Shallow Gas Diffusion Experiments through a Phreatic Fractured Aquifer

We conducted gas diffusion experiments in 2018 in a granite quarry in Carroll, New Hampshire to study gas migration through fractured bedrock. The goal of this study is to understand the influence of groundwater on the migration of radionuclides through the subsurface. Wells were drilled in 2016 to depths of ~12 m and the explosive charges were detonated in the wells to generate fracture networks that mimic the fractures that would be created during an underground nuclear explosion. In July of 2018, hydraulic permeabilities were characterized using aquifer testing including pump tests and slug tests. One well was tested both before and after detonation, and a 46-fold increase in permeability was found from 9.01x10-12 to 4.13x10-10 m2. Two tracer gas injections were conducted into the sealed explosive cavities located below the water table (one in September 8 and the other on October 31) using a mixture of tracer gases (50% SF6 and 50% Xe) to compare the transport of the gases through the explosion-generated fracture network. We found that both gases had comparable breakthrough times. The experiment in September showed diurnal variations in gas concentration at the surface which were well correlated to changes in atmospheric temperature (which varied between 3°C and 38°C). The temperature variations were less significant during the October experiment (between -2°C and 15°C). During this period, the gas migration was more strongly correlated to atmospheric pressure variations than temperatures. Breakthrough was consistent with barometric pressure driven transport. Tracer gas is released from aqueous storage by an increase in temperature which may cause an increase in observed gas concentration at the ground surface. Pressure decreases accompanied by temperature increases are favorable for post-detonation Xe detection. The experimental data will be used to help constrain a PFLOTRAN model to compare gas transport in saturated versus unsaturated conditions.