Two boreholes per siting region were investigated for dissolved helium in porewater (JO: BOZ1, BOZ2; NL: BUL1, STA3; ZNO: TRU1, MAR1), with the results summarised here and discussed in more detail in Rufer et al. (2024).
General observations
The porewater helium profiles from all six boreholes (Fig. 4‑126) show generally smooth trends above the Muschelkalk – interpreted to be indicative of diffusive transport – and are largely comparable in shape to the δ2H and δ18O profiles (Section 4.6.2). Over the low-permeability, clay‑mineral-rich Lias – Dogger sequence, all profiles show similar concentrations of 4He in the porewater and predominantly flat profile shapes with depth. As an exception, the BOZ2 profile is influenced in the upper part of this section by the groundwater of the Hauptrogenstein aquifer. 3He/4He porewater signatures from the central aquitard section show similarly flat profiles with well ‑constrained isotope ratios. It is noteworthy that helium shows no discernible anomalies in the «Herrenwis Unit» of NL.
Interactions with groundwaters
All helium profiles extend to the Keuper aquifer (Klettgau Formation) or beyond. In two boreholes, transmissivities in the Klettgau Formation were sufficiently large to sample groundwater (BOZ2, TRU1; see Fig. 4‑85, Section 4.5.3.10). The associated 4He porewater profiles show a curvature from higher 4He concentrations in the hanging units towards lower values close to the groundwater, indicating helium transport in this direction in the past. In BOZ2, the groundwater has a lower 4He concentration compared to the adjacent porewater, supporting such an interpretation also at the present time. In TRU1, however, the presently measured 4He groundwater value exceeds that of the adjacent porewater by an order of magnitude, contrasting the observed curvature of the 4He porewater profile (see discussion below). The 3He/4He ratios of the porewater in both boreholes match the 3He/4He ratios of the Keuper groundwater. In the other four boreholes, hydraulic transmissivities in the Klettgau Formation were too low to obtain groundwater samples, and the 4He porewater profiles show no apparent anomalies. The 3He/4He profiles of the BOZ1 and MAR1 porewaters, however, do show clearly higher values for samples measured from the Klettgau Formation, indicating some exchange with an isotopically different groundwater, consistent with the observations of the δ2H and δ18O profiles (Section 4.6.2). In NL, while a singular slightly higher porewater 3He/4He value was obtained from the Keuper in the BUL1 borehole, the limited spatial resolution of these profiles in both boreholes does not allow a conclusive interpretation.
Towards the top, all helium profiles (except MAR1) extend to the depth of the Malm (NL, ZNO) or Hauptrogenstein (JO) aquifer, and groundwater samples could be obtained in all investigated boreholes except BOZ1. The small anomalies in the porewater 4He value of BOZ2 towards the lower groundwater value, and the lack of any excursion in the 3He/4He porewater profile, suggests an exchange between the porewater and – in terms of helium – an isotopically similar but lower concentration groundwater. No excursion is seen in BOZ1. In NL and ZNO, the STA3 and TRU1 porewaters tend to higher 4He concentrations and lower 3He/4He values with shallower depth. In both boreholes, the 3He/4He value of the Malm groundwater is similar (~ 1 × 10-7) and matches the porewater 3He/4He values at the same depth. This congruence, however, contrasts with the mismatch between the 4He concentrations of the groundwater and adjacent porewater, with the groundwater concentration being roughly 10 times lower in STA3 and 10 times higher in TRU1 compared to the porewater. These observations could indicate a past exchange with a groundwater having a 3He/4He signature similar to the present-day groundwater but a different 4He concentration (higher in STA3 and lower in TRU1). As the 3He/4He ratio of the groundwater is lower than that of the porewater in the central aquitard, the helium of the groundwater must originate from a source with a more radiogenic (lower 3He/4He ratio) signature than the sampled aquitard lithologies (see discussion below). BUL1 shows somewhat divergent profile shapes, with porewater 4He concentrations being elevated in the lower Malm, but then returning to values near identical to the groundwater in the aquifer. The 3He/4He ratio of the present-day Malm groundwater in BUL1 is also lower than that of the central aquitard porewater, again indicating an external source for (at least a part of) the helium in the groundwater. The porewater 3He/4He ratios, however, increase from the central aquitard towards the groundwater, suggesting past exchange with a groundwater characterised by a higher 3He/4He ratio. A former contact with a groundwater with an admixture of air-saturated water (3He/4He of 1.38 × 10-6; Kipfer et al. 2002) or with helium derived from a subcontinental mantle-like reservoir (3He/4He of 6.5 – 8.5 × 10‑6, Ballentine et al. 2002 and references therein) would allow this observed 3He/4He increase to be explained.
At depth, five of the porewater helium profiles intersect the Muschelkalk aquifer, from which groundwater with roughly one (JO), two (NL) and three (ZNO) orders of magnitude lower 4He concentration compared to the respective central aquitard porewater was encountered. These five contiguous 4He porewater profiles show steep gradients towards very low values close to the groundwater depth, generally the lowest observed porewater 4He concentrations in the profiles. These gradients mimic the δ2H profiles (Section 4.6.2) and are also similarly observable in the anion tracer profiles (Section 4.6.3). They are interpreted to be the result of either very low porosity (Mazurek et al. 2023a) and diffusivity in the anhydrite-rich lithologies in that interval and/or of recent changes in the groundwater composition. The corresponding porewater 3He/4He profiles from the central aquitard to the Muschelkalk groundwater are mostly flat and roughly match the 3He/4He ratio of the groundwater in NL and ZNO. In JO, the basal part of the porewater 3He/4He profiles trend to lower values towards the Muschelkalk, again matching the groundwater signature in the aquifer.
Discrepancy between 4He concentrations in the Keuper and Malm groundwater and adjacent porewater in the TRU1 borehole
In this borehole, the sampled 4He concentration in the Malm and Keuper groundwater exceeds that of the adjacent porewater samples by roughly a factor of 10, while the corresponding 3He/4He signatures are nearly identical. With the groundwater 4He concentration also exceeding the values of the central aquitard porewater, it cannot be explained by diffusive flux from the aquitard into the groundwater as the sole source but requires a helium source external to the investigated aquitard/aquifer sequence. A pervasive helium flux from depth across the Mesozoic sediment stack can be excluded because it would contradict the observed low 4He concentrations in the aquitard porewaters and – at least recently – be restricted by the Muschelkalk groundwater with its very low 4He concentration. In consequence, this external helium must be supplied to the different groundwater levels in a manner that largely bypasses the aquitard lithologies between them. Based on the groundwater chemistry, gas and isotope compositions of the groundwaters in the Malm, Keuper and Muschelkalk aquifers, a possible pathway along a steep tectonic fault crosscutting the entire sediment stack, such as the NW-SE-trending Neuhausen Fault situated at distance of less than 1 km to the northeast was suggested (Rufer et al. 2024) and references therein). In such a scenario, potential source lithologies for such a radiogenic (low 3He/4He) helium component could be the units of the Permo-Carboniferous Trough and the crystalline basement.
Enrichment of the different groundwaters by such a helium component migrating upwards along a fault would, to a large extent, be controlled by the relation between the ascending helium flux and the groundwater flux. Groundwaters with low flow rates and high residence times would become more enriched in helium and have their 3He/4He signature lowered compared to faster flowing aquifers with lower residence times. With the groundwater residence times gradually increasing from the Muschelkalk to the Keuper and Malm aquifers in ZNO, the observed groundwater concentrations and 3He/4He signatures qualitatively fit with this conceptual enrichment model.
In-situ production of 4He, minimum accumulation times and diffusion timescales of the observed porewater helium system
The calculated in-situ production of radiogenic 4He shows broadly comparable ranges over most of the stratigraphic stack (Rufer et al. 2024), with notably lower production rates (compared to the more clay-mineral-rich central aquitard) in the evaporitic strata of the Bänkerjoch and the Zeglingen Formation and in the dolostones and limestones of the Schinznach Formation and the Malm. In the carbonate dominated lithologies, these lower 4He production rates are compensated by higher release coefficients of helium from the rock matrix to the porewater. As a result, 4He accumulation rates in e.g. the limestones of the «Herrenwis Unit» in NL are similar to those in more clay-rich units (Rufer et al. 2024). Over all three regions, both, 4He in-situ production and accumulation rates of specific lithotypes are very comparable between boreholes and correspond well to established data such as e.g. from the BEN borehole (Rufer & Waber 2015) or the compilation given in Waber & Traber (2022).
Minimum necessary accumulation times for the presently observed 4He in the porewater over the stratigraphic column clearly indicate that in-situ produced helium is diffusively redistributed along the concentration gradient, generally from the units of the Lias – Dogger via the over- and underlying units into the bounding groundwaters with significantly lower 4He concentrations (at least in the past for Malm and Keuper in TRU1). As such, the porewater of all these aquifer/ aquitard units must be considered an open system with respect to diffusive transport of dissolved gas.
Based on calculated diffusion timescales, helium would require at least 1.8 Myr to diffuse from the centre of the Opalinus Clay to the under- or overlying aquifers (assuming an aquitard thickness of 300 m). These values are much shorter than the minimum 4He porewater accumulation times in the Opalinus Clay (~ 115 Myr in JO, ~ 149 Myr in NL and ZNO; Rufer et al. 2024). This, together with the observed flat 4He and 3He/4He porewater profiles in the clay-mineral-rich central sections, implies that the porewater helium inventory across the Lias – Dogger aquitard is the result of a long-term evolution of the porewater system driven by He production and diffusion towards the boundaries.
Fig. 4‑126:Profiles of 4He concentrations and 3He/4He ratios in porewater (pw) and groundwater (gw) for the BOZ2 and BOZ1 (JO), STA3 and BUL1 (NL) and MAR1 and TRU1 (ZNO) boreholes
For the groundwater, open rectangles show packer intervals, closed rectangles most likely inflow zones. DAO: Dogger Group above Opalinus Clay.