The present invention relates to a method for treating groundwater in situ in rock or soil. An elongate permeable upgradient zone and an elongate permeable downgradient zone, each in hydraulic communication with a permeable subsurface treatment zone and having a major axis parallel to a non-zero component of the general flow direction, are provided in the subsurface by any of a number of construction methods. The upgradient zone, downgradient zone, and treatment zone are situated within the subsurface medium and have permeabilities substantially greater than the adjacent subsurface medium's permeability. Groundwater is allowed to move from the subsurface medium adjacent to the upgradient zone into the upgradient zone, where the groundwater refracts and moves to a treatment zone. After being treated in the treatment zone by an in situ treatment process, such as a process employing air sparging, sorption or reaction with zero-valent iron, the groundwater moves into, through, and out of the downgradient zone into the subsurface medium adjacent to the downgradient zone. The method does not require pumping. A method for directing groundwater around a particular location to prevent contamination of the groundwater by a contaminant located at the particular location, to prevent migration of a contaminant located at the particular location, to reduce the flow velocity of groundwater in the particular location, or to increase the residence time in an in situ treatment center located downgradient from the particular location is also disclosed.