Degassing of deep groundwater in fractured rock around boreholes and drifts

Deep groundwater contains dissolved gases that may come out of solution if the water pressure is lowered. Water pressures are often decreased down to atmospheric pressure when water is withdrawn from deep boreholes and drifts in the bedrock, for example, in investigations regarding nuclear waste dis posal. Groundwater degassing may then contribute to the development of a lo cal, unsaturated zone around the borehole or drift, which may affect inflow and, as a consequence, the outcome of hydraulic and tracer tests in fractu red rock. Laboratory experiments with gas-saturated water in rock fracture replicas have demonstrated that degassing causes considerable hydraulic con ductivity reduction under certain conditions. Degassing has also been hypot hesized to be the cause of observed flow reductions in the field; however, supporting field experiments have so far been lacking. We report results th at constitute the first field support of the development of an unsaturated zone and, as a consequence, hydraulic conductivity reduction due to groundw ater degassing around a borehole. The borehole tests were conducted approxi mately 450 m below the groundwater table at the Aspo Hard Rock Laboratory i n southeastern Sweden. No hydraulic conductivity reductions were observed a t gas contents of about 1%, whereas a 50% reduction in hydraulic conductivi ty was observed at a gas content of 13%. Formal hypothesis testing, based o n all available field and laboratory degassing tests, supports degassing as the actual cause of the observed hydraulic conductivity reduction at the h igher gas content. We also show analytically that hydraulic conductivity re ductions due to degassing may occur at much lower gas contents around drift s than around boreholes.