Pumping tests in a low permeability rock: Results and interpretation of a four-year long monitoring of water production flow rates in the Callovo-Oxfordian argillaceous rock

Hydraulic conductivity of the Callovo-Oxfordian argillaceous rock ranges between 10−14 and 10−12 m/s. In spite of these low values, long term ‘pumping tests’ covering a range of head differentials have been performed in several boreholes in the Andra Meuse/Haute-Marne Underground Research Laboratory (URL). Specifically designed experimental setups made it possible to monitor water production flow rates ranging from 0.5 to 50 mL/day, during more than 4 years. Long term pumping tests provided an alternative to the pressure test method for evaluating hydraulic conductivities at the −430 m and −505 m depth levels in the URL. The obtained values were close to 0.8 × 10−13 m/s and 1.3 × 10−13 m/s, respectively. The lowest flow rate measured under well-established hydraulic head boundary conditions was close to 2.5 mL/day. It corresponds to a calculated hydraulic gradient value less than 120 m/m. This result indicates that water can flow in the Callovo-Oxfordian with such a hydraulic gradient value.