Dynamics of Saltwater Intrusion Processes in Saturated Porous Media Systems

Abstract

The overarching goal of this dissertation is to contribute towards a better understanding of the dynamics of saltwater intrusion and the associated transport processes in coastal groundwater systems. As a part of this effort, we have also investigated the impacts of various climate-change impacted variables such as average sea level and regional recharge fluxes on saltwater intrusion processes. Climate change effects are expected to substantially raise the average sea level. It is widely assumed that sea-level rise would severely impact saltwater intrusion processes in coastal aquifers. In the first phase of this study we hypothesize that a natural mechanism, identified here as the ‘‘lifting process,’’ has the potential to mitigate, or in some cases completely reverse, the adverse intrusion effects induced by sea-level rise. A detailed numerical study using the MODFLOW-family computer code SEAWAT was completed to test the validity of this hypothesis in both confined and unconfined systems. Our conceptual simulation results show that if the ambient recharge remains constant, the sea-level rise will have no long-term impact (i.e., it will not affect the steady-state salt wedge) on confined aquifers. Our transient confined-flow simulations show that a self-reversal mechanism, which is driven by the lifting of the regional water table, would naturally drive the intruded saltwater wedge back to the original position. In unconfined systems, the lifting process would have a lesser influence due to changes in the value of effective transmissivity. A detailed sensitivity analysis was also completed to understand the sensitivity of this self-reversal effect to various aquifer parameters. The outcomes of the first phase of this research indicated that the changes in groundwater fluxes due variations in rainfall patterns is one of the major climate-change-induced hydrological variable that can impact saltwater intrusion in coastal aquifers. In the second phase of this study, we use a combination of laboratory experiments and numerical simulations to study the impacts of changes in various types of groundwater fluxes on saltwater intrusion dynamics. We have completed experiments in a laboratory-scale sand tank model to study the changes in two types of groundwater fluxes— areal-recharge flux and regional flux. The experimental results were modeled using the numerical code SEAWAT. The transient datasets collected in this experimental study are found to be useful benchmark data for testing numerical models that employ flux-type boundary conditions. Also, based on the experimental observations we hypothesized that when the fluxes are perturbed it would require relatively less time for a salt wedge to recede from an aquifer when compared to the time required for the wedge to advance into the aquifer. This rather counter-intuitive hypothesis implies that saltwater intrusion and receding processes are asymmetric and the time scales associated with these processes will be different. We use a combination of laboratory and numerical experiments to test this hypothesis and use the resulting dataset to explain the reason for the difference in salt wedge intrusion and recession time scales. In coastal aquifers, presence of a salt wedge divides the groundwater flow system into two distinct regions which includes a freshwater region above the wedge and saltwater region below the wedge. Typically, the freshwater transport processes occurring above a wedge are much faster than the saltwater transport processes occurring beneath the wedge. Recently, many modeling and laboratory studies have investigated the movement of salt wedges and the associated transport processes. Most of these transport studies, however, have focused on understanding the groundwater plume transport above a wedge. As per our knowledge, so far no one has completed controlled laboratory experiments to study the transport processes occurring beneath a saltwater wedge. In this study, we have completed contaminant transport experiments to map the dynamics of saltwater flow patterns beneath a wedge and relate it to the freshwater flow patterns present above the wedge. We used a novel experimental approach that employed variety dyed neutral density tracers to map the mixing and transport processes occurring above and below a salt wedge. The experimental datasets were simulated using the SEAWAT code. The model was then used to investigate contaminant transport scenarios occurring beneath a saltwater wedge in larger field-scale problems.