Coastal Hydrogeology is a branch of Hydrogeology that focuses on the movement and the chemical properties of groundwater in coastal areas. Coastal Hydrogeology studies the interaction between fresh groundwater and seawater, including seawater intrusion, sea level induced groundwater level fluctuation, submarine groundwater discharge, human activities and groundwater management in coastal areas.
The freshwater-seawater interface is a dynamic boundary where freshwater mixes with seawater. An interface in Coastal Hydrogeology refers to the location that freshwater from aquifer meets seawater. Steady freshwater-seawater interface is an equilibrium stage where the boundary locates in a relatively fixed location, while seawater intrusion or a strong recharge rate breaks the equilibrium, leading to an unsteady freshwater-seawater interface. Mixing of groundwater and seawater creates a special chemical system that is a good indicator to show the interaction and the interface.
Human activities such as pumping of groundwater and land reclamation break the equilibrium, leading to seawater intrusion, development of a seepage zone or pollution of the ocean. The interaction between groundwater system and the ocean is complex. Preventive actions and engineering measurements are adopted to mitigate the impacts.
Freshwater-Seawater Interface
Freshwater – Seawater interface or saltwater interface is an essential aspect in Coastal Hydrogeology that tries to figure out the location of the transition zone or mixing zone between freshwater and seawater. A sharp interface is formed when the transition zone is thin or narrow. Coastal aquifers can be classified in the same three categories as all aquifer systems: Sedimentary Aquifers, Hard Rock Aquifers and Limestone Aquifers.
Ghijben – Herzberg Principle
The Ghijben – Herzberg Principle formulated the water table and the boundary of groundwater in an island or unconfined aquifer should form a lens-shape. This principle can estimate the bottom boundary of the interface which provides a general idea of the capacity of an aquifer. By estimating the capacity of an aquifer, we can find out the available groundwater resource in some regions.
Freshwater-Seawater Interface in Coastal Aquifers
It is assumed that the aquifers below are homogeneous in the different models so that the hydraulic conductivity of each aquifer is uniform.
The above figures simulate possible coastal aquifers. In reality, it is complex. Due to complex geology - non-uniform rock layers and weathering, both confined and unconfined aquifers can be found within a coast. It is possible to have multiple confined aquifers at the bottom and an unconfined aquifers at the top of a coast.
Seawater Intrusion
Seawater intrusion is a process where seawater intrudes into a freshwater aquifer. Natural or anthropogenic factors can cause seawater intrusion. Salinization of a freshwater aquifer can be caused by the inflow of seawater due to a change in groundwater pressure, reduction in water recharge, reduction in groundwater discharge, or other sources of salt entering the aquifer. The transition zone or the interface would shift either to the land or shift vertically. Sea level rise, flooding or tsunami also leads to saltwater intrusion into the freshwater aquifer.
Pumping-Induced Saltwater Up-coning
Pumping of groundwater from a well near the interface could cause seawater up-coning where seawater intrudes vertically to the aquifer. When the pumping rate exceeds the maximum pumping rate or critical pumping rate, seawater would be pumped out.
Effects of Heterogeneity and Anisotropy
Aquifers should be anisotropic and heterogeneous. A high permeability layer is associated with the freshwater flow. If the hydraulic conductivity of an aquifer in a vertical direction is lower than the horizontal direction, the transition zone would shift more in a horizontal direction.
Tides
Tides can push the transition zone towards the land and widen the transition zone. Tides can strengthen the freshwater seawater mixing due to dampening of the tidal pressure forming the non-uniform flow of groundwater. The greater the tidal amplitude, the greater the mixing effect. Less dampening in an aquifer would lead to a more uniform flow.