Abstract: A subsurface water treatment system is provided which may be used to produce purified water using ambient subsurface water as a source fluid. The system includes one or more ultrafiltration membrane units which use ambient water as the source fluid and to produce therefrom an ultrafiltrate substantially free of solid particulates having a largest dimension greater than 0.1 microns. An electrochemical unit provides an antifoulant solution comprising hypohalous acid species. A backwash unit allows periodic cleaning and defouling of a non-producing ultrafiltration membrane unit. The system may also include a nanofiltration unit and a reverse osmosis membrane unit which may be used to produce purified water having any of a wide range desired total dissolved solids content. The novel systems and methods described may be used to stimulate the production of hydrocarbon fluids from a hydrocarbon reservoir.