PHOSPHORUS AND METAL GEOCHEMISTRY OF BOTTOM SEDIMENTS IN EAGLE CREEK RESERVOIR, CENTRAL INDIANA

The eutrophication, or nutrient loading, of drinking water reservoirs is a critical issue as water resources are scarce and valuable. One result is nuisance algal blooms, which produce taste and odor issues that degrade water quality. Quantifying how the external and/or internal supply of phosphorus (P), an algal limiting nutrient, contributes to eutrophication is vital to reservoir management policies. Primary sources of P to a reservoir come from the natural weathering of apatite rich rocks, from cropland run-off, and wastewater drainage. Accumulation of P in lake sediments may increase the rate of eutrophication when anoxic bottom waters allow for the redissolution and subsequent redistribution of orthophosphate, a principal form of biologically available P. To test this hypothesis, geochemical analysis of 2 bottom sediment cores were conducted from the Eagle Creek Reservoir (ECR), a primary drinking water source for over 80,000 Indianapolis IN residents. The cores, 55 cm (water depth 7.2 m) and 109 cm (water depth 16.5 m) in length, consisted largely of clay. In the shorter core a sharp, pre-flood (1967) contact is evident at 48 cm. Refrigerated subsamples were taken within 24 hours of coring, at 1 cm intervals, freeze dried, and analyzed for detailed P and metal chemistry using a modified SEDEX method for P, and strong acid digestion. LOI ranged from 2-12 %. The four fractions of P isolated by the SEDEX method maintained consistent concentrations downcore. In the longer core as much as 78% of the total P is associated with iron (Fe) within the top 3 cm. Monitoring of the physical and chemical depth profiles in ECR showed that hypolimnetic anoxia occurred seasonally from April to October 2003. Sample dates in July through August showed hypolimnetic anoxia at a thermally stratified station 8.5 m deep. On July 2 and July 28, anoxia coincided with elevated [Total P] in the bottom waters, 2 to 4 times greater than epilimnetic water (avg. 74 ug/L), respectively. Given the potential for redissolution of Fe-bound P during seasonal thermal stratification and hypolimnetic anoxia, P-loading from sediment-flux could provide a significant amount of biologically available P to ECR.