Groundwater Flow In Coastal Aquifer Research Articles

Vertical distribution of groundwater age in and around the freshwater–saltwater transition zone determined using noble gases and carbon isotopes

The distribution of groundwater ages in coastal aquifers are complicated because of the heterogeneity of the hydrogeology and the mixing of terrestrial and marine water sources. Therefore, multiple studies verifying the groundwater age distribution via borehole surveys are required to better understand groundwater flow in coastal aquifers. This study examined groundwater age distributions in coastal aquifers adjacent to a high-elevation volcano (Mt. Fuji, Japan) by analyzing geochemical and isotopic tracers retrieved from drilling 350- and 160-m-deep boreholes. Brackish groundwater samples collected from the boreholes had high helium isotope ratios, indicating a large contribution of mantle-derived helium. Groundwater ages determined from carbon and helium isotopes indicate that terrestrial groundwater recharged at high elevations flows from the volcano to the coast, mixing with seawater in a transition zone. The groundwater age at the freshwater–saltwater transition zone and the upper part of the saltwater zone is older than that of the underlying saline water. The distributions of the groundwater age in the study area confirm that the conventional groundwater flow system in coastal aquifers is applicable even in deep aquifers. The complex patterns of groundwater age and mixing derived from geochemical and isotopic tracers can guide sustainable use of groundwater resources and the subsurface.

Tidal controls on coastal groundwater conditions: field investigation of a macrotidal system

Previous studies of coastal groundwater resources have not properly explored the hydraulic conditions at coastal and estuarine boundaries, despite recognised influences of oceanic and estuarine water-level fluctuations (e.g. tides and waves) on groundwater near these boundaries. Such influences may have important implications for determining submarine groundwater discharge and seawater intrusion. In this paper, oceanic and estuarine controls on the hydrology of coastal aquifers are characterised for a macrotidal system—the Pioneer Valley coastal plain, northeastern Australia. The tidal water-table over-height (tide-induced increase in average water-table height at the coastline) is quantified at three locations and compared with theoretical estimates, which assume simplified physiographical conditions compared with those encountered at the field sites. The results indicate that local geological conditions, beach morphology and characteristics of tidal forcing control the behaviour of nearshore groundwater within the system. Existing analytical and numerical solutions that are commonly applied as first-pass estimates are found to be insufficient for predicting observed tidal water-table over-height in the Pioneer Valley, due to the sediment heterogeneities, non-uniform beach slopes and large tidal ranges of the system. The study reveals the spatial and temporal variability in tidally influenced hydraulic heads at the estuarine and coastal boundaries of the aquifer, and provides estimates of tidal water-table over-height up to 2.41 m during spring tides. These findings highlight the complexity of coastal groundwater systems, and the need to incorporate appropriate nearshore and near-estuary boundary conditions in models of regional groundwater flow in coastal aquifers.

Fresh and saline groundwater discharge to the ocean: A regional perspective

Studies of groundwater flow in coastal aquifers often focus strongly on freshwater and investigate flow in a narrow (<5 km) zone surrounding the coastline. This work was designed to place coastal flow in a regional context and to compare fresh and saline submarine groundwater discharge (SGD) for regional flow systems. Numerical flow and transport models were developed to estimate SGD associated with topography‐driven flow, seawater recirculation, and geothermal convection in a passive margin setting. Simulations were based on two cross sections of North Carolina, and sensitivity studies were used to explore the impact of varying hydraulic and transport parameters. Results suggest that saline flow associated with seawater recirculation and geothermal convection should be considered in studies of SGD. Studies limited to shallow topography‐driven flow may be justified in using small study areas, but flow systems contributing to SGD may span at least 20 km surrounding the coast and continental slope.

Experimental Investigations on Groundwater Flow in Coastal Aquifers

Experiments were conducted using laboratory models to study salt water intrusion phenomena in coastal aquifers. Two model tanks filled with sand were used. The smaller model tank could produce steady-state cross-sectional flow in a reasonably short time. The bigger tank model produced a more interesting threedimensional flow field compared to the smaller one, but it also took much more time to reach a steady state condition, and posed difficulties in observing results. A cross-sectional model was used to investigate efreshwater lensi phenomena. A few pumping scenarios were tested. Contamination of the pumping well due to excessive withdrawal was observed. The effectiveness of a scavenger well was studied. The bigger model was used to study the steady-state lateral intrusion problems. Various attempts were made to measure the interface position in the sand.

Tides as phase-modulated waves inducing periodic groundwater flow in coastal aquifers overlaying a sloping impervious base

This paper presents procedures for analysing the interactions between tidal waves and coastal aquifers overlaying a sloping impervious bed, using Boussinesq equation and wave equation. Fourier series solutions of a linearised Boussinesq equation are presented subject to a periodic boundary condition (BC). The periodic BC is a phase-modulated periodic solution of the wave equation which has been shown to satisfactorily simulate uneven twin peaks of semi-diurnal tides as observed in a study area on the east coast of Queensland, Australia. Numerical analyses show that the Fourier series solutions of Boussinesq equation subject to periodic BCs reveal two important features of the tidal waves. First, the tidal waves damp towards landward, and second, the half amplitude of the tide above the mean sea level is greater than that below it. While the first feature is clearly expected, the second feature is physically more meaningful and important, and is confirmed by the field data.