A permeable wall for the in situ treatment of groundwater flowing therethrough is formed by a plurality of hexagonal cells removably placed in a frame in watertight relation to the frame and each other. The frame is mounted in a gate between barriers or pilings in the flow path of the groundwater. Each cell is formed by a hexagonal shell of water impermeable material defining a central chamber opening therethrough and containing a permeable water treatment material. Sealing tongues project from a portion of the shell, and sealing grooves are defined in another portion. The tongues on one cell are received in sealing grooves on an adjacent cell when the cells are mounted in the frame. Removable panels are mounted on the frame to prevent water flow when the cells are being inserted or removed. The same construction can be utilized for other fluid-solid reactors.