CHARACTERIZATION OF P DISTRIBUTION IN SEDIMENT CORES FROM AN ALUM TREATED LAKE

Irondequoit Bay, a large embayment on the southern coast of Lake Ontario, has exhibited degraded conditions since the early 1900s. Considerable effort to reduce phosphorus loading into the Bay has occurred including removal of treated sewage effluent emptying into the bay watershed and alum treatment. Studies in the 1980's determined that internal P loading from sediment during periods of anoxia sustained eutrophic conditions even after external loading was reduced. Phosphorus cycling in lakes may be strongly influenced by the release of sediment bound P during periods of anoxia. Phosphorus has a strong affinity for sorption onto surfaces of Fe and Mn oxyhydroxides; therefore, P associated with these phases is subject to changes in sediment redox conditions.

Sediment cores were collected at three locations within the deep basin using a KB piston corer in early May 2003 prior to lake stratification. Cores range from 65 cm to 84 cm in length and exhibit layered structures. Cores were frozen prior to removal from corer sleeves, and sectioned into 5 or 10 cm intervals. Characterization of sediment P distribution provides information on its potential mobility. Practical characterization of P distribution may be viewed as operationally defined by the methods used for extraction. Total sediment P, Fe and Mn were determined using USEPA method SW846 3050B. Inorganic P constitutes the majority of the P in the sediment samples. Fractionation of the inorganic P was completed using a four-step procedure (Kuo, 1996). This procedure fractionates P into soluble and loosely bound P, amorphous Al and Fe phosphates, reductant soluble-P and residual Ca phosphates. Analyses of each extract were completed using ICP-AES. Total P ranges from 462 to 1846 mg/kg. Results of total analyses show no general trends for the entire data set. Within individual cores, weak correlations exist between P and the sum of oxyhydroxides forming metals Fe, Mn and Al. The core sample from the deepest portion of the bay shows a general decrease in Al, Fe and Mn with increasing depth. This same pattern does not hold for P concentrations. Fractionation of P shows varying values for each fraction with depth. Overall, at least 60% of all P is associated with reductant soluble or residual phases.