Numerical simulation of flow and chemical transport in integrated surface-subsurface hydrologic systems

Stream water quality is affected by baseflow contributions from groundwater, and by contributions from precipitation events. Defining the source and pathway of stream water inputs is a prerequisite to understanding the impact of contaminants originating in rainfall, from industry and agriculture, and from urban runoff Numerical models provide a useful tool in evaluating possible flowpaths and the timing and magnitude of stream inputs from various sources.; An integrated numerical model is developed and evaluated in this work. This numerical model considers the flow of water and transport of multiple solutes on the two-dimensional land surface and three-dimensional, dual continua subsurface. The numerical model is modular is tailored towards irregular geometries, and utilizes discretization and solution techniques. A new prism-based discretization is introduced and shown to be consistent with two and three-dimensional finite elements. An adaptive temporal-weighting scheme is presented which partitions the flow and transport equations into active and inactive zones.; Observed surface discharge volumes and timings are simulated with reasonable accuracy. The observed dynamic response is a nonlinear function of multiple parameters, affected by subsurface permeability, surface roughness, topography, and initial conditions.; Simulations of the transport of a conservative tracer introduced with rainfall indicate that processes affecting solute concentrations in the surface water are restricted to a relatively thin region adjacent to the land surface. While having little affect on the flow solution, subtleties in rainfall boundary condition assignment in the discrete equations impact predictions of tracer concentrations in discharge water and, therefore, also affect interpretations of water origin.; Application of the coupled surface-subsurface model to the transport of conservative tracer in the field-scale experiment re-enforces the conclusion that mixing processes occurring at the land surface interface dominate tracer concentrations in stream discharge. Successful simulation of coupled surface-subsurface transport depends on the accurate representation of the spatial and temporal variability of water exchange processes (i.e. advection) and diffusive-type processes associated with concentration differences between continua.; The field-scale simulations clarify the role of the capillary fringe on streamflow generation. The simulated response of the capillary fringe to rainfall suggests that increased subsurface head gradients do not cause significant groundwater seepage. Rather, infiltration rates along the stream axis are reduced, with runoff formed largely by excess rainfall over a dynamic contributing area. The corresponding transport simulations suggest that rainfall tracer dilution occurs largely by diffusive processes as water flows over the land surface to the stream. The simulations suggest that hydrograph separation theory is fundamentally flawed if diffusive modification of tracer concentrations in surface water is prevalent in nature. (Abstract shortened by UMI.)...