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Hydrogeologic modeling of submarine groundwater discharge in the Gulf of Mexico near southeastern Louisiana Thompson, Craig E.


Submarine groundwater discharge (SGD) has been the subject of increasing amounts of research in the past decade. It is defined as the total mixture of seawater and fresh groundwater flowing out from the seabed, into the coastal water, through the underlying sediments. Since the early recognition of its ecological significance in the mid-1960's, numerous studies have been performed whose goal was its quantification. It has been studied using a variety of techniques. These include direct methods, geochemical tracers, and numerical models. A number of studies have incorporated multiple techniques with limited succes. These efforts have found that results obtained using geochemical tracers and direct methods are in general agreement; while those obtained through modelling methods vary to some degree. The goal of this study was to quantify the rate of offshore groundwater discharge in the coastal zone of southeastern Louisiana near the Mississippi delta. A previous study indicated that SGD may be important within the region, with the volume discharged reaching approximately 10% of the Mississippi River discharge. Numerical groundwater flow modelling and the geochemical tracer radium were used in this study to examine the interpretation of the previous results. Quantifying the volume of groundwater discharged in this region is of particular significance due to the presence of hypoxic bottom waters that develop each summer. Two cross-sectional models and a three-dimensional model were developed in order to understand the groundwater flow systems of the region. The model region extended inland from the upland recharge zone out to the edge of the continental shelf. The hydrogeologic units present in the study region are part of the Coastal Lowlands aquifer system. The system is comprised of a thick package of unconsolidated sediments that extend kilometres into the subsurface. The models were simulated using the densitydependent numerical code FRAC3DVS. A number of two-dimensional simulations were performed using a variety of parameter and boundary conditions to determine if there was any scenario under which a significant volume of SGD may be expected. A single three-dimensional model was run. In all simulations, it was found that the groundwater flow regime could be described by three flow systems. These were an onshore system, a seawater recirculation system, and an offshore convection system. Results of all of the simulations showed that little SGD is expected within the study region. No groundwater discharge was predicted adjacent to the coastline. Insufficient topographic relief is present within the system to drive fresh groundwater discharge offshore. Instead, saline groundwater recharge was predicted due to the seawater recirculation system. This system resulted in saline groundwater being present in the deeper parts of the aquifer system over a hundred kilometres inland from the shore. Further offshore, a low rate of discharge was predicted due to the offshore convection system. The predicted volume of SGD was approximately 0.1% of Mississippi River discharge. A second research group from the University of East Carolina, Tulane University, and the University of Miami performed a companion study aimed at quantifying SGD using the geochemical tracer radon. Results from their work showed again that little offshore discharge was occurring within the study region. Computed discharge rates were less than 1 cm/d during all sampling periods. These results confirmed the results of the numerical modelling. The results from these studies are significantly different to what was previously found. This may be due to the low groundwater concentrations of radium that were assumed when calculating the rate of offshore discharge. If higher radium concentrations had been used, as found in deep saline formation waters, then results from both sets of research would be match more closely. Further research is necessary to determine offshore groundwater pathways in order to quantify the concentration of tracer present in any offshore discharge.

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