HR: 0830h
AN: H41D-1027    [PDF]
TI: Reliability of Groundwater Flow Models of the Hyporheic Zone of Small Mountain Streams
AU: * Wondzell, S M
EM: swondzell@fs.fed.us
AF: US Forest Service, Pacific Northwest Research Station, Olympia Forestry Sciences Lab 3625 93rd Ave., S.W., Olympia, WA 98512 United States
AU: LaNier, J
EM: lanierju@science.oregonstate.edu
AF: Oregon State University, Department of Geosciences, Corvallis, OR 97331 United States
AU: Haggerty, R
EM: haggertr@geo.orst.edu
AF: Oregon State University, Department of Geosciences, Corvallis, OR 97331 United States
AU: Gooseff, M N
EM: gooseff@cnr.usu.edu
AF: Utah State University, Dept. of Aquatic, Watershed, and Earth Resources, Logan, UT 84322 United States
AB: Several recent studies have used numerical groundwater flow models to simulate exchange flows of stream water through the hyporheic zone. Physically-based groundwater flow models can provide quantitative predictions of the amount of exchange flow and the residence time distribution of that water in the subsurface - both of which help determine the effect of hyporheic exchange flows on water quality. However, reliability of these predictions have not been well validated. We ask the questions: 1) How reliable are hyporheic groundwater models in typical applications examining hyporheic exchange flows? and 2) How does the reliability change with increased data availability and model sophistication? We developed groundwater models of the hyporheic zone for two mountain streams in the HJ Andrews Experimental Forest, Oregon. The models are based on geomorphology as quantified by surveyed topography, with saturated hydraulic conductivity conditioned to in-situ measurements from slug tests and observed water levels in more than 25 wells per stream. For each stream, we developed several models using different methods to estimate two of the most uncertain parameters - hydraulic conductivity and the thickness of the saturated alluvium. We tested the reliability of each model in predicting in-well tracer breakthrough data from injections of NaCl. We found that, for the two mountain streams, increased model sophistication does not necessarily lead to improved reliability, since the model with homogeneous hydraulic conductivity functioned almost as well as the heterogeneous models. However, while general trends in solute breakthrough were correct in the model, hydraulic conductivity data from even 25 wells was insufficient to characterize detailed arrival times accurately without conditioning conductivity to concentration data itself. This suggests that geomorphic data may be sufficient to predict water fluxes through the subsurface and approximate travel times. However, for detailed analysis of solute transport pathways and breakthroughs, intensive sampling of the subsurface may be necessary.
DE: 1829 Groundwater hydrology
DE: 1860 Runoff and streamflow
SC: Hydrology [H]
MN: 2003 Fall Meeting