Shallow Subsurface Characterization of Gas Transport in a Playa Wetland

Abstract

Controls on vadose-zone gas transport beneath and adjacent to a southern High Plains ephemeral lake (playa) were investigated. Under dry conditions, vertical gas permeability and tortuosity were enhanced by cracks and root tubules in the upper 2.5 m. Below this depth, the horizontal components of both permeability and tortuosity tensors were dominant. Both atmospheric pumping and pneumatic tests were used to determine the gas permeability tensor; whereas gas tracer tests were used to estimate the tortuosity tensor. The field data were elevated in a multilayer numerical model. The results suggest that gas movement can be dominated by both advection and diffusion, with vertical movement dominating in the shallow zone under dry conditions. After a large precipitation event, vertical gas permeability was reduced and large pressure differentials (as high as 1.5 kPa) were produced, creating driving forces for advective gas transport.