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1. Research Questions and Goals

This study is investigating the relative contributions of heterogeneity and anisotropy in sediment permeability, and current-topography and channel morphology driven flow towards mass and energy transport along and across the interface between a river and its bed and banks. We proposed to accomplish these goals mainly through numerical modeling using geostatistically-generated permeability fields. Analog experiments will further support our findings.

2. Accomplishments During the Second Year

The results from year 1 have now been published in Water Resources Research with the Phd student supported by this project (Audrey Sawyer) as lead author. The paper was in the top 5 most downloaded papers in WRR for the week it became available on-line. The associated results have also been presented at two meetings- the annual Geological Society of America conference and the fall American Geophysical Union meeting. Audrey Sawyer was also the lead-author and presenter in these meetings.

We have conducted heat transport simulations using the permeability distribution we developed in year 1. The thermal patterns in heterogeneous cross-bedded sediment are little affected by the permeability structure of the sediment. We are therefore not pursuing further simulation work on this since the typical magnitude of heterogeneity for cross-bedded sediment is expected not to affect heat transport so much due to strong conduction/dispersion relative to advection of heat.

Our flume construction was delayed further delayed but it was finally delivered just this summer (June 2009). In its absence, we pursued alternative opportunities to use other flumes. My research group was fortunate to have had the opportunity to run experiments at the Outdoor Stream Laboratory (OSL) of the National Center for Earth-surface Dynamics at the St. Anthony Falls Laboratory of the University of Minnesota. The OSL is a full-scale but fully-controlled river and floodplain. Our group, the PI and 3 graduate students, installed more than 150 piezometers at the OSL for mapping the water table, permeability tests, and water sampling. We basically ran experiments that are similar to those originally proposed but at a larger scale.

The water table indicated flow through the pointbar with river water infiltrating at the upstream section of the pointbar and partly returning to the river along its downstream section.  We mapped the permeability structure of the pointbar aquifer and determined its control on exchange processes between the river and the pointbar aquifer by tracer tests. LiCl was released in the channel and water samples were collected in the piezometers for analysis with ICP-MS. Initial analyses indicated that we detected the Li peak as it traveled through the pointbar. Another interesting result from the experiments at the OSL is that we found that the permeability of the pointbar aquifer, which is heterogeneous to begin with, has increased possibly due to winnowing of fine-grained sediment out of the aquifer. This is the first time for this process to be documented. In addition to these observations, we also deployed sixty thermistors in the pointbar to monitor temperatures and to potentially use heat as a tracer of fluid flow through the pointbar aquifer.

We also pursued mapping streambed temperatures in the field. A student conducted fieldwork in New Mexico to map permeability and temperature fields at the same time. We found that current-topography interaction is a more dominant controlling factor for heat transport through the sediment under a pool-riffle-pool sequence than streambed heterogeneity.

Since we have refocused our efforts on real heterogeneous permeability fields at two scales now (meter scale for studies from year 1 and fieldwork from year 2) and channel-floodplain scale for the OSL, we will no longer pursue the portion of this study where we would generate synthetic permeability fields. Working with a variety of real permeability fields has been more telling and has provided the same or more insight compared to what we might get with synthetic fields. When this project was proposed, the PI was not aware of high-resolution permeability fields suitable for testing our hypotheses hence the idea to generate stochastic permeability fields to be used for hypothesis-testing. But in the course of further literature review, we found one real permeability field that was perfect for this project and generated another one from pictures using a technique Audrey Sawyer, a student supported by this project, developed.

3. Goals for the third year (extension) of grant funding

Our remaining goal for the last year of the project (on extension) is to analyze results from OSL experiments and from the field work in New Mexico. The OSL portion includes analysis of thermal patterns and analysis of Li transport during the tracer test. The New Mexico portion entails further calculation of fluid fluxes from the temperature data.

4. Impacts on the investigator and students

This starter grant is the PI’s 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. I have proposed to further use the permeability fields we developed for testing hypothesis regarding biogeochemical reactive transport in stream sediment. This proposal was submitted as a CAREER grant to the National Science Foundation. Leveraging what she learned from working on this project, Audrey Sawyer also submitted applications for student grants. She recently got notified that she will be receiving the prestigious Horton Research Grant from the American Geophysical Union to conduct experiments in our flume. The students support for this grant was distributed to three students for summer support (along with matching internal grants from my institution) who participated in various ways in the flume experiments at the Outdoor Stream Laboratory and in field work. All of these students will be writing up their results as manuscripts for publication.