DIEL CYCLES IN STREAM STAGE AND TURBIDITY IN A SMALL MIDWESTERN AGRICULTURAL STREAM: IMPLICATIONS FOR WATER QUALITY ASSESSMENT

Water quality assessment is typically based on spot sampling and does not account for natural temporal or spatial variability of stream conditions. The South Branch of Rice Creek, a small perennial agricultural stream flanked by wetlands in south-central Michigan, is currently on the EPA nonattainment list for suspended solids. Stream studies beginning in 2002 have indicated diel cycling of many hydrological parameters, including pH, stage, chlorophyll, specific conductance, water temperature, and turbidity (a proxy for suspended solids). In this study, continuous monitoring (15-minute intervals) of stream stage and turbidity and groundwater levels in adjacent wetlands reveal clear diel cycling which should be accounted for in water quality assessment.

Monitoring of stream turbidity using YSI optical sensors in the central and upper reaches of the South Branch shows synchronous diel fluctuations, with nighttime turbidity levels 2-3 times the daytime levels. Turbidity maxima (~ 10-30 NTU) occur between 4-6 a.m. and minima (~ 5-10 NTU) between 3-6 p.m. Turbidity cycles monitored continuously during flashy storm events recover quickly but show an increase in amplitude. Groundwater levels in wetland wells, monitored using Solinst pressure transducers corrected for barometric pressure, show gradual lowering of the water table during a period of no precipitation. Superimposed on the decline are diel cycles of drawdown (up to 8 cm) and recharge (up to 5 cm) with minima at 10 p.m. and maxima at 8 a.m. Presumably the wetland water table cycling is affecting stream discharge. Stream stage, also measured with Solinst pressure transducers, exhibits irregular diel cycles. Daily fluctuations (~ 1-3 cm) are asynchronous at upstream and downstream sites and appear to occur earlier upstream. Stage fluctuations at the downstream site are similar to the timing of groundwater fluctuations in the adjacent wetland.

Our preliminary hypothesis is that diel cycling in turbidity and stage may be a result of the combined effect of groundwater pumping driven by evapotranspiration in wetlands during the day, followed by groundwater sapping at night during the period of recharge. We believe the irregular timing of stage fluctuations is due to the stream's complex interaction with groundwater along the Southern Branch.