Identifying temporally and spatially changing boundary conditions at an aquifer - aquitard interface using helium in porewater

Helium concentrations and He-3/He-4 isotope ratios of porewater, groundwater and rock were measured on samples collected from a Jurassic sediment sequence at the Mont Terri underground rock laboratory (Northern Switzerland). Porewater He data of rock samples collected from borehole BDB-1 at high spatial resolution across a karstic limestone unit (Passwang Formation) into the underlying claystone sequence (Opalinus Clay, Staffelegg Formation) describe a continuous profile from the water-conducting zone in the limestone into the clay-rich rocks of low permeability. Concentrations of 4He, 3He and their parent nuclides in the rock allow calculating insitu production and accumulation terms. Since the time of sedimentation, 90%-97% of the in-situ produced He-4 has been released to the porewater. Today only 2.5% of the maximum possible accumulated He-4 is still retained in the porewater while the major part of in-situ produced He-4 was removed from the system presumably by porewater-groundwater exchange. The porewater He-4 concentrations show a diffusion profile from the aquitard towards the aquifer, reflecting a) a transient state between He-4 in-situ production and porewater-groundwater exchange, b) a transient state from previously higher He-4 concentrations in the porewater, and c) a spatially variable boundary in the karstic limestone unit. Evolutionary models of porewater He-4 concentration profiles in combination with constraints from independent chemical and isotopic tracers allow deciphering a complex palaeo-hydrogeological history of the system over about the last 30 ka. A local excursion from the general profile towards higher He-4 concentrations and He-3/He-4 ratio in a limestone layer in the Opalinus Clay cannot be further constrained in time based on the present sample frequency, but appears to represent a hydrogeological signal.