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.Jiao, J., & Post, V. (2019). Coastal hydrogeology. Cambridge University Press.
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. +Types of aquifersDescriptionSedimentary AquifersSedimentary aquifers consist of different kinds of sediments including coarse-grained, fine-grained sediments and clay minerals. Permeability varies in these aquifers.Jiao, JJ., Wang, Y., Cherry, J.A., Wang, X.S., Zhi, B.F., Du, H.Y. & Wen, D.G. (2010). Abnormally high ammonium of natural origin in a coastal aquifer-aquitard system in the Pearl River Delta, China. Environ Sci Technol 44: 7470–7475. Low-energy depositional environments and flat relief are common in coastal areas. Fine-grained sediments are common in the sea. Those sediments are less porous with low permeability. The permeability of this system drops towards the seaside. Water movement slows down with the change of permeability and the seawater-freshwater density. Igneous, sedimentary or metamorphic rocks can be the bedrock below the sedimentary aquifer system.Anderson, H.R. (1978). Hydrogeologic Reconnaissance of the Mekong Delta in South Vietnam and Cambodia. US Government Printing Office, Washington, DC.Hard Rock AquifersHard rock aquifers are composed of igneous rock or metamorphic rock or both. Low porosity hard rock with different orientation of joints and fractures provides space for the flow of groundwater, forming a hard rock aquifer. Groundwater flow direction is controlled by the orientation of fractures and geological structures like dykes.Custodio, E., & Bruggeman, G. A. (1987). Groundwater problems in coastal areas : a contribution to the International Hydrological Programme. Unesco.Limestone AquifersA Limestone aquifer is an aquifer that is made of carbonate minerals namely marine limestone or bioclastic limestone. Fine-grained limestone has low porosity and permeability while bioclastic limestone is the antithesis. As groundwater may dissolve carbonates aquifer forming an extensive dissolution network which is a karst aquifer. Water flows quickly and water can be stored in a karst aquifer. Groundwater would be rich in carbonic acid under a limestone or karst aquifer.
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. +Types of AquifersIllustrationsDescriptionUnconfined Continental Coastal AquiferGroundwater replenishment of this type of aquifer can be either from rainfall or from terrestrial groundwater. The transition zone is inside the landmass. Due to the density difference between freshwater and seawater, the freshwater tends to flow upwards to the land surface.Unconfined Continental Aquifer with Seepage zoneThis aquifer system is similar to an unconfined continental coastal aquifer. There is a seepage zone in this case. Seepage would shift the interface towards the sea. The seawater within the seepage zone may have a slightly different chemical composition compared with seawater away from seepage zone.Unconfined Circular Island AquiferIsland is a case of an unconfined aquifer. As an island is surrounded by the sea, it is a coastal aquifer. Precipitation and infiltration recharge the groundwater with an island. If rainfall recharge of an island aquifer is significant, the seepage zone might form and shift the interface towards the sea. Ghijben – Herzberg Principle can be used to estimate the depth of groundwater.Fetter, C.W. (1972). Position of saline water interface beneath Oceanic Islands. Water Resources Research. 8(5): 1307Confined Continental Coastal AquiferRegional groundwater flows can end up with confined aquifers in the coastal region and have an interface between seawater.Confined Continental Coastal Aquifer with Seepage zoneFreshwater always flows into the sea. Forming seepage. If the hydraulic conductivity of the aquifer is low or the discharge of the aquifer is large, the seepage zone can push further to the sea. 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.Klassen, J., & Allen, D. M. (2017). Assessing the risk of saltwater intrusion in coastal aquifers. Journal of Hydrology (Amsterdam), 551, 730–745. 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.Muskat, M. (1937). The flow of homogeneous fluids through porous media, by M. Muskat with an introductory chapter by R. D. Wyckoff. McGraw-Hill, 1937.Dagan, G., & Bear, J., (1968). Solving the problem of local interface upconing in a coastal aquifer by the method of small perturbations. Journal of Hydraulic Research 6: 15–44. 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.Kerrou, J., & Renard, P. (2010). numerical analysis of dimensionality and heterogeneity effects on advective dispersive seawater intrusion processes. Hydrogeology Journal, 18(1), 55–72. Pool, M., Post, V. E. A., & Simmons, C. T. (2014). Effects of tidal fluctuations on mixing and spreading in coastal aquifers: Homogeneous case. Water Resources Research, 50(8), 6910–6926.
Tides Tides can push the transition zone towards the land and widen the transition zone.Underwood, M.R., Peterson, F.L., & Voss, C.I. (1992) Groundwater lens dynamics of Atoll Islands. Water Resources Research 28: 2889–2902. 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.
Other factors Geological structures that cross-cut different layers such as faults or dykes can affect the direction and velocity of seawater intrusion. Dykes, an igneous unit that is impermeable or low in hydraulic conductivity might stop the intrusion of seawater. Faults also affect the direction of water flow which is studied by Fault Zone Hydrogeology. Both regular and catastrophic flooding can cause downward intrusion of seawater recreating the transition zone of the fresh-salt water.Villholth, K.G., & Neupane, B. (2011). Tsunamis as long-term hazards to coastal groundwater resources and associated water supplies. Tsunami – A Growing Disaster (Mokhtari M, ed.). InTechOpen, 87–104. Diffusion would also be responsible for seawater intrusion, as there is a net flow of solutes from high concentration to low concentration. For example, a complete salinization of a 10 m thick layer of freshwater takes thousands of years through diffusion. Other factors can include pumping-induced seawater intrusion.
Submarine Groundwater Discharge
Submarine groundwater discharge (SGD) is the groundwater flows across the interface of the aquifer and the sea. It refers to the flow of water shifting towards the sea.Burnett, W. C., Aggarwal, P. K., Aureli, A., Bokuniewicz, H., Cable, J. E., Charette, M. A., Kontar, E., Krupa, S., Kulkarni, K. M., Loveless, A., Moore, W. S., Oberdorfer, J. A., Oliveira, J., Ozyurt, N., Povinec, P., Privitera, A. M. G., Rajar, R., Ramessur, R. T., Scholten, J., … Turner, J. V. (2006). Quantifying submarine groundwater discharge in the coastal zone via multiple methods. The Science of the Total Environment, 367(2), 498–543. Nearshore submarine groundwater discharge is defined as a range of 0 to 10 m, embayment SGD is defined as 10 m to 10 km and offshore SGD is defined as more than 10 km. Topography, salinity and temperature convention and tidal pumping are responsible for the driving force for the submarine groundwater discharge.
Topography-Driven Flow The topography and geology of an area affect the permeability and flow network of groundwater. For an unconfined aquifer, groundwater discharges would be near shore and discharge decreases with the propagation towards the sea. For confined aquifers, groundwater can flow further towards the sea to the embayment zone or even develop submarine springs.
Groundwater Tidal Dynamics