A method for remediating aquifers and groundwater contaminated, for example, by toxic halogenated organic compounds, certain inorganic compounds, and oxidized heavy metals and radionuclides, using the introduction of an innocuous oil, preferably an edible, food grade oil such as soybean oil, formulated into a microemulsion preferably by mixing with a natural food-grade emulsifier (such as lecithin) and water.