The accurate knowledge of water and chemical mass balances is the key to proper management of water resources in a watershed. After decades of field investigations and modeling, groundwater storage, fluxes between surface and subsurface reservoirs, and fluxes between land surface and atmosphere over a basin/watershed still remain highly uncertain. A breakthrough in characterization, monitoring, and modeling approaches is needed to resolve this issue. In this proposal, we propose a novel hydraulic tomography approach to characterize, monitor, and predict the hyporheic zone and beyond, using spatial-temporal variations in river stages and the corresponding groundwater responses near a river. This new technology allows three-dimensional imaging and determination of fluxes across the river/groundwater boundaries and characterization of the hyporheic and subsurface system in a manner that is presently not feasible. If this proposed proof-of-concept study is successful, the technology can ultimately be extended to basin/watershed scale problems. The goal of the study is to demonstrate that hydraulic tomography based on river stage fluctuations is a valid concept and a viable technology for detecting large-scale subsurface heterogeneity near the river. We intend to show that the concept and technology can accomplish its goal in unconfined aquifers that interact with rivers. Proposed tasks include: 1) the modification of existing hydrologic tomography inversion algorithm to accommodate temporally and spatially varying boundary conditions, which represent the temporally and spatially varying river stages; 2) the examination of the effects of temporal and spatial varying river stage on groundwater level in synthetic heterogeneous unconfined aquifers through numerical simulations; 3) the use of these simulated well hydrographs in the modified inversion algorithms to test its ability to detect heterogeneity within the hyporheic zone; and 4) the conduct of laboratory studies using two existing sandboxes with a synthetic heterogeneous unconfined aquifer. Results from the study should demonstrate that while river stages may change rapidly in time and space, the tomographic inversion is a valid and logical approach as long as the variation of the river stages is well characterized.
