Two-dimensional analytical and three-dimensional finite-element method modeling of the interactions between wetlands and groundwater

Simulating saturated/unsaturated flow between wetlands and the contiguous subsurface is complex, because it involves the modeling of wetland storage changes that in turn cause changes in pertinent hydrologic boundary conditions. Existing groundwater models are incapable of simulating the flow between a wetland and an aquifer, if small changes in wetland storage cause significant vertical and lateral movements of the wetland boundary. In addition to the simulation complexities introduced by a change in wetland geometry and storage, periodic and nonperiodic fluctuations of the wetland stage induce corresponding elevation changes in the phreatic surface of the contiguous aquifer. As these induced waves propagate through an aquifer, friction causes a loss of energy, which is manifested as spatially dampened potentiometric head perturbations along the inland direction. An analytical model was developed that describes subsurface flows around a wetland induced by any nonperiodic fluctuations in surface-water stage, such as a flood wave. Not considered, however, are any changes in wetland storage, or wetland geometry as a function of surface water stage. The analytical model was validated using a finite-difference numerical groundwater model that is based on the governing equation expressed in radial coordinates. Comparisons of analytical and numerical results show excellent agreement.; Two numerical models were specifically designed to simulate wetland-aquifer interactions. The first model is a saturated groundwater flow model that incorporates adaptive simulation technologies to permit real-time simulation of moving wetland boundaries and their associated local influence on the phreatic surface. This model is numerically efficient and uses deformable hexahedral finite elements to trace phreatic surface changes in real-time. The second model also uses deformable hexahedral finite elements; however, this model simulates variably saturated groundwater flow. For both models, the numerical elements deform as required to characterize the horizontal and vertical extent of the moving wetland boundary (which serves to define a location within the porous system, where the pressure is known). Simulation results from both models were applied to a field site where water was rapidly withdrawn from an isolated wetland to observe system response and evaluate the wetland aquifer hydraulic connection.