A method for remediating a hydrocarbon-contaminated region of a subterranean body of groundwater (10) to destroy or reduce the initial concentration levels of hydrocarbon contaminants. A plurality of mutually spaced wells (12, 14, 16, 18) are provided which intersect the groundwater (10) region. A treating flow of acetic acid is provided from one or more of the wells into the groundwater region, to establish acidic conditions. A turbulent flow of an aqueous solution of ferrous ion into the groundwater (10) region is generated for mixing with the acidified groundwater (10), thereby providing a catalyst for disassociation of hydrogen peroxide. A treating flow of hydrogen peroxide solution from one or more of the wells (12, 14, 16, 18) into the groundwater region is then provided, so that the hydrogen peroxide may undergo a Fenton-like reaction in the presence of the acidic conditions and ferrous ion to generate hydroxyl free radicals for oxidizing the contaminants. An apparatus system for practising the method is also disclosed.