Megan M. Hill

"An Investigation of carbon, nitrogen, and sediment loading at Griffy Reservoir, Bloomington, IN"
BSES Senior Research 2004

It was hypothesized that since the creation of Griffy Reservoir in 1924, sediment and nutrient loading rates have increased due to increased development in the watershed. We investigated the rate of sediment loading and the distribution of grain sizes, total carbon, total organic carbon, and total nitrogen in the sediments of Griffy Reservoir, Bloomington, IN.

Three sediment cores and eight pairs of grab samples (an early spring set and a late summer set) were collected from the lake and creek flowing into the lake, respectively. The cores were dated using visual observations and radiometric dating techniques of Cs-137 and Pb-210. Weight percents of total organic carbon, total carbon, and total nitrogen were determined using elemental analyzers.

The data suggest that the sedimentation rate is greatest near the inflow into the lake (7.95mm/yr) and decrease with distance from the inflow to a minimum rate of 3.75mm/yr near the dam. Sediment grains appear to be normally distributed such that larger grains are preferentially deposited first, near the inflow, and small grains are preferentially deposited later, near the dam. The data show organic carbon makes up most of the total carbon in the system and that total organic carbon values range from 0.2 -3% in the cores and .06-6% in the grab samples. Nitrogen values ranged from 0.05-0.25% in the cores and 0.05-.45% in the grab samples. Both carbon and nitrogen showed decreasing concentrations with increasing depth in the cores. The pattern of decrease of nitrogen mimics that of carbon, and it is suggested that nitrogen buried in the sediments is organic nitrogen transported with organic carbon. Furthermore, nutrient concentrations were generally higher in the late summer samples than in the early spring samples suggesting a seasonal variation in the input of carbon and nitrogen into the system. Overall, the data suggest that the rate of sediment and nutrient loading into Griffy Reservoir has remained fairly constant over the past 80 years.