WATERSHED CONTRIBUTION TO THREE CENTRAL INDIANA RESERVOIRS: CHARACTERIZATION OF SUSPENDED SEDIMENT AND DISSOLVED LOADS

Water quality issues and nuisance causing taste and odor compounds are among the most challenging problems facing the Indianapolis water supply. In order to resolve these issues, it is imperative to develop an understanding of drinking water quality as it relates to watershed dynamics. Therefore, a comprehensive watershed-scale research program was developed and began in the winter of 2003 on the three watersheds that most directly impact Indianapolis’ drinking water supply: Eagle Creek, Fall Creek, and Cicero Creek watersheds. Watershed drainage areas range from 420 km² to 567 km² and are dominated mostly by agriculture. Water and sediment samples have been and continue to be collected quarterly from tributaries in each watershed during non-event periods and following rainfall events, allowing for characterization of both base flow and storm flow (runoff) loadings from the watersheds. Samples were analyzed for a suite of chemical and biological parameters including nitrogen, phosphorus, and E. coli.

Watershed contributions of both dissolved and suspended components vary significantly in magnitude and composition on a seasonal basis and with respect to land use in sub-watersheds. Stream water discharge of contributing tributaries and sediment concentration suggest that some sub-watersheds contribute disproportionately higher sediment and chemical loads relative to their drainage area. The highest suspended and dissolved components were often in watersheds undergoing development. For example, Thorpe Creek sub-watershed has one of the smallest drainage areas (24.8 km²) within Fall Creek watershed yet it has about six times the averaged suspended sediment concentration value during winter base flow (6.39 g/m³). Changes in land use impact the sediment and dissolved loads in each sub-watershed. Although the change in land use in the last 15 years varies across the three watersheds, agriculture has decreased the most while grassland and urban development has increased. The monitoring of physical and chemical conditions in the watersheds throughout the year will provide critical information about seasonal loadings in the drinking water reservoirs to aid in the development of both reservoir and watershed management programs.