PIs: Aaron Packman (Northwestern University), Douglas Jerolmack (University of Pennsylvania), Jud Harvey (U.S. Geological Survey) Overview: Solute transport in rivers is a complex problem influenced by both hydrologic and geomorphic dynamics. It is important to be able to predict the interactions of hydrologic dynamics, geomorphic dynamics, and solute transport in order to improve assessment of long-term average biogeochemical processing rates in river systems. Stream-subsurface hydrologic interactions are modulated by surface morphology and strongly affect solute transport in both streams and pore waters, thereby controlling a very wide range of biogeochemical processes. Here we propose to obtain a unique new data set encompassing observations of both fluvial morphodynamics and solute transport in a sand-bed river subject to frequent high-flow and sediment-transport events. We also propose to develop new theory to allow scaling relationships previously observed in both geomorphology and surface-groundwater interactions to be incorporated into a model for long-term average solute transport in rivers. The model will be applied to the study site by incorporating observed statistics of stream flow variations and channel morphology, and will be tested using direct observations of both solute penetration into the subsurface and net downstream solute transport averaged over a range of time scales. Intellectual Merit: The intellectual merit of this work lies in advancing fundamental understanding of the linkage between stream hydrology, fluvial morphodynamics, surface-groundwater interactions, and solute transport in rivers. This is an exceedingly complex problem as it involves not only turbulent flow-boundary interactions, but also sediment transport and pore fluid flow in the highly heterogeneous near-stream environment. Here we will synthesize numerous recent observations and hypotheses developed separately in hydrology and geomorphology in order to ascertain how the linkage between flow dynamics and morphodynamics in rivers controls solute transport over various spatial and temporal scales. While it is now generally recognized that such synthesis needs to be done in order to assess the behavior of environmental systems in an integrated fashion, such integration has rarely been achieved in practice. By developing new theory that takes advantage of commonality in geomorphic and hydrologic scaling relationships, we enable explicit evaluation of the integrated process that control solute transport dynamics over a variety of spatial and temporal scales in river corridors. Broader Impacts: The proposed effort will contribute very broadly to environmental science and the associated management of water resources and aquatic ecosystems. Further, through a variety of synergistic activities we will also realize numerous broader impacts in human resource development and in the scientific community at large. The project results will provide significant, necessary capability to evaluate the migration of a wide range of important solutes in river systems. There is a critical need for tools that can be used to predict the linkage of morphological variations (e.g., land-use changes) and long-term average transport behavior under variable hydrologic forcing (natural flow variability and alternative climate change scenarios). Therefore the project results can be expected to not only contribute detailed understanding of river dynamics, but also to have very broad impacts in supporting analysis of the migration and processing of carbon, nutrients, contaminants, and a range of other important substances in river systems. Essentially, the project efforts provide a critical step towards scientifically based sustainable management of freshwater resources. We will further enhance these general contributions by supporting the broader scientific community working on these problems. We will archive the unique field data set that we will acquire, and make it publicly available to other theoreticians and modelers to facilitate broader development and testing of models for flow and transport in river corridors. We will also facilitate scientific discourse, and particularly synthesis of scientific results to address policy and management questions, by convening multiple, directed interdisciplinary sessions at major technical conferences, and by translating project results directly into the broader Hydrologic Synthesis activity currently sponsored by NSF. Finally, we will realize broad contributions to human resource development by training numerous young investigators through this work, by engaging in synergistic and collaborative international activity, and by using project results to further our ongoing efforts to encourage pre-college students, and particularly students from under-represented minorities, to enter careers in science
In this project, researchers from Northwestern University, the University of Pennsylvania, and the U.S. Geological Survey collaborated to investigate how the morphology and structure of river systems controls the motion of water between the river and the underlying subsurface sediments. This process is extremely important to river ecosystems and to the natural cycling of a variety of priority materials, including carbon, nutrients, and contaminants. We conducted experiments to measure water flow and the associated migration of dissolved solutes and suspended particles in a series of small streams in North Carolina and Virginia. We also performed experiments to observe similar processes under more controlled conditions in the Saint Anthony Falls Hydraulics laboratory, located at the University of Minnesota. The laboratory experimental work was done in partnership with the National Center for Earth-Surface Dynamics. From these experiments, we learned that the complex, fractal geometry of rivers plays an important role in river-groundwater interactions. Because rivers have repeated patterns in morphology over a wide range of scales (from a few centimeters to tens or hundreds of meters in the systems we studied), porewater and groundwater flow under and around rivers also occurs over a wide range of scales, and this causes the water to be transported and stored in the subsurface for a very wide range of times. This provides substantial opportunity for reaction of stream-borne materials delivered into the subsurface, but also means that flushing of the subsurface can be very slow. We developed a model to predict these effects, and used it to show that most assessments of rivers do not consider a sufficiently wide range of subsurface flows. We also observed how the transport of dissolved and suspended material in rivers varies with stream flow. Most studies only consider a narrow range of flow conditions, but flows in most river systems vary considerably over the course of a year, and occasional storms can produce large changes in river flow and sediments. We observed that floods can rapidly change streambed properties and also substantially change patterns of subsurface flow and associated transport of dissolved and suspended material. We also observed that fine organic carbon particles are readily trapped in streambeds during low flow conditions, but rapidly remobilized and exported during floods that erode streambed sediment. Based on this, we recommend collection of additional data to better understand how the rates and patterns of river-groundwater interaction vary in important types of rivers. Further, we conclude that river-groundwater interactions and river flow variations are both extremely important to the dynamics of suspended particles in rivers, and changes in flow and sediment erosion/deposition over time will control the processing of particulate organic carbon in rivers.
