HR: 1340h
AN: H23F-1497    [Abstracts]
TI: Mixing of Groundwater and Surface Water Under Diurnal and Seasonal River Stage Cycles
AU: * Waichler, S R
EM: scott.waichler@pnl.gov
AF: Pacific Northwest National Laboratory, P.O. Box 999, MS K9-36, Richland, WA 99352 United States
AU: Yabusaki, S B
EM: yabusaki@pnl.gov
AF: Pacific Northwest National Laboratory, P.O. Box 999, MS K9-36, Richland, WA 99352 United States
AB: Water flow and tracer transport in a river-aquifer-vadose zone hydrologic system at the arid Hanford Site along the Columbia River, Washington, USA were investigated using a vertical, 2-D model. Field observations of hydraulic head and uranium contamination indicate that flow and transport are strongly linked to diurnal and seasonal river level fluctuations. The Subsurface Transport Over Multiple Phases (STOMP) simulator was used to model the system for a one-year period at hourly temporal resolution and 0.5 m spatial resolution. The mean regional gradient is toward the river, but river levels have a strong diurnal variation due to hydropower management, and the mean 24-hour change in river stage (0.48 m) was more than three times the regional head drop across the model domain. Large swings in simulated groundwater velocity magnitude and direction resulted from these imposed diurnal changes in river stage. Maximum pore velocities in the base case ranged from 4 m/d farthest from the river to more than 20 m/d next to the river during typical river-stage fluctuation. Groundwater velocities were typically not sustained for more than a few hours because of the short cycles of river level change. The high velocities and frequent fluctuations enhanced the intrusion and mixing of river water with groundwater, and thus controlled the overall size of the simulated mixing zone of aquifer and river water. When the boundary conditions were defined by daily or monthly averages instead of hourly values, the lack of large and frequent fluctuations drastically reduced the incursion of river water into the aquifer and limited the size of the zone where river water mixed with groundwater. Although the large, high-frequency fluctuations in river stage were essential for developing the mixing zone over the long-term, the short duration of the fluctuations resulted in little diurnal change in the size of the mixing zone. However, seasonal changes in average water level did lead to significant changes in the mixing zone size. The identified transport behaviors are thought to be important to uranium mobility, which is controlled by the variation in solution chemistry between the river and aquifer end members.
UR: http://www.pnl.gov/main/publications/external/technical_reports/PNNL-15125.pdf
DE: 1830 Groundwater/surface water interaction
DE: 1832 Groundwater transport
DE: 1847 Modeling
SC: Hydrology [H]
MN: Fall Meeting 2005