FRACTIONATION OF PHOSPHORUS IN THE SEDIMENT OF A WETLAND AFTER A DECADE OF RECEIVING SEWAGE EFFLUENT

Sediment deposited in traps positioned along a pollution gradient in a wetland receiving sewage effluent was fractionated with respect to phosphorus (P) to understand the processes of P removal. Significant differences in P fractions were found between sites. Close to the point of effluent discharge, virtually all of the P deposited was in organic form; whereas downstream of a confluence with a wetland not impacted by the sewage, inorganic P constituted 50 to 80% of the total P. More than 80% of the inorganic P occurred in forms extractable by 0.1 M NaOH. This form of P was highly correlated (r2 almost-equal-to 0.5-0.8) with oxalate-extractable Fe. Natural wetlands unimpacted by sewage effluent were shown to be the main source of the Fe. Hydrologic conditions regulating the delivery of Fe to the sewage-impacted wetland were implicated as the main factors determining the relative concentrations of inorganic P within the impacted wetland system. While it is not clear whether adsorption or precipitation reactions were responsible for the P deposition, we conclude that the present rate of P removal is sustainable. Our results may explain why, in some cases, the P removal achieved in wastewater wetland systems is much greater than can be predicted solely from soil adsorption maxima.