A system is installed in the subsurface to control the hydraulic gradient between the area within a containment system and the area outside the containment system, and provides an easy-to-maintain treatment system. Preferably, the treatment system includes a sufficiently sized conduit between the inside-area and the outside-area that is accessible via a manhole. This conduit can be filled with remedial material or other treatment equipment, the manhole-sized access providing a relatively easy means of providing, maintaining, and replacing this material and equipment. Because the conduit is appropriately sized, hydraulic pressures on either side of the open conduit can be expected to be substantially equal, thereby reducing the potential of leakage of contaminants caused by hydrostatic pressure. Gates and screens are provided on either side of the conduit, to facilitate flow control, with manhole accessways to facilitate maintenance of these gates and screens.