Water balance and long-term inflow estimates from <i>in situ</i> sampling of the HE-E heater test

The establishment of robust water mass balances is important for the characterization and understanding of bentonite barrier systems. This paper presents a water balance and long-term inflow estimate based on borehole sampling of the HE-E heater experiment. The HE-E heater experiment has been running for over 14 years at the Mont Terri Underground Research Laboratory in Switzerland. The experiment is in a 10-m long section of a micro-tunnel in the shaly facies of the Opalinus Clay and includes two 4-m long heater sections. Heater 1 is in a sand/bentonite (65/35) buffer and Heater 2 is in a granular bentonite buffer. This research uses results from the recent sampling of the granular buffer material (GBM) with a telescoped drilling procedure performed without interruption of the running of the experiment. The measured water content and dry density have been combined with long-term monitoring data to estimate the water mass and total inflow to the GBM buffer section since emplacement. The inflow estimates are then compared with modelling predictions and the measured pre-emplacement inflow under open ventilated conditions. The results demonstrate that very little water has migrated into the GBM from the Opalinus Clay and that full saturation of HE-E is likely to take many decades.