Phosphorus characterization in sediments impacted by septic effluent at four sites in central Canada

Characterization of phosphorus (P) enriched solids was undertaken in the sediments below four mature septic system infiltration beds, where previous monitoring of phosphate (PO4) 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 NH4+ to NO3−. 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 Fe2+ in solution and subsequent conversion of Fe2+ to Fe3+ may promote the precipitation of ferric or ferrosoferric PO4 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.