Modeling to Predict the Salt Transport between a Scale Model Estuary and the Unconfined Aquifer with Groundwater Flow

Abstract

In many field studies, variation in fluid density may have important effects on controlling groundwater flow and solute transport processes. These studies include: Indian River Lagoon (IRL), a coastal estuary, located on the east coast of Florida, saline disposal basins, radioactive waste disposal and contaminant spills. In the environment, where the fresh groundwater region is in direct contact with the saline seawater region in coastal aquifers, dynamic equilibrium normally exists between these two regions, but there are numerous human and environmental factors that can adversely impact the equilibrium and lead to severe saltwater contamination of the freshwater region. Intrusion of saline water is the common groundwater pollution problem. The problem can occur due to seepage of saline wastes from the surface, invasion of seawater in coastal aquifers and upward movement of geologic origin saline waters in other aquifers (Gupta, 2017). Variations in concentration and/or temperature can play an important role in the transport of solutes in such groundwater systems. This study has five key objectives: (1) to explore how the nature of saline surface water changes as the groundwater flow rate changes and the subsequent effect on the salt transportation through the porous media, (2) to analyze Bahkmat and Elrick’s (1970) equation to predict the source depletion curve with existence of groundwater and modify the equation to include groundwater flow, (3) to explore how the nature of a breakthrough curve changes as the groundwater flow rate changes, (4) to examine the feasibility of using the 2- D SEAWAT model to predict the source depletion and breakthrough curves under different groundwater flow rates, and (5) to study the movement of salt into the aquifer under time varying recharge scenarios. Ten experiments were carried out in a fully saturated, homogeneous sand column. Measurements were taken at the source and five ports distributed vertically along the column, located at fairly uniform distances below the source. Results for the first eight experiments show that an increase in the groundwater flow rate leads to an increase in the rate of decrease in the source salt concentration and breakthrough curves with lower peak concentrations and earlier peak breakthrough curves concentration with time. The main transport mechanism was simply molecular diffusion because the salt concentration at the lower ports would never get higher than that at the higher ports. The Bachmat and Elrick (1970) equations are applicable for a period of time but the Modified Bachmat equations are applicable for a longer period of time than the Bachmat and Elrick (1970) equations. Detailed numerical experiments were completed using the MODFLOW-family computer Code SEAWAT to study the effect of changing the groundwater flow rate on salt migration in porous media. Different methods were used to compare the measured and model-predicted values which included visual comparison and statistical analysis. The visual and statistical comparisons were made by comparing model-predicted and measured normalized salt concentration at the source and at the five ports locations. By visual comparisons, the model was very good in predicting the source. By statistical comparison, the model was very good in predicting the source and at the ports except for ports 1 and 2 where the NSE Index values were out of the acceptable level.