic (²¹Ne) noble gas isotopes in situ together with the effects of diffusion are considered. Fluid samples were collected from depths between 180 and 2480 m below surface, allowing us to compare the modelled values with the measured concentrations along a vertical depth profile. The results show that while the concentrations in the upper part are likely affected by diffusion, there is no indication of diffusive loss at or below 500 m depth. Furthermore, no mantle derived gases were found unequivocally. Previous studies have shown that distinct vertical variation occurs both in geochemistry and microbial community structuring along the drill hole, indicating stagnant waters with no significant exchange of fluids between different fracture systems or with surface waters. Therefore in situ accumulation is the most plausible model for the determination of noble gas residence times. The results show that the saline groundwaters in Outokumpu are remarkably old, with most of the samples indicating residence times between ∼20 and 50 Ma. Although being first order approximations, the ages of the fluids clearly indicate that their formation must predate more recent events, such as Quaternary glaciations. Isolation within the crust since the Eocene–Miocene epochs has also direct implications to the deep biosphere found at Outokumpu. These ecosystems must have been isolated for a long time and thus very likely rely on energy and carbon sources such as H₂ and CO₂ from groundwater and adjacent bedrock rather than from the ground surface.
A multi-tracer study of groundwater origin and transit-time in the aquifers of the Venice region (Italy)
2014, Applied Geochemistry
Located in the northeastern region of Italy, the Venetian Plain (VP) is a sedimentary basin containing an extensively exploited groundwater system. The northern part is characterised by a large undifferentiated phreatic aquifer constituted by coarse grain alluvial deposits and recharged by local rainfalls and discharges from the rivers Brenta and Piave. The southern plain is characterised by a series of aquitards and sandy aquifers forming a well-defined artesian multi-aquifer system. In order to determine origins, transit times and mixing proportions of different components in groundwater (GW), a multi tracer study (³H, ³He/⁴He, ¹⁴C, CFC, SF₆, ⁸⁵Kr, ³⁹Ar, ⁸⁷Sr/⁸⁶Sr, ¹⁸O, ²H, cations, and anions) has been carried out in VP between the rivers Brenta and Piave. The geochemical pattern of GW allows a distinction of the different water origins in the system, in particular based on ${HCO}_{3}^{-}, {SO}_{4}^{2 -},Ca / Mg, {NO}_{3}^{-}$ , ¹⁸O, ²H. A radiogenic ⁸⁷Sr signature clearly marks GW originated from the Brenta and Tertiary catchments. End-member analysis and geochemical modelling highlight the existence of a mixing process involving waters recharged from the Brenta and Piave rivers, from the phreatic aquifer and from another GW reservoirs characterised by very low mineralization.
Noble gas excesses in respect to atmospheric equilibrium occur in all samples, particularly in the deeper aquifers of the Piave river, but also in phreatic water of the undifferentiated aquifers. ³He–³H ages in the phreatic aquifer and in the shallower level of the multi-aquifer system indicate recharge times in the years 1970–2008. The progression of ³H–³He ages with the distance from the recharge areas together with initial tritium concentration (³H + ³He_trit) imply an infiltration rate of about 1 km/y and the absence of older components in these GW. SF₆ and ⁸⁵Kr data corroborate these conclusions. ³H − ³He ages in the deeper artesian aquifers suggest a dilution process with older, tritium free waters. ¹⁴C Fontes–Garnier model ages of the old GW components range from 1 to 12 ka, yielding an apparent GW velocity of about 1–10 m/y. Increase of radiogenic ⁴He follows the progression of ¹⁴C ages. ³⁹Ar, radiogenic ⁴He and ¹⁴C tracers yield model-dependent age-ranges in overall good agreement once diffusion of ¹⁴C from aquitards, GW dispersion, lithogenic ³⁹Ar production, and ⁴He production-rate heterogeneities are taken into account. The rate of radiogenic ⁴He increase with time, deduced by comparison with ¹⁴C model ages, is however very low compared to other studies. Comparison with ¹⁴C and ¹³C data obtained 40 years ago on the same aquifer system shows that exploitation of GW caused a significant loss of the old groundwater reservoir during this time.

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^☆: This paper is published as part of a series reporting results of the International Stripa Project.

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Atmospheric and radiogenic gases in groundwaters from the Stripa granite☆

Abstract

Chem. Geol.

J. Hydrol.

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Appl. Geochim.

Geochim. Cosmochim. Acta

J. Hydrol.

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Radon and helium solution in an HDR geothermal reservoir

The composition of dissolved gases in deep groundwaters and groundwater degassing

Environmental isotope studies in two aquifer systems: A comparison of groundwater dating methods

Stable isotopic evidence for palaeo-recharge conditions of groundwater. Palaeoclimates and Palaeowaters: A Collection of Environmental Isotope Studies

Palaeoclimatic trends deduced from the hydrochemistry of a Triassic Sandstone aquifer, U.K.

Empirical laws for dilute aqueous solutions of non-polar gases

J. Chem. Phys.

On the detection of faults along the Sverdlorsk DSS profile from high concentrations of He in underground water

Phys. Solid Earth