NUTRIENT CYCLING AND WATER QUALITY IN AN URBAN DRINKING WATER RESERVOIR

The eutrophication of a drinking water reservoir burdens a municipal economy in the short term, as costly algaecide treatments are applied to control productivity, and possibly the long term as well, as chemical treatments have to be carefully balanced with ecosystem health and public concerns. Uncovering the sources of excess nutrients driving eutrophication, either internal to the reservoir via bottom sediment reflux and/or external via watershed runoff, and eventually controlling these sources, is the objective of a research project focused on central Indiana watersheds serving the rapidly expanding greater Indianapolis area. Our research here highlights geochemical data from the Eagle Creek Reservoir, on the northwestern edge of Indianapolis in central Indiana. The Eagle Creek watershed area is 419.58 km2, 52% of which is used for agriculture; the reservoir has a water surface area of 5.46 km2, and a maximum depth of 16.46 m, allowing some regions to become thermally stratified, creating the potential for anoxic bottom waters during the summer and winter. Phosphorus (P), a primary limiting nutrient in lake productivity, has been historically and routinely added to croplands to increase yield. We examined 97 surface sediment samples from the reservoir, and found that they are generally high in P (0.3-4.5 mg/g) in relation to background soil levels (0.5-2 mg/g). The spatial distribution of P closely follows that of organic matter, displaying algal productivity concentrated in discrete areas of the reservoir. The deeper areas of the reservoir consistently exhibited elevated P concentrations, attributed both to concentration of light organics and fine particulates in deeper depositional basins, and lack of dilution from marginal clastic sedimentation. This deep concentration of P and organic matter is potentially significant during a thermal stratification event when hypolimnial suboxia may cause the dissolution of iron oxyhydroxide particulates in the sediments, allowing the reflux of some of the adsorbed P to fuel algal blooms internally. To test this internal loading hypothesis we will examine downcore profiles of P distribution, and detailed extraction geochemistry to determine the potential reactivity of P in the sediments.