Vertical and lateral distribution of helium in porewater of the Mesozoic sedimentary sequence of northern Switzerland – A comparative investigation across multiple boreholes

Abstract

Helium concentrations and 3He/4He ratios were measured on porewater extracted from the low-permeability rock matrix and groundwater in bounding aquifers in Northern Switzerland. The samples were collected from six deep boreholes, which were drilled across the Tertiary Molasse and the underlying Mesozoic sediments at three different study areas within the context of Nagra's deep drilling programme for the underground disposal of radioactive waste. Porewater helium data obtained from each borehole at high spatial resolution describe continuous profiles over multiple aquifer-aquitard intervals across the Jurassic to Triassic sediment sequence. Given the lateral distribution of the investigated areas (∼20 km apart) and having two boreholes in each area (a few km apart), this allows to investigate both vertical and lateral changes of He in the porewater and groundwater system at high resolution and over large scales.In the about 300–400 m thick Dogger–Lias aquitard sequence including the Opalinus Clay, porewater He concentrations are nearly identical and have very similar 3He/4He signatures in all boreholes. This suggests a common history in the development of these systems despite the boreholes being located in three study areas with different hydrogeological setting and tectonic history since their last burial and diagenesis. Proximal to the groundwaters, He profiles show that the porewater system is controlled by diffusive redistribution of in-situ produced He from high concentrations in the aquitards towards lower concentrations in the bounding groundwaters. At some locations, the influence of evaporitic strata with very low diffusivity acting as transport barriers becomes obvious. Close to the aquitards, differences between the profile shapes of helium and other conservative tracers such as δ2H in the different boreholes and areas indicate different boundary conditions for the different tracers over the more recent geological past, as well as, for some boreholes, the potential contribution of helium from the groundwater to the porewater in the nearby low-permeability rock matrix.In one study area, cross-formation transport of helium could be identified close to a deep reaching tectonic structure related to the Hegau-Lake Constance fault system. This basement derived gas is subsequently laterally distributed via groundwater in the different aquifers and influences the porewater of the adjacent aquitard formations, as evidenced by the helium concentrations and 3He/4He ratios in the groundwater and porewater in the two boreholes of this siting area.