Modelling of Seawater Intrusion

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Modelling of Seawater Intrusion
C. P. KUMAR
National Institute of Hydrology
Roorkee (India)
17 June, 2005
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• Seawater Intrusion
• A natural process that occurs in virtually all
  coastal aquifers.
• Defined as movement of seawater inland into fresh
  groundwater aquifers, as a result of
• higher seawater density than freshwater
• groundwater withdrawal in coastal areas

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Densities

• Freshwater 1000 kg m-3
• Seawater 1025 kg m-3
• Freshwater 0 mg L-1
• Seawater 35,000 mg L-1

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Salt Water Intrusion
Pumping causes a cone of depression and...
draws the salt water upwards into the well.
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PROPER MANAGEMENT WILL PREVENT
SALINIZATION OF WELLS! Not PREVENTING sea
water intrusion, but CONTROLING sea water
intrusion.
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Presence of salinity in coastal aquifers can be
detected by (a) Geophysical methods -
Resistivity method (b) Geochemical
investigations - Chemical composition of
groundwater - Isotope studies (age of water to
identify the source of salinity)
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• Field surveys (geophysical and geochemical
  studies) can only reveal the present state of
  seawater intrusion but can not make impact
  assessment and prediction into the future.
• Mathematical models are needed for these
  purposes.
• Ghyben-Herzberg relation is a highly simplified
  model.
• Dynamic movement of groundwater flow and solute
  transport needs to be considered.
• A density-dependent solute transport model
  including advection and dispersion is needed for
  the modelling.

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• Most popular models for seawater intrusion
• SUTRA
• SEAWAT
• HST3D
• FEFLOW
• Recently released Visual MODFLOW Pro 4.1 now
  integrates SEAWAT-2000 to solve variable density
  flow problems, such as seawater intrusion
  modeling projects.

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USGS

• HST3D
• Three-dimensional flow, heat, and solute
  transport model
• MOCDENSE
• Fluid density and viscosity are assumed to be a
  linear function of the first specified solute.
• SEAWAT
• A computer program for simulation of
  three-dimensional variable-density ground water
  flow
• SHARP
• A quasi-three-dimensional, numerical
  finite-difference model to simulate freshwater
  and saltwater flow separated by a sharp interface
  in layered coastal aquifer systems
• SUTRA
• 2D, 3D, variable-density, variably-saturated
  flow, solute or energy transport

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Others

• 3DFEMFAT
• 3-D finite-element flow and transport through
  saturated-unsaturated media. Combined sequential
  flow and transport, or coupled density-dependent
  flow and transport. Completely eliminates
  numerical oscillation due to advection terms, can
  be applied to mesh Peclet numbers ranging from 0
  to infinity, can use a very large time step size
  to greatly reduce numerical diffusion.
• FEFLOW
• FEFLOW (Finite Element subsurface FLOW system)
  saturated and unsaturated conditions.  FEFLOW is
  a finite element simulation system which includes
  interactive graphics, a GIS interface, data
  regionalization and visualization tools. FEFLOW
  provides tools for building the finite element
  mesh, assigning model properties and boundary
  conditions, running the simulation, and
  visualizing the results.
• FEMWATER
• - 3D finite element, saturated /
  unsaturated, density driven flow and transport
  model.

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• Numerical Dispersion
• Numerical approximations of the derivatives of
  the non-linear solute transport equation may
  introduce truncation errors and oscillation
  errors.
• The truncation error has the appearance of an
  additional dispersion-like term, called numerical
  dispersion, which may dominate the numerical
  accuracy of the solution.
• Oscillations may occur in the solution of the
  solute transport equation as a result of over and
  undershooting of the solute concentration values.
  
• If the oscillation reaches unacceptable values,
  the solution may even become unstable.

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• The complex density-dependent ground water flow
  and mass transport models provide stable and
  accurate results when employed with proper
  spatial and temporal discretization.
• The grid Peclet Number (ratio of the spatial
  discretization and the dispersion length) and the
  Courant Number (ratio of the advective distance
  during one time step to the spatial
  discretization) should match the following
  constraints

where Px, Py and Pz are the Peclet Numbers Cx,
Cy and Cz are the Courant Numbers ?x, ?y and ?z
are the grid spacings ?L and ?T are the
longitudinal and transverse dispersivity,
respectively and ?t is the time step.
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• Expertise and Studies at NIH
• Modelling of Seawater Intrusion
• Dr. Anupma Sharma
• Dr. S. V. N. Rao
• Mr. C. P. Kumar
• Dr. Vijay Kumar
• Mr. P. K. Majumdar
• Dr. M. K. Jose (on deputation)
• Nuclear Hydrology Group
• Kakinada Regional Centre

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• UNDP Training
• Two scientists were trained under UNDP Project
  (Vijay Kumar, 1997 C. P. Kumar, 1998) -
  Application of SUTRA model.
• Ph.D. Thesis
• Numerical Modelling of Seawater Transport in
  Coastal Aquifers (Anupma Sharma, University of
  Roorkee, 1996)
• Planning Models for Water Resources Management in
  Coastal and Deltaic Systems (S. V. N. Rao, IIT
  Madras, 2003)
• Research Project
• Freshwater-Salinewater Interrelationships in
  Multi-Aquifer System of Krishna Delta, Coastal
  Andhra Pradesh
• (Hydrology project in collaboration with Ground
  Water Department, Andhra Pradesh)

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Recent Publications (excluding national
conferences) Simulation of Sea Water Intrusion
and Tidal Influence C. P. Kumar ISH Journal of
Hydraulic Engineering, March 2001. New MOC Model
of Seawater Transport in Coastal Aquifers Anupma
Sharma, Deepak Kashyap and G. L. Asawa Journal of
Hydrologic Engineering, September/October 2001.
Numerical Simulation Models for Seawater
Intrusion C. P. Kumar Journal of Indian Water
Resources Society, July 2002. Simulation of
Seawater Intrusion in Ernakulam Coast Dipanjali
D. Bhosale and C. P. Kumar International
Conference on "Hydrology and Watershed
Management", 18-20 December 2002, Hyderabad.
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Modelling Strategies to Simulate Miscible
Transport of Seawater in Coastal Aquifers Anupma
Sharma, Deepak Kashyap and G.L. Asawa Hydrology
Journal of IAH, March-June 2003. SUTRA and HST3D
Modeling and Management of Saltwater Intrusion
from Brackish Canals in Southeast Florida Manfred
Koch and Anupma Sharma The Second International
Conference and Workshop on Saltwater Intrusion
and Coastal Aquifers Monitoring, Modeling, and
Management (SWICA-M3), March 31-April 2, 2003,
Mexico. Effect of Various Parameters on the Size
of Fresh Water Lens in Home Island Vijay Kumar
and John L. Luick AHI Journal of Applied
Hydrology, 2004.
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Constraints in the Numerical Modelling of Salt
Water Intrusion C. P. Kumar Journal of Soil and
Water Conservation, December 2004. Aquifer
Restoration from Seawater Intrusion A Field
Scale Study of Minjur Aquifer System, North
Chennai, Tamilnadu, India. S. V. N. Rao 18th
Seawater Intrusion meeting in Cartagena, Spain
Few other papers on groundwater development and
management in coastal aquifers by Dr. S. V. N.
Rao
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• SIMULATION OF SEA WATER INTRUSION AND TIDAL
  INFLUENCE
• Objective Simulation of sea water intrusion in
  Nauru Island and examine the effect of tidal
  forcing on the fresh water resources.
• Nauru Island is a coral island in the
  central Pacific Ocean, very near the equator and
  occupies a land area of 22 km2.
• The Nauru aquifer was simulated in
  two-dimensions using vertical section with SUTRA.

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• The water table is at an average elevation of 0.3
  m above sea level and ground water flows radially
  outward to the sea.
• The island is underlain by a lens of fresh water
  as much as 7 m thick with average thickness of
  4.7 m. Fresh water overlies a thick mixing zone
  which in turn overlies sea water.
• The unusually thick mixing zone of brackish water
  is due to the high hydraulic conductivity of the
  limestone.

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• Quantitative estimates of hydraulic conductivity
  have not been undertaken in Nauru Island, but by
  analogy with similar raised limestone islands
  elsewhere, hydraulic conductivity is estimated to
  be about 800 - 1,000 m/d.
• Tidal fluctuations may also have some effect on
  the distribution of salinity in the mixing zone,
  particularly in areas near the coastline.
• Oceanic tides have an amplitude of 0.8 m.
• Mean annual rainfall is 1,994 mm and annual
  rainfall has a high degree of variability.
• For this study, a uniform recharge rate of 540
  mm/year was assumed.

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• Discretization
• A vertical section of the aquifer along the line
  C-C - 6400 m long and 120 m deep, was
  discretized to 832 rectangular elements and 891
  (27 x 33) nodes.
• The horizontal spacing was kept constant as 200
  m. The vertical spacing was made variable, being
  2, 3, 5 and 10 m from top of the aquifer to
  depths of 20, 35, 60 and 120 m, respectively,
  below mean sea level (MSL).

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• Boundary Conditions
• A no-flow boundary condition is specified along
  the bottom of the mesh at a depth of 120 m where
  the limestone is considered to be impervious.
• A recharge boundary due to rainfall is specified
  at the top of the aquifer.
• Along the left and right vertical boundaries, a
  hydrostatic pressure defined by p ?s g d was
  imposed. Here, p is the hydrostatic pressure, ?s
  is the density of sea water, g is the
  acceleration due to gravity, and d is the depth.
• Any inflow, occurring through the specified
  pressure boundaries, has a sea water
  concentration of 35,700 mg/L TDS (i.e. C
  0.0357 kg TDS/kg fluid).

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• Model Parameters
• The Nauru aquifer is reported to be not under any
  major stress such as pumping, it was therefore
  assumed to be in a steady state condition.
• Only one set of salinity data, measured during
  1987, was available.
• No measurement of hydraulic parameters has been
  undertaken in the island and therefore estimated
  by trial and error using relevant information
  from similar cases.

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Values of Hydraulic Parameters for Nauru Island
for Simulation with SUTRA Hydraulic
Parameter Value Horizontal hydraulic
conductivity, Kh 900 m/d Anisotropy,
Kh/Kv 50 Recharge rate 540
mm/year Porosity 0.30 Longitudinal
dispersivity, ?L 65 m Transverse dispersivity,
?T 0.15 m Molecular diffusivity 1.0x10-10
m2/s
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• The following fixed values were used in the
  computations
• Fresh water density ? 1,000 kg/m3
• Sea water density ?s 1,025 kg/m3
• Fluid viscosity ? 10-3 kg/m/s
• Coefficient of fluid density change with
  concentration ?/C 700 kg/m3

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• Simulation of Ground Water Salinity
• The 1997 version of SUTRA (2D) was used for the
  simulation.
• To obtain a steady state solution, the simulation
  run was divided into 1,000 time steps of 15 days
  each, which corresponds to a total simulation
  period of about 41 years.
• Figure 4 presents the measured salinity
  concentrations along section C-C and figure 5
  presents the ground water salinity obtained in
  the present study.
• The ground water salinity contours for the
  concentrations 0.005, 0.01, 0.02 and 0.03 in
  figure 5 are found to compare well with measured.

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• The results indicate that the model represents
  the behaviour of the aquifer quite well under the
  existing conditions.
• The model is very sensitive with respect to
  changes in hydraulic conductivity and recharge.
  Higher values of hydraulic conductivity
  facilitate intrusion of sea water, whereas
  increased recharge has the opposite effect,
  diluting saline water within the aquifer.
• The model is also sensitive to changes in
  porosity, anisotropy and dispersivity but less
  sensitive to changes in molecular diffusivity.

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• Tidal Influence
• The tidal signal is manifested as a pressure wave
  that propagates inside from the coastal
  boundaries towards the centre of the model area.
• Sinusoidally varying pressures were applied at
  the boundaries to simulate tidal forcing.
• The amplitude of sine wave function (assumed for
  sea water tides) was taken as 0.80 m with
  frequency of two cycles per day.
• The tidal influence on sea water intrusion has
  been shown in figure 6 which can be compared with
  figure 5 (without tidal forcing).

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• A dramatic reduction of the fresh water lens was
  observed when tidal influence is also considered.
• The area of fresh water (concentration less than
  0.0005 i.e. 500 mg/L TDS) was reduced by
  approximately one half in figure 6 (with tidal
  forcing).
• This result highlights the importance of
  including tidal forcing in numerical studies of
  coastal and island aquifers.

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Location of the earthquakes / tsunami
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Tsunami Animation
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The 26-12-04 tsunami has affected groundwater
systems in the low-lying coastal zones of the
stricken areas. Schematic representation of the
possible effects of the 26-12-04 tsunami on
coastal groundwater systems
Upconing of brackish groundwater under
abstraction wells, Intrusion of brackish
or saline water from ponds,
Fingering of brackish water from pools,
Reduction in
freshwater volume due to shoreline retreat, etc.
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• There are three primary modes through which the
  saltwater may enter the underlying aquifers.
• The first is direct contamination of wells, both
  large-diameter dug wells and small-diameter
  tubewells that were either open at the top or
  damaged during the flooding.
• The second contamination pathway is widespread
  infiltration of seawater into the aquifer from
  the land surface through the unsaturated zone,
  the quantity controlled by the permeability of
  the surface sediments and the depth to the water
  table.
• Following drainage to the sea, some seawater may
  remain inland as surface-water bodies in local
  low-lying areas. It acts as long-term point
  sources of saltwater to the groundwater system.
• Numerical models can be used to analyse the
  impact of tsunami on groundwater resources.

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• Potential Remediation Approaches
• Widespread infiltration of a dense non-reactive
  contaminant is difficult to remediate.
• Removal of bodies of standing saline water and
  purging of wells.
• Allow natural recharge to flush salt from the
  aquifer.
• If the seawater is isolated in a particular
  aquifer horizon, it may be pumped out of the
  aquifer and discarded. However, application of
  this method near the coast may induce classical
  seawater intrusion.
• If saltwater contamination is contained in
  shallow aquifers which are isolated from deep
  aquifers by confining units, the deep confined
  aquifers may become an alternative source of
  fresh water through installation of deeper
  tubewells.

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• Future Action
• Data collection and long-term monitoring is
  necessary to assess and manage the impact of the
  tsunami-induced saltwater contamination.
• Measurements of well salinity levels over time as
  well as salinity profiles with depth at selected
  locations should be obtained.
• Generic cross-sectional or three-dimensional
  numerical groundwater models of variable-density
  flow and solute transport can be constructed to
  better understand contamination mechanisms and
  the effectiveness of different remediation
  strategies.

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THANKS