RELATIONSHIPS BETWEEN SEDIMENT-BOUND PHOSPHORUS AND PRIMARY PRODUCTIVITY IN STREAMS: NATURAL VARIABILITY AND IMPACTS FROM TREATED WASTEWATER

A year and a half long study is being conducted in Little Kickapoo Creek in McLean County, Illinois to determine the role of sediments in phosphorous (P) cycling in the creek. A wastewater treatment plant recently went on line which discharges into the creek, providing a new input of phosphorus into the system. This study quantifies how sediment-bound phosphorus changes with respect to this new influx and also how it varies seasonally, and is part of a larger watershed-scale investigation of the role of P cycling on biological productivity and diversity. Other data that is being collected includes P concentration in the water column and measurements of primary productivity on sediments.

Sediments samples are collected from two sites upstream and three sites downstream from the plant on a bi-monthly basis. Several rounds of background samples were collected at each site prior to the start of the plant. Sequential chemical extractions quantify the amount of phosphorus stored in four fractions; MgCl2 for weakly adsorbed P, NaOH for Fe/Al hydroxides, HCl for P co-precipitated with carbonates, and ashing for organic P.

The results to date indicate that most of the sediment-bound P resides in the Fe/Al hydroxide and carbonate fractions, with only a few percent in the weakly bound and organic fractions. P concentrations in the Fe/Al fraction increase during the winter through early spring and decrease from spring through early summer. We interpret this as P additions from winter storms, and then desorption and uptake as biological productivity increases in the spring. The carbonate fraction is highly variable probably due to the amount of carbonate in each sample collected. Prior to start up of the treatment plant, P concentrations in the water column were often nondetectable, indicating rapid uptake after desorption from the sediments. Since startup, P concentrations (Dissolved Reactive Phosphorus) in the water column have increased to as much as 3 mg/L, and may be contributing to the increased primary productivity at the downstream sites.