Effects of Saltwater Intrusion on a Costal Aquifer;
Henry’s Problem and a Coastal Region of Hersonissos,
Crete
Andrew Davis, Clarkson University Honors Program, Class of 2010;
Biomolecular Science / Mathematics Double Major
Research Mentor: Kathleen Fowler, Assistant Professor of Mathematics
An aquifer is a subsurface, water bearing region. The management and maintenance of aquifers
is a necessity to insure the region’s water quality. Coastal aquifers undergo a unique type of water quality
deterioration because of a phenomenon known as salt-water intrusion [3]. As fresh water flows from the
aquifer toward the ocean, a saline solution moves inward, polluting a section of the aquifer near the
coastline [1]. Eventually, dynamic equilibrium is reached between the fresh and salt water [3]. This
equilibrium is dependent on the physical characteristics of the aquifer, such as soil porosity and hydraulic
conductivity [1]. The saltwater intrusion is also affected by properties of the water within the aquifer,
such as hydraulic head and Darcy’s velocity [1]. The installation of wells within costal aquifer has the
potential to disturb this equilibrium [3]. If the extraction rate is to large it could cause a drop in the
subsurface water table and will reduce the amount water flowing from the costal aquifer toward the
shoreline [3]. This will cause deterioration of the water quality in the aquifer due to an increase in
saltwater intrusion [3]. This drop in quality is quantified by the concentration of chloride ions in the
aquifer [3]. By mathematically modeling the movement of seawater into a costal aquifer, it is possible to
optimize the rate at which water can be pumped out of the costal wells without polluting the aquifer.
One way of simulating saltwater intrusion is using the Sharp Interface Approach. In this method
it is assumed that the saltwater and freshwater are two immiscible fluids [3]. The angle of the sharp
interface is determined by the difference in the heads of the salt water and the fresh water [3]. If the
hydraulic head of the freshwater is high, then the sharp interface will be almost vertical, however as the
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hydraulic head decreases the saltwater will move inland and intrude upon the aquifer [3]. To further
understand saltwater intrusion and how the freshwater – saltwater interface acts a community problem
was used to show the steady state of a hypothetical aquifer.
Henry's problem is a common benchmarking model used to understand how saltwater behaves
near a coastline. We used Henry's model to gain insight into coastal seawater intrusion. The model
specifications were taken from the “User’s Guide to SEAWAT: A Computer Program for Simulation of
Three-Dimensional Variable-Density Ground-Water Flow” [1]. In this problem there was a two meter by
one meter by one meter box that represented a cross-section of the aquifer [3]. The top and bottom of the
cross-section was set as a no-flow boundary. The left boundary had a constant flux of 5.702 m3/d per
meter with a concentration of zero. The right boundary had a constant head that varied linearly; the top
the aquifer had a head value of zero, while the bottom had a head value of one meter. The bottom 0.7
meters of the right boundary had a constant concentration of 35 ppm. The hydraulic conductivity was 864
meters per day, the longitudinal and transverse dispersive was zero meters, the porosity was 0.35, and the
density of seawater and freshwater was 1,025 and 1,000 kilograms per cubic meter respectively. These
boundary conditions were input into Groundwater Modeling System (GMS) using both the
MODFLOW/MT3D simulator and the FEMWATER simulator. GMS is a graphical user interface used to
set up problems in a variety of groundwater simulators. MODFLOW is a United States Geological
Survey (USGS) simulator that runs on a three dimensional grid [4]. The MODFLOW simulator created
and three dimensional grids that were used to help model the ground water flow [4]. The data output
from MODFLOW was then used in the MT3D simulator, which was developed by the Environmental
Protection Agency (EPA), to model the saltwater movement by treated the salt as contaminate [5]. These
two GMS simulators were able to show the salt water intruding inland and contaminating a portion of the
aquifer. The FEMWATER simulator, developed by the army corps of engineers, was able to use a three
dimensional mesh to simulate the groundwater movement and show the changes in hydraulic head [2].
However, it was unsuccessful in simulated the movement of the salt as contaminate.
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Henry’s problem provides a hypothetical situation that isolates saltwater intrusion. However, this
understanding of saltwater intrusion can be applied to real costal aquifers. The simulators used to model
Henry’s problem will also be used to model the Coastal Region of Hersonissos, Crete [3]. This aquifer
covers 17.9 km2 with a coastal line of 3.8 km in the west east direction and 4.7 in the north south direction
[3]. Data has been collected about the region, included major geological features and soil properties, such
as the presence of limestone, hydraulic conductivity, and the permeability of the soil [3]. This
information, along with boundary conditions of information such as hydraulic head, and recharge, can be
used to model the groundwater movement in the region using the same GMS software. Using the well
package it is possible to determine the effects that extracting fresh water from the aquifer will have over
time. It will then be possible to optimize the amount of fresh water that can be pumped from the region
without causes too much drop in the subsurface water table, thus polluting the aquifer. It is also possible
to optimize the location of the wells within the aquifer. The amount of water that a well can pump
without polluting the aquifer is dependent on its location; therefore it is possible to find solutions to the
problem that only work when wells are located in specific locations.
Works Cited
[3] Guo, W. and C.D. Langedvin. 2002. User’s Guide to SEAWAT: A Computer Program for Simulation
of Three-Dimensional Variable-Density Ground-Water Flow. U.S. Geological Survey.
[1] Hsin-Chi, J.L., D.R. Richards, C.A. Talbot, G. Yeh, J. Cheng, H. Cheng, N.L. Jones. FEMWATER;
A three-dimensional finite element computer model for simulating density-dependent flow and
transport in variable saturated media.
[2] Karterakis, S.M., G.P. Karatzas, I.K. Nikolos, and M.P. Papadopoulou. Optimization Methodologies
for the Solution of Coastal Subsurface Water Management Problems Subject to Environmental
Criteria.
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[4] McDonald, M.G. and A.W. Harbaugh. Programmer’s documentation for MODFLOW-96, an update
to the U.S. geological survey modular finite difference groundwater flow model. U.S. Geological
Survey Open-File Peport 96-486, 1996.
[5] Zheng, C. and P. Wang. MT3DMS: A Modular Three-Dimensional Multispecies Transport Model for
Simulation of Advection, Dispersion, and Chemical Reactions of Contaminants in Groundwater
Systems; Documentaion and User’s Guide, December 1999.
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