ASSESSMENT OF HYDRAULIC CONDUCTIVITY, HEAD DIFFERENTIAL, AND DARCY'S VELOCITY IN THE LAKEBED: NEW POTENTIOMANOMETER APPLICATIONS

Potentiomanometers are routinely used for delineating fluxes across the groundwater and surface water interface. Their previous use was limited to evaluating head differences between the hyporheic zone or shallow substrate and the surface water body. Darcy's velocity was often estimated by using hydraulic conductivity (K) that has been measured in an adjacent location. Due to subsurface heterogeneity, this procedure introduced uncertainty in the estimated Darcy's velocity. Alternatively, the potentionmanometer's screen could be retrieved and a new screen inserted in the same location. This created bias in results due to high-K skin and substrate disturbance.

A simple constant-head injection test (CHIT) for estimating hydraulic conductivity (Cardenas and Zlotnik, 2003) was refined and incorporated into the piezomanometer design of Winter et al. (1988). After measuring the surface and subsurface head differences using the piezomanometer, the manometer was replaced with a calibrated vessel while maintaining the piezometer in the same position. This prevented disturbance of the substrate. Discharge measurements from the CHIT were converted to K using the acting head and corresponding shape factor. Estimating Darcy's seepage velocity concluded the data interpretation. This technique is valid in a broad range of conditions, including applications in a relatively low-K lakebed mud at Alkali Lake in the Sandhills, Nebraska.