Prediction of hyporheic exchange and solute transport dynamics in a headwater tributary of the Illinois and Mississippi River systems

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0408744PackmanProject Summary: Water and solute exchange between streams and the surrounding subsurface(hyporheic exchange) has recently been recognized as a critical process in the cycling of many importantsubstances in watersheds. Hyporheic exchange has particularly been shown to influence nutrient andcarbon dynamics, and the fate and transport of contaminants in surface waters. These issues have risen tothe forefront in the Midwestern U.S. because substantial nitrate loads from agriculture and urban centerscontribute to chronic hypoxia in the Gulf of Mexico. Despite the clear link that exists between hydrologicprocesses in headwater streams, denitrification efficiency in removing stream nitrate, and nitrogen loadsreaching the Gulf, our current understanding of these processes in low-gradient, sand-bed streams islargely empirical. Most glaring is the lack of, fundamentally-based, predictive models for hyporheicexchange in sand-bed streams, which has prevented the transfer of experimental observations gained atone site to other sites, or even to the same site under different flow conditions.Our objective is to collect a definitive data set from a low-gradient, sand-bed stream and test theapplicability of several fundamental, process based models of hyporheic exchange that have provensuccessful in laboratory flumes. We seek to both advance our fundamental understanding of hyporheicexchange processes in real stream, but also to assess the improvements in prediction of solute transportthat are possible for a variety of flow and geomorphic conditions. The stream selected for intensive studyis Sugar Creek, a headwater stream in the agricultural region along the northern Illinois-Indiana border.Sugar Creek is representative of many low-gradient streams in agricultural headwater areas that aretributary to the Illinois and Mississippi River systems. We propose to conduct detailed tracer experimentsand related physical measurements that will be notable because they will resolve hyporheic flow paths,solute transport, and controlling processes with an unprecedented level of detail. In-stream soluteinjections will be undertaken in several seasons in a 3 to 4 kilometer long reach of Sugar Creek. Stream-tracerdata will be used to assess bulk transport integrated over 8 to 10 sub-reaches (50 300 m long)representing variable geomorphic conditions. Several of those sub-reaches will be thoroughlycharacterized in terms of their streambed topography and morphology, cross-channel flow variability,sediment characteristics, and subsurface movement of the solute tracer. This unprecedented level ofdetail will be made possible using state of the art measurement technologies. The resulting data sets willsupport the application of a suite of fundamental, predictive, process-based models of hyporheic exchangeand its effects on downstream solute transport. The models will be evaluated in terms of their ability topredict not only reach-averaged tracer concentration data, but also point estimates of interfacial hyporheicflux and porewater tracer concentrations. Based on fundamental theory, we will develop and testapproaches for upscaling rates of hyporheic exchange to predict solute transport in the entire study reachunder seasonally varying conditions. In sum, we will apply several modeling approaches of differingsophistication in order to 1) evaluate our current ability to predict hyporheic exchange in sand bed streamsfrom first principles, and 2) develop reasonable approaches for upscaling the computations so that solutetransport can be predicted with a specified level of uncertainty for longer stream reaches.Because this study is focused on fundamental processes that occur widely, this work will have verybroad scientific impacts and will be used to address multiple pressing societal concerns. Detailedunderstanding of the relationship between stream/sedimentary conditions and hyporheic exchange willfacilitate improved understanding of nutrient dynamics, carbon cycling, and releases from contaminatedsediments. Our goals for predictions (in terms of uncertainty and spatial resolution) are compatible withfuture applications in reach and basin-scale water-quality models. In particular, the choice of Sugar Creekas the intensive study site will provide critical hydrologic information to support complementary studieson nitrate fluxes and sedimentary denitrification in the headwaters of the Mississippi River basin. Theproject's broader impacts will also be increased by the synergistic activities of the PI's. Both PI's have astrong history of involvement in scientific organizations, and the project's themes will be reflected inactivities such as special sessions at major technical meetings. We will particularly seek to broaden thegeneral contribution of this work by encouraging interdisciplinary and international communication andinter-site comparisons. The project will also contribute considerably to human resource development,both through direct training of students and via the use of the field site for educational demonstrations.