DEVELOPING A TURBIDITY-SUSPENDED SEDIMENT RATING CURVE FOR BUCK CREEK, SPRINGFIELD, OHIO

Turbidity measurements are increasingly prevalent in water quality studies as a proxy for suspended sediment load, heavy metals, and chemical and microbial contamination. Optical turbidity probes are part of a telemetric high-frequency hydrologic and water quality monitoring network on a 9-km reach of Buck Creek in Springfield, Ohio, including four water quality stations, two gaging stations, and a weather station, the purpose of which is to monitor changes in water quality resulting from lowhead dam modifications. As part of this study, a turbidity-suspended sediment rating curve has been developed for one of the water quality stations.

Suspended sediment samples were collected adjacent a water quality station using an ISCO 3700 Flow Sampler during storm events, filtered using a Millipore funnel and Zefon 934-AH glass microfiber filters, and weighed to determine suspended sediment concentration. The relation between suspended sediment concentration (SSC), in mg/l, and turbidity (T) in nephalometric turbidity units (NTU) follows a power function SSC = 0.0018T^0.95 (R²=0.85). This curve indicates a direct positive relationship, as the turbidity increases, so does the suspended sediment.

Hydrographs of storm events in the Buck Creek watershed are double-peaked, representing an initial influx of stormwater from an urbanized area of the watershed, which is located nearer the outlet, and a secondary influx of water from a predominantly agricultural part of the watershed. Turbidity parallels this response and exhibits hysteresis. Turbidity representing agricultural input displays a clockwise hysteresis as expected, higher on the rising limb of the hydrograph and lower on the falling limb. Turbidity associated with the urban part of the hydrograph shows a counter-clockwise hysteresis which we attribute to delayed contributions from combined sewer overflows.