https://h2ogeo.upc.edu/es/
Groundwater Hydrogeology and geochemistry webinar.

Autor: Josep M. Soler (CSIC)
Grimsel Test Site – LongTerm Diffusion (GTSLTD). Radionuclide transport experiments in granite with and without fractures.

Abstract:
A field diffusion experiment in unfractured granitic rock (monopole 2) was performed at the Grimsel Test Site (Switzerland). Grimsel groundwater containing several tracers (3H as HTO, 22Na+, 134Cs+, 133Ba2+, 36Cl) was continuously circulated through a packedoff borehole and the decrease in tracer concentrations in the liquid phase was monitored for a period of 1266 days (05/03/2014 – 22/08/2017). Additionally, tracer breakthrough was monitored in an observation borehole a few cm away.
Core samples from new boreholes were collected immediately after the end of the experiment, allowing the measurement of tracer distributions in the rock. The observed patterns for the nonsorbing tracers (HTO, 36Cl) showed clear preferential transport directions, consistent with advective flow towards the gallery from which the boreholes were drilled. 3D modeling of the experiment can explain the measured concentrations in the boreholes and in the rock. Tracer transport for the conservative tracers (HTO, 36Cl) is affected by both diffusion and advection through the granitic rock matrix. Also, in situ accessible porosities (about 0.0014) are smaller than those measured in rock samples (about 0.009), pointing to unloading and destressing of the rock samples after drilling. The effect of advection for weakly sorbing tracers (22Na+) is only minor, and it is practically negligible for the strongly sorbing tracers (134Cs+, 133Ba2+).
A new experiment involving groundwater flow along a fracture between two boreholes together with matrix diffusion is currently underway. Initial results (breakthrough curves), corresponding to a pulse injection of HTO, uranine and 22Na+, point to the effects of heterogeneous advection (multiple fractures or channels) and matrix diffusion.