HR: 15:25h
AN: H23G-08    [Abstracts]
TI: A Site-Specific Analysis of Hyporheic Mass Transfer and Residence Times
AU: * Saenger, N
EM: nsaenger@stanford.edu
AF: Department of Civil and Environmental Engineering Stanford University, Terman Engineering Center 380 Panama Mall, Stanford, CA 94305-4020 United States
AU: Kitanidis, P K
EM: peterk@Stanford.EDU
AF: Department of Civil and Environmental Engineering Stanford University, Terman Engineering Center 380 Panama Mall, Stanford, CA 94305-4020 United States
AU: Street, R L
EM: street@stanford.edu
AF: Department of Civil and Environmental Engineering Stanford University, Terman Engineering Center 380 Panama Mall, Stanford, CA 94305-4020 United States
AB: Hyporheic exchange is an ecologically important process, controlling the nutrient supply in the upper river bed sediments and with this benthic habitat quality. To analyze the controlling parameters and their influence on mass transfer and residence times, hydraulic exchange at a riffle-pool-sequence in the River Lahn, Germany, was analyzed using HEC-RAS to simulate the surface water flow as a boundary for the subsurface flow and MODFLOW, MODPATH, and MT3DMS to reproduce the transport in the subsurface. Field studies provide the basic data for input to these models. Hydraulic heads within the interstices, solute transport, and residence times of surface water in the subsurface were simulated. To reproduce spatial and temporal changes in fluvial sediment properties, we determine sediment parameters for different surface water flow rates and distribute the parameters smoothly over the longitudinal profile of the river reach. The sediment parameters are calibrated to fit measured hydraulic heads and tracer throughflow curves from the pool-riffle-sequence. The results show that although the surface water flow plays an important role for the hyporheic exchange at the study site, the influence of the sediment properties on the exchange is larger. About the same percentage of surface water enters the hyporheic zone for each of the different flow conditions. Densely packed sediments reduce the exchange. Anisotropy increases the hyporheic exchange for low and medium surface water flow and has the opposite effect for high flow. Residence times were short at higher flow and long at low flow. Finally, we investigated the effect on the exchange processes of modifications of the river bed in a hypothetical case study. The effect of the local replacement of sediment by small cross-section sills was analyzed in comparison to the natural river bed conditions. In general, the presence of the sills increased the exchange and focused it around the sills.
DE: 1829 Groundwater hydrology
DE: 1831 Groundwater quality
DE: 1860 Runoff and streamflow
DE: 1871 Surface water quality
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting