Abstract: A method of in situ biodenitrification of a contaminated site comprising providing to a contaminated site a phosphorous source, namely a polyphosphate or a trimetaphosphate. The phosphorous source is provided to a saturated zone or an unsaturated zone and a carbon source is also provided, most preferably acetate. The invention is also of an in situ biodenitrification apparatus (FIG. 3) comprising at least one extraction well for extracting subsurface water from a contaminated site, a container for mixing nutrients into the extracted water, and an injection well for re-introducing the extracted water to the subsurface. Preferably, at least three extraction wells are placed approximately on a circle having the injection well as a center, and a phosphorous source is introduced to the container, preferably a polyphosphate or a trimetaphosphate.