CONSTRAINING INFILTRATION RATE THROUGH AN AQUITARD BY MODELING THE GENERATION OF METHANE GAS POCKETS

A 2D multiphase flow model has been developed to simulate the production of biogenic methane “drift gas” pockets in an aquitard to put a geochemical constraint on the amount of groundwater recharging the aquifer. The aquitard is characterized by fine-grained glacial till matrix with sand and gravel lenses, where the “drift gas” accumulates. Noble gas concentrations from the underlying aquifer indicate the groundwater has been degassed at some point. It is believed that as total dissolved gas pressure increases, due to actively occurring methanogenesis within organic rich, buried paleosols, hydrostatic pressure is exceeded and methane and other dissolved gases are exsolved from solution. It is hypothesized by using the NUFT code the model will be able to constrain the amount of groundwater infiltrating through the aquitard by simulating the production of these “drift gas” pockets. An atmospheric neon source component is included in the model in order to simulate the noble gas loss observed in underlying aquifer. The model output will be calibrated to known gas composition (methane, nitrogen, and all noble gases) in the drift gas and associated dissolved groundwater concentrations, and to hydraulic head values. Parameters to be varied include recharge rate, methane production rate, and hydraulic conductivities of the till matrix and sand and gravel features. A preliminary model has shown that the majority of methane produced in the paleosol zone is transported downwards in its dissolved phase until it reaches the sand/gravel lenses, where it is then released into free gas phase. This is in agreement with radiocarbon data on drift gas samples that suggests the source of the methane is younger than the depth at which it is found. The drift gas produced is about 70% methane with air and a small amount of neon comprising the rest. Drift gas composition varies, but the majority of samples contain between 70 to 90 percent methane. Preliminary findings show that dissolved methane transport and exsolution are very dependent on a sensitive relationship between recharge and methane production rates. In addition to constraining recharge through the aquitards, this model could quantitatively assess methane production rates and time scales for production of drift gas pockets.