A near-surface collection manifold for extracting and/or venting sub-surface gas from beneath a geomembrane includes a plenum defining an enclosure with an upper portion. A conduit extends upwardly from the upper portion of the plenum, the conduit communicating with the interior volume of the plenum and has external threads for receiving a threaded nut thereon. The conduit is adapted and provided for extending through an aperture in the geomembrane for withdrawing sub-surface gas from within the interior volume of the plenum and through the geomembrane. A gasket having an opening formed therein is slipped over the conduit and above the geomembrane so that the geomembrane is sandwiched between the gasket and the upper portion of the plenum. A threaded nut is fitted to the conduit and above the gasket for securing the gasket against the geomembrane, thereby sealing the geomembrane to the upper portion of the plenum.