46655-G8
Current-Topography Driven Exchange Processes between Water Columns and Heterogeneous Permeable Sediments
1. Research Questions and Goals: This study is investigating the relative contributions of heterogeneity and anisotropy in sediment permeability, and current-topography driven flow towards mass and energy transport along and across the sediment-water interface (SWI). We proposed to accomplish these goals mainly through numerical modeling using geostatistically-generated permeability fields. Flume experiments will help verify our findings. 2. Accomplishments During the First Year: We have run simulations of flow and transport through sediment with heterogeneous permeability fields and compared them with equivalent isotropic and anisotropic cases. However, instead of focusing on geostatistically-generated synthetic permeability fields, we used real mapped permeability fields. When this study was proposed, we were not aware of other detailed permeability distributions for the scales we are interested in. Further research turned up some relevant data. We also generated useable permeability fields via image analysis of pictures of cross-bedded sediment taken by our colleagues. The fields we used include a detailed map of the Massillon Sandstone and two developed based on image analysis of cross-bedded sediment deposited by the Brazos River in Texas. We have also conducted our own permeability measurements of modern cross-bedded sediment along the Brazos River using a portable syringe-based air mini-permeameter. Our simulations show that early-time behavior of solute exchange across the SWI is sensitive to heterogeneity in permeability for the Massillon Sandstone and for Brazos River deposits). Bimodal permeability fields, ie, silt and sand, have a stronger influence on transport than permeability fields that have an almost equally broad but continuous smooth distribution, ie, an assortment of grain sizes but mostly sand-sized. However, late-time transport and exchange of solutes converges to a similar power-law type residence time distribution indicating that late-time behavior is controlled by the boundary conditions rather than internal variations in the permeability field. The area of exchange is sensitive to the permeability tensor of equivalent homogeneous fields. Exchange zones are shallower when the principal axis of the permeability tensor is horizontal; they eventually deepen when the principal axis becomes more inclined with respect to the horizontal direction. However, exchange flux increases regardless of orientation of the tensor. It is therefore critical that the permeability tensor is considered completely since flow and transport in cross-bedded sediment are typically not dominantly aligned along the principal axes of the coordinate system (x and y). Our initial results are now being written up. The paper will be submitted to Water Resources Research. I expect at least two more papers to result from this work upon completion of our goals. 3. Goals for the second year of grant funding: Flume experiments- We faced delays with starting our flume experiments since construction of our flume has been delayed. However, construction is in full swing and we expect delivery by September 2008. Soon after this, we will conduct our flume experiments. Heat transport simulations- We will be conducting heat transport simulations using our flow simulations. Further sensitivity and geostatistical analysis- We have mostly achieved our goal of showing when heterogeneity is important by using two contrasting types of heterogeneity. However, a broader sensitivity analysis would provide deeper understanding. We will still attempt to generate synthetic random fields and use those in our simulations. 4. Impacts on the investigator and students: This starter grant is the PIs first external and competitive grant. It has provided much needed experience on managing research funding, making sure research goals are met, and supervision of research assistants. The grant has opened up new avenues of research. It became apparent that we would need to make our own observations on permeability fields. We are likely the first group to attempt this on unconsolidated fluvial sediment at this level of detail. We are using image analysis to translate digital pictures to permeability fields since it is practically impossible to map permeability fields of modern cross-bedded sediment at a scale and resolution suitable for our purposes. This capability allows us to quantitatively analyze many small-scale features. As I accomplish more of the goals I set out to do, I expect to not only find answers but unravel more questions.
