NUTRIENT TRANSFORMATIONS IN A NATURAL WETLAND RECEIVING SEWAGE EFFLUENT AND THE IMPLICATIONS FOR WASTE TREATMENT

The processes influencing nutrient (phosphorus and nitrogen) renovatio n in a natural wetland which had received oxidation pond effluent for twelve years were studied, and compared with current literature percep tions. Mass transport studies showed that 30-70% of the influent P was removed from the water column, which was much greater than published values suggest could be predicted for this highly loaded (approximatel y 34 g p m-2 y-1) system. Sediment traps studies showed that depositio n of particulate P immediately downstream of confluences with arms of the wetland not impacted by sewage effluent (natural wetlands) was the dominant cause of P removal. Separation of the deposited-P into chemi cally definable fractions along with studies on the water chemistry, s uggested that P deposition was associated with iron-organic complexes contributed dominantly from the natural wetlands. Considerable spatial and temporal heterogeneity was also demonstrated for nitrogen transfo rmations. During summer most of the influent-N was in nitrate form whi ch was all transformed during passage through the wetland. Isotope (N- 15) dilution studies indicated that approximately 60-70% was denitrifi ed, 25-35% converted to ammonium (dissimilatory reduction), and 5-10% assimilated. For most of the year, however, influent N was mainly in r educed forms. Despite this, significant quantities of nitrate were exp orted from the wetland especially at higher flows in spring-early summ er. Assays on the sediment showed that there was a marked increase in nitrification activity at the confluence with natural wetlands. It is suggested that marked changes in sediment redox potential at these con fluence sites provide ideal conditions for nitrification of sorbed amm onium which is subsequently flushed from the system in 'flood events'. The distribution and type of nutrient processing observed in this wet land are attributable to its configuration. The implication of these r esults to the sustainability of nutrient renovation in wetland treatme nt systems is discussed.