Author(s): Claudio I. Meier; Farhad Jazaei; Alba Argerich
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Abstract: A 45 m-long by 15 m-wide medial gravel bar was instrumented with 39 nests of PVC mini -piezometers (236 wells in total), in order to describe in high-resolution the upper boundary as well as the physical drivers of hyporheic exchange flow (HEF) under the bar. The bar is located on Bull Creek, a creek draining a 102 km2 karstic watershed in the Ozark Region of Southern Missouri, with a flashy hydrologic regime. The nests were located on a regular, rectangular grid, with separation distance of 5 m in the longitudinal direction, and 2.5 m in the transversal direction. Six mini-piezometers were located at each nest: one screened along its full length, in order to determine the position of the water table (phreatic surface), and five others screened at their end (over a 5 cm length), in which piezometric heads were measured at depths of 40,60,80,100, and 120 cm below the bar surface level. Part of the bar was also instrumented with similar piezometer nests but at a much higher resolution of 1 x 1 m2, with each well instrumented with temperature sensors (iButtons) measuring every 1 min (a total of more than 200 wells). Hot as well as cold water was injected in some selected wells, and the temperature records were used to track travel times between pairs of locations. Quasi-permanent head tests were conducted at these wells, to estimate the radial, horizontal saturated hydraulic conductivity of the alluvial sediments. From the two sets of measurements, gradients and Ksat on one hand, and temperature in the other, we obtained two independent estimates of the travel times under the bar. The water table under the bar - the upper boundary for HEF - is not planar, but instead shows steep gradients at the bar’s head. Similarly, hydraulic gradients are much steeper at the head of the bar, where strong downwelling occurs. We compare our different estimates of travel times, and discuss the spatial variability of Ksat.
Year: 2018