Multicomponent Groundwater Flow And Submarine Groundwater Discharge In Coastal Multilayered Aquifer Systems

Coastal areas are usually characterized by dense population and booming economies and influenced by not only continental but also oceanic hydrogeological processes, leading to various serious and complex problems ralated to water resources, ecology and environment such as the beach oil spills and the red tides that occurs frequently in recent years. As a result, the study of coastal zone becomes more attractive to researchers than ever before and there appear many fundamental, applied scientific problems and new research fields that need to be solved urgently, providing opportunities for original, novel and advanced studies.Beaches and intertidal zones are located in the center of intersection of ocean and inland, and are important zones for interactions between seawater and groundwater. Beach aquifer is a complex and dynamic zone affected by the combined actions of physical, chemical and biological factors. Groundwater flow and solute transport in this zone are influenced by the combination of various complex and nonlinear factors there, and are very difficult to quantify comprehensively. Therefore, it is necessary to understand and learn the hydrodynamics in coastal aquifers, which is influenced by various factors. In the last few decades, numerous analytical, numerical, and experimental studies have been conducted to investigate the groundwater flow and solute transport in coastal aquifers. Despite of this, gravel beaches were paid much less attention than sandy beaches. In reality, oil spills tend to be persisting in gravel beaches. This paper investigated the reasons for this.Estimations of the submarine groundwater discharge (SGD) and of net inland recharge (spatial and temporal distribution) in coastal areas are of great importance for evaluating contaminant transport from aquifer to open water bodies, for solving the problem of water pollution and ecological degradation, and for the quantitative assessment of the role of groundwater played in the global water cycle. SGD has been attracted much attention from hydrogeologists recently. Most of previous studies considered the cases when the coastal aquifers are superficial and homogeneous. The spatial scale of the considered domain was limited near the coastline where the seawater-freshwater interface is located. The long-term observations of the hydraulic head in temporal scale were not considerd. In particular, there is great uncertainty in estimating the percentage of net, land-originated groundwater recharge of SGD. The studies of heterogeneous, multilayered, deep aquifer systems in the spatial scale of tens of kilometers have not been considered yet. We quantitatively investigated this situation.Analytical and numerical methods were used to study the spatial and temporal distribution of groundwater flow, salinity and submarine groundwater discharge in coastal aquifers. For the small-scale gravel beach aquifer, the groundwater hydraulics and solute transport have been conducted by field investigation, laboratory experiment and numerical modeling. For the large-scale coastal multilayered aquifer systems, two general models are considered to study the SGD theoretically. The main content and results are summarized as follows:We first investigated a gravel beach aquifer, which is located in Knight Island, Prince William Sound, Alaska. We performed field observations and laboratory experiment of beach morphology, lithology, rainfall, water table,pore water salinity, and tracer (lithium). A two-layered beach structure and the boundary condition of inland were determined. The two dimensional finite element model MARUN (MARine UNsaturated model) was used to simulate observed water table, salinity and tracer (lithium) concentration. The MARUN model can simulate water flow and solute transport in variably-saturated porous media, taking into account the effects of salt concentration on fluid density and viscosity. It quantitatively characterized the influences of important factors such as beach permeability, heterogeneity, beach slope, tides, the inland recharge, and the density difference between saltwater and freshwater on groundwater flow, salinity and tracer (lithium) of the intertidal zone. The beach was contaminated in 1989 by the Exxon Valdez oil spills. But no oil residues were found in the considered transect. The sensitivity of the beach to oil spills was discussed. Based on field inspection and model calibration, we obtain the following the main results:(1)The behavior of water table variation with time indicates that the beach needs to be viewed as made up of two layers, which are characterized by a high-permeability surface layer underlain by a low-permeability lower layer. Due to the high-permeability surface layer, the water level falls at the same speed as the falling tide for a certain depth, and then became almost constant until subsequent flood tide. The permeability of lower layer is so low such that the water within it practically does not fall off with time or falls very slowly with time. The water level at inland was higher than the tidal level most of the time, which suggests that the beach was consistently filled from inland freshwater recharge.(2) Time series of salinity are found to be influenced by inland freshwater recharge from rainfall runoff. Large quantity of freshwater caused by rainfall runoff can be regarded as a fresh groundwater flood from inland. Due to the seaward movement of the front of the fresh groundwater flood, two stages of salinity variation formed clearly at each well. The first is the "high-salinity" stage before the freshwater front arrived.The second is the "low-salinity" stage when and after the freshwater front arrived.(3) The tracer injection experiment indicated that the lithium (which can also be regarded as nutrient) applied along the transect would be washed to the sea very quickly (within less than one tidal cycle) by the combination of the two-layered beach structure, the tidal fluctuation and the freshwater flow from inland.(4) The salt wedge seaward of the low tide line was almost invariant in comparison with the strong fluctuations of the saline plume in the surface layer of the intertidal zone.The presence of the two layers prevented the presence of a freshwater discharge "tube" between the upper saline plume and salt wedge. This is in contrast with the previous works where the beach sediments are homogeneous and the freshwater discharge tube was observed.(5)The distribution of the averaged pore water velocity within the transect demonstrates that the bulk of the freshwater comes from the inland side, and the flow direction is seaward. The velocity in the surface layer is much higher than that in the lower layer due to the permeability difference between the two layers. Time series of pore water velocity at location 0.1m below the interface indicates that the groundwater flow is seaward for the majority of time as the seaward distance increases. The patterns of inflow and outflow rates show that seawater-groundwater circulation mainly occurred in the middle and high intertidal zone. The tide-induced Submarine Groundwater Discharge (SGD) is estimated around 9 m3d-1m-1.Compared with previous SGD of homogeneous sandy beaches, our SGD is larger than theirs. The large value is probably due to the large tidal range of～4.8m and the very permeable surface layer.(6) The simulations reproduced the observed water table, salinity, and lithium concentrations. Sensitivity analysis reveals that the estimated parameters are well determined. Both the water table and salinities are sensitive to the hydraulic conductivities of the upper and lower layers. The salinities are more sensitive to dispersivity and hydraulic conductivities of the loose sediments within the pits dug for well installation than the water table.(7) The simulated water table of the intact beach is higher than the interface between the surface and lower layers, which prevented Exxon Valdez oil from penetrating into the lower layer in 1989. However, a beach 100 meter south of it remains heavily polluted. Once the oil enters the lower layer, due to the strong adhesion and capillary effects and low permeability of the fine material, the tidal and freshwater washing effect on the oil become very weak and is limited within the lower layer, leading to the oil persistence.In the studies of submarine groundwater discharge from the multilayered coastal aquifer system, analytical and numerical methods were used. The possibility for the multilayered coastal aquifer system to provide considerable land-originated SGD was investigated using a non-linear analytical model in the spatial scale of tens of kilometers. The model comprises an unconfined aquifer, a confined aquifer and an aquitard between them with a vertical sea-land interface at the coastline. The effects of tides and the density difference between seawater and freshwater are neglected in the model. The model considers constant rainfall recharge uniformly distributed in inland area. The approximate analytical solutions of water table in the unconfined aquifer and hydraulic head in the confined aquifer are derived based on the following assumptions:(i) flow is horizontal in the aquifers and vertical in the aquitard, and (ii) flow in the unconfined aquifer is described by nonlinear Boussinesq equation. Approximate analytical expressions of SGD from the unconfined and confined aquifers are derived. The analytical solution is compared with two-dimensional numerical solutions. The results indicated that the assumptions used for the analytical solution are valid.Typical hydrogeological parameters of coastal multi-layered aquifer systems are taken into account in the discussions. The larger the rainfall recharge, the higher the water table in the unconfined aquifer and hydraulic head in the confined aquifer. For any given rainfall recharge, the water table decreases and the hydraulic head increases as the specific leakage of the aquitard increases. The water table of the unconfined aquifer is greater than the hydraulic head of the confined aquifer for all the cases considered, leading to downward groundwater flow through the aquitard. When rainfall infiltrates into the unconfined aquifer, part of it flows into the sea driven by the water table gradient. The rest leaks into the confined aquifer through the aquitard, and flows into the sea through confined aquifer driven by the head gradient.For a typical coastal aquifer which extends 50 km landward, as the aquitard's specific leakage increases from 10-9 1/d to 10-1 1/d and the inland rainfall recharge increases from 18.2 mm/yr to 182 mm/yr, the SGD from the confined aquifer increases from 1.87 m2/d to 10.37 m2/d. It indicates that SGD from the confined aquifer increases with the specific leakage of the aquitard and inland rainfall recharge. The amount of SGD from the confined aquifer may account for considerable portion of the total SGD derived from inland, a situation that is not considered previously. The analytical solution in this paper can be applied to verify the numerical codes related to long-term inland recharge. The solution is also helpful to understand the role played by the leakage of the aquitard in SGD from confined aquifer in the multi-layered aquifer system.The analytical method is used to study the multilayered submarine aquifer system extending under the sea infinitely. Based on a linear analytical model in temporal scale of seasons, we investigated the possibilities for the deep, multilayered submarine aquifer system to provide considerable land-originated SGD. The aquifer system comprises a semi-permeable layer (seabed), two confined aquifers separated by a semi-permeable layer. The inland recharge is described by a time-independent component (yearly average) plus a sinusoidally periodic part representing the seasonal variation. The analytical expressions of the hydraulic head in the two confined aquifers are derived, and both of them are expressed as a steady-state solution and a periodic solution. For the four base cases, the SGD per unit along-shore length is calculated according to Darcy's law on the seafloor. The distributions of SGD from deep confined submarine aquifers along the Winyah Bay transect perpendicular to the coastline in the South Atlantic Bight are discussed using our analytical solution. All the layers are assumed to extend under the sea with offshore width of about 120 km. The results demonstrate the following important facts:For the upper confined aquifer (the Floridan aquifer system and Tertiary sand aquifer), both the constant inland recharge and seasonal periodical variation in inland recharge may lead to considerable SGD, and the width of the zone of SGD ranges from about 0.8 km to about 8.0 km. No matter the inland recharge is in terms of constant or seasonal period, or their combinations, a head difference of 1.0 m at the coastline between the upper aquifer and the sea will lead to a SGD rate from 1.82 m2/d to 18.3 m2/d as the leakance of the seabed varies from 0.001 1/d to 0.1 1/d. Therefore, there may be great possibility for the Floridan and Tertiary sand aquifer to provide large, land-originated SGD. In particular, when both the average inland recharge and the amplitude of the seasonal variation of the inland recharge equal 45.45 m2/d, the analytical model predicts a maximum head of 5.0 m at the coastline. For this situation, seasonally-variable SGD equals 90.9 m2/d in summer and 0.0 m2/d in winter, providing a consistent explanation of the seasonal variation of 226Ra observed by Moore [2007] (Journal of Geophysics,2007,112, C10013,doi:10.1029/2007JC004199).The potential contribution of the constant inland recharge from the lower Black Creek aquifer to SGD is effectively limited by the small leakance of the thick semi-permeable layer overlying that aquifer. The potential contribution of the seasonal periodic inland recharge from the lower aquifer to SGD is effectively reduced by the large elastic storage of that aquifer. Overall, the whole contribution of the lower aquifer to SGD is only 1.2%-12%of that of the upper aquifer. In the model, the aquifers deeper than the two most upper ones are neglected. It can be concluded that neglecting the deeper aquifers is reasonable in the sense that the deeper the aquifer, the smaller the contribution of the aquifer to the SGD.The density-effect is neglected in the model. To quantify the validity of the assumption of neglecting the density-effect, the use of numerical methods seems inevitable. Nevertheless, our analytical solution has its own significance in the sense that it provides detailed description of the relationship between groundwater flow and hydrogeological parameters when neglecting density-effect. At the same time, it can be used as a benchmark to quantify the density-effect through comparisons with various numerical solutions that include the density-effect.In conclusion, a combination of methods based on field inspection, laboratory experiments and numerical modeling were used to analyze multi-component groundwater in a gravel beach. The study emphasized on analyzing factors that influenced water table, salinity and submarine groundwater discharge. The reason why there is no oil spills persisted in the beach was that the water table is higher than the interface between the surface and lower layers. The freshwater-seawater dynamics revealed here may provide new insights into the complexity, intensity and time-scales of mixing between fresh groundwater and seawater in tidal beaches.We investigated the coastal multilayered aquifer systems that may provide considerable land-originated SGD using the analytical and numerical methods. It is of importance to solve real problems.In reality, hydrogeological conditions of multilayered aquifer systems beneath the seafloor are difficult to obtain. The theoretical results of this thesis may provide some guides to analyze the hydrogeological conditions of the submarine aquifer from groundwater head observations in inland wells.