Characterization of phosphorus (P) enriched solids was undertaken in the sediments below four mature septic system infiltration beds, where previous monitoring of phosphate (PO 4 ) concentrations in the groundwater had indicated that substantial retention of P was occurring in the vadose zone. At each site, zones of sediment P enrichment were identified by an acid extraction procedure. Acid extractable sediment P concentrations were found to be 2-5 times higher than background values, within narrow discrete zones generally 10-30 cm in thickness, located within one meter of the infiltration pipes. Back scattered electron images of the P enriched zones indicated that the P solids occurred as distinct authigenic grains (up to 300 μm diameter) and as grain coatings. Microprobe analyses indicated predominantly Fe-P in calcareous sediments (Cambridge and Langton) and Al-Fe-P in non-calcareous sediments (Muskoka and Harp Lake). Porewater analyses indicated that the zones of P accumulation were closely associated with zones of redox change characterized by the conversion of effluent NH 4 + to NO 3 - . The data suggests that a substantial amount of the septic derived P is being attenuated by mineral precipitation reactions that occur rapidly after the effluent encounters subsurface sediments. Reductive dissolution of ferric (oxy)hydroxide minerals as a consequence of reducing environments near the infiltrations pipes, the release of Fe 2 + in solution and subsequent conversion of Fe 2 + to Fe 3 + may promote the precipitation of ferric or ferrosoferric PO 4 minerals. In sediments with limited buffering capacity (calcite deficient), the decrease in pH resulting from effluent oxidation may cause Al (oxy)hydroxide dissolution and subsequent precipitation of Al-P rich phases. These precipitation reactions appear to have the capacity to immobilize a substantial amount of septic derived P (25-99% at these sites) for a considerable period of time.