PHOSPHORUS CHARACTERIZATION IN SEDIMENTS IMPACTED BY SEPTIC EFFLUENT AT 4 SITES IN CENTRAL CANADA

Characterization of phosphorus (P) enriched solids was undertaken in t he sediments below four mature septic system infiltration beds, where previous monitoring of phosphate (PO4) concentrations in the groundwat er had indicated that substantial retention of P was occurring in the vadose zone. At each site, zones of sediment P enrichment were identif ied by an acid extraction procedure. Acid extractable sediment P conce ntrations were found to be 2-5 times higher than background values, wi thin narrow discrete zones generally 10-30 cm in thickness, located wi thin one meter of the infiltration pipes. Back scattered electron imag es of the P enriched zones indicated that the P solids occurred as dis tinct authigenic grains (up to 300 mu m diameter) and as grain coating s. Microprobe analyses indicated predominantly Fe-P in calcareous sedi ments (Cambridge and Langton) and AI-Fe-P in non-calcareous sediments (Muskoka and Harp Lake). Porewater analyses indicated that the zones o f P accumulation were closely associated with zones of redox change ch aracterized by the conversion of effluent NH4+ to NO3-. The data sugge sts that a substantial amount of the septic derived P is being attenua ted by mineral precipitation reactions that occur rapidly after the ef fluent encounters subsurface sediments. Reductive dissolution of ferri c (oxy)hydroxide minerals as a consequence of reducing environments ne ar the infiltrations pipes, the release of Fe2+ in solution and subseq uent conversion of Fe2+ to Fe3+ may promote the precipitation of ferri c or ferrosoferric PO4 minerals. In sediments with limited buffering c apacity (calcite deficient), the decrease in pH resulting from effluen t oxidation may cause Al (oxy)hydroxide dissolution and subsequent pre cipitation of AI-P rich phases. These precipitation reactions appear t o have the capacity to immobilize a substantial amount of septic deriv ed P (25-99% at these sites) for a considerable period of time. (C) 19 98 Published by Elsevier Science B.V. All rights reserved.