An integrated framework for modeling contaminant transport in conjunctive stream-aquifer systems.

摘要

Groundwater and streams are closely integrated in the hydrologic cycle in which they interact in a wide variety of physiographic and climatic landscapes. Consequently, the study of interactions between groundwater and streams has been an important issue for water resources management. In the past decade, the study of stream-aquifer systems has undergone substantial development, resulting in the creation of many models for simulation of such systems. Currently, however, none of these stream-aquifer models is capable of modeling complex stream-aquifer interactions that involve solute transport.; This dissertation addresses this problem by presenting an integrated framework for conjunctive stream-aquifer transport modeling that incorporates the coupling of new and state-of-the-art models encompassing flows and solute transport in streams, groundwater, and the interactions between these two water bodies. The conjunctive model's modular structure contains three well-documented and widely-used USGS models: MODFLOW handles the groundwater flow in the aquifer; DAFLOW accurately computes unsteady streamflow by means of the diffusive wave routing technique, as well as stream-aquifer exchange simulated as streambed leakage; and MOOD computes solute transport in the groundwater zone. In addition, an explicit finite difference package was developed for solute transport in streams, which can also handle the hyporheic exchange by incorporating features from current development in modeling one-dimensional transient storage. The quadratic upstream interpolation (QUICK) algorithm is employed for its accuracy in spatial differencing for the finite difference solution to the stream transport equations. To improve the accuracy and efficiency of the model, the adaptive stepsize control for the Runge-Kutta method (ASCRK) is incorporated as the quality control scheme for numerical solution for stream transport. Special considerations are given for tributary and diversion junctions between branches. Altogether, this conjunctive model is able to simulate solute transport in a riverine aquifer system with a complex stream network, which involves dynamic exchange between the stream water and groundwater, up to the regional scale.; In a special application of this conjunctive model, the realistic representation of streams, underlying aquifers, and the interaction between the two water bodies, as well as some of the model's favorable features, can all be used to overcome the shortcomings of current practice in transient storage modeling. As evidence of this model's capabilities, several hypothetical examples are presented.; In the end, potential improvements for the current version of this conjunctive model are explored, so the continually growing interest and demand for realistic, efficient, and accurate stream-aquifer modeling systems can be fulfilled. In particular, an adaptive mesh refinement method is proposed to replace the ASCRK method for one-dimensional stream transport and preliminary results show promising advances in model efficiency.