SHALLOW SUBSURFACE CHARACTERIZATION OF GAS-TRANSPORT IN A PLAYA WETLAND

Controls on vadose-zone gas transport beneath and adjacent to a southe rn High Plains ephemeral lake (playa) were investigated. Under dry con ditions, vertical gas permeability and tortuosity were enhanced by cra cks and root tubules in the upper 2.5 m. Below this depth, the horizon tal components of both permeability and tortuosity tensors were domina nt. Both atmospheric pumping and pneumatic tests were used to determin e the gas permeability tensor; whereas gas tracer tests were used to e stimate the tortuosity tensor. The field data were elevated in a multi layer numerical model. The results suggest that gas movement can be do minated by both advection and diffusion, with vertical movement domina ting in the shallow zone under dry conditions. After a large precipita tion event, vertical gas permeability was reduced and large pressure d ifferentials (as high as 1.5 kPa) were produced, creating driving forc es for advective gas transport.