The Impact of Streambed Sediment Size on Hyporheic Temperature Profiles in a Low Gradient Third-Order Agricultural Stream

To develop an understanding of the impact of sediment size on temperature profiles within the hyporheic zone of an agricultural stream located in McLean County, Illinois, both statistical and numerical modeling techniques were utilized. The field data were collected by two temperature probe grids along two riffles, one featuring predominantly gravel (d₅₀ = 3.9 mm) and the other predominantly sand (d₅₀ = 0.94 mm). Each grid consisted of five wells centered in the stream's thalweg, with three wells placed parallel to stream flow (longitudinal profile), and three wells placed perpendicular to stream flow (longitudinal profile). Each well was equipped with four temperature loggers recording at 15 minute intervals over a 6 month period, positioned upon installation uniformly at 0.3, 0.6, 0.9, and 1.4 meters below the streambed surface. Two additional temperature loggers recorded surface water temperatures.

Statistical methods involved general summary statistics, and time series cross-correlation with prior smoothing through the use of a 24-hour averaging filter. Numerical modeling was completed using VS2DH software, a 2-D heat transport model developed by the USGS.

Initial findings indicate that analysis by statistical methods may provide a clearer mode of comparison between sites than numerical modeling methods. Numerical modeling attempts appear severely restricted by the use of a 2-D model, by low data resolution across each site, and by the likely presence of a high degree of subsurface heterogeneity.

Descriptive statistics reveal that the gravel site features a higher degree of hyporheic thermal heterogeneity, resulting from greatly varying degrees of stream-hyporheic connectivity. Stream-hyporheic connectivity at the sand site appears more consistent throughout, resulting in less thermal heterogeneities. Ultimately, our results suggest that physical heterogeneities such as a greater range in sediment size or a less even stream channel are reflected as thermal heterogeneities in the subsurface.