Mechanisms of Phosphorus Control in Urban Streams Receiving Sewage Effluent

Urban streams often are a major source of phosphorus (P) to rivers, primarily due to large inputs of sewage effluent. A good example of this is Chicago (Illinois, USA) area streams, which make up most of the flow of the upper Illinois River. Even though streams in this section of the Mississippi River basin are characteristic hard-water systems and exhibit high calcium and carbonate concentrations, the precipitation of Ca–P minerals is minimal and phosphate is not removed from the water column. The objective of this study was to determine the chemical mechanisms controlling P activity in Chicago area streams. Measurement of dissolved ion activities on filtered surface water samples demonstrated that an average of 79% of P in the study streams was dissolved and the remaining was particulate (<0.05 μm and >1.0 μm in diameter, respectively). Neither a P colloidal-size fraction nor a correlation between dissolved and particulate Fe and P was observed. Thermodynamic modeling and SEM-EDS analysis of particulate matter in filter residues indicated that dissolved P may adsorb and co-precipitate on the surface of calcite rather than precipitating in a pure Ca–P mineral phase. Although SEM-EDS results also suggested that P was adsorbed to silicate minerals, organic residues likely dominated the P-containing particulate fraction. Sediment extraction results indicated that organic P was one of two major P components in the stream bottom. The Fe-associated P fraction represented the largest sediment-P fraction, and with little association between Fe and P in the overlying water, dissolved inorganic P may have aided in the authigenic formation of an Fe–P sediment phase. Overall, results suggest that pH combined with Ca and Mg activity are the dominant chemical controls on P chemistry in this P enriched system.