Modelling of a Coastal Aquifer using FEFLOW

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Modelling of a Coastal Aquifer Using FEFLOW
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Seawater Intrusion

• In coastal regions, overexploitation of
  groundwater has caused the encroachment of
  seawater into freshwater aquifers.
• Saltwater is heavier, hence tends to move
  underneath the freshwater layer.
• The freshwater, however, has a hydraulic gradient
  downward towards the coast, hence will flow to
  the sea.
• This outflow momentum force can counter balance
  the density-driven seawater.
• Without it, seawater will continue to move inland
  until the entire aquifer below sea level is
  occupied by it.
• Since such a hydraulic gradient always exists due
  to the precipitation recharge inland, an
  equilibrium position will establish. The toe then
  marks the maximum extent of intrusion.

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Seawater Intrusion in Coastal Area of North Goa

• Coastal tracts of Goa (India) are rapidly being
  transformed into settlement areas.
• The poor water supply facilities have encouraged
  people to have their own source of water by
  digging or boring a well.
• During the last decade, there have been
  large-scale withdrawals of groundwater by
  builders, hotels and other tourist
  establishments.
• Though the seawater intrusion has not yet assumed
  serious magnitude, but in the coming years it may
  turn to be a major problem if corrective measures
  are not initiated at this stage.
• It is necessary to understand how fresh and salt
  water move under various realistic pumping and
  recharge scenarios.

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Objectives of the Study

• Simulation of seawater intrusion in a region
  along Goa coast.
• Evaluation of the impact on seawater intrusion
  due to various groundwater pumping scenarios.
• Sensitivity analysis to find the most sensitive
  parameters affecting the simulation.
• Suggestions for remedial measures.

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• Study Group Mr. C. P. Kumar (NIH, Roorkee)
• Dr. Sudhir Kumar (NIH, Roorkee)
• Dr. B. K. Purandara (NIH, Belgaum)
• Dr. A. G. Chachadi (Goa University)
• Duration 3 Years (April 2004 to March 2007)
• Budget NIH Funding

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• Study Area
• The study area is bound by rivers Chapora and
  Mandovi in north and south directions
  respectively, besides Arabian sea in the west.
• Bardez Taluk of North Goa
• (catchments of Baga Nerul rivers 74 km2)
• Coastal tract from Fort Aguada to Fort Chapora
  (15 km)
• One third tourists visit Bardez coastal areas
• (Anjuna, Baga, Calangute beaches)
• Area of interest 30 km2

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• Work Plan
• Establishment of technical collaboration with
  participating agency (Goa University) and
  identification of a part of the coastal area in
  Bardez taluk of North Goa for the present study.
• Collection of relevant data, literature and maps.
  
• Field investigations
• Identification of 20 observation wells.
• Measurement of monthly groundwater level data in
  observation wells (September 2004 to August
  2005).
• Collection of groundwater samples in September,
  November 2004, January, March, April, June 2005.

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• Laboratory analysis
• Measurement of salinity for collected
  groundwater samples in the laboratory.
• Based upon bi-monthly measurements of
  salinity, groundwater quality in all the
  observation wells was found to be reasonably
  fresh, both in pre- and post-monsoon periods.
• It can be attributed to the fact that the
  transition zone of fresh water-saline water lies
  below the shallow open wells, as evidenced by
  vertical electrical soundings.

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• Resistivity Profiling and Sounding
• To know the distribution of salt-water/fresh-water
  interface -
• Resistivity profiling Apparent electrical
  resistivity measured along four profiles (Anjuna,
  Baga, Calangute and Candolim) from sea coast to
  525 meters inland.
• Vertical electrical sounding Carried out at 7
  monitoring well locations upto a depth of 20
  metres.
• Results of resistivity profiling and vertical
  electrical sounding have been presented in
  forthcoming slides.

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• Modelling Software Package
• A sophisticated finite-element package for
  simulating 3D fluid density-coupled flow and
  contaminant mass (salinity) in the subsurface.

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• Model Setup
• Digitization of the study area map
• Creation of finite element mesh
• Design of slices and layers (3D model)
• Flow data (initials, boundaries, materials)
• Transport data (initials, boundaries, materials)

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Digitized Map Observation Wells
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Finite Element Mesh
(Initially, the model was setup with 734 nodes
and 607 elements)
Nodes 32053 Elements 52656 Total Area
73.796 km2
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Topography
Reference zero elevation (datum) 50 m below MSL
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Initial Head Distribution
Reference zero elevation (datum) 50 m below MSL
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Hydraulic Conductivity
Measured at 6 locations in the study area (3.3 to
31.6 m/d or 0.381e-4 to 3.657 e-4 m/s) - Data
regionalization through Akima inter/extra-polation
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Groundwater Draft Recharge
Density 25 wells / km2 Average annual gw draft
per structure 0.65 ha-m Rainfall
Recharge Laterite 7 Alluvium 10
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• Initially, the model was setup with 734 nodes and
  607 elements and trial simulation run was made.
• However, it caused numerical oscillations.
  Therefore, the entire mesh was re-created with
  32053 nodes and 52656 elements and all the
  subsequent steps were repeated.

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FEFLOW Problem Summary Dimension Three-Dimens
ional Type Saturated media (groundwater) Numbe
r of Layers 6 Number of Slices 7 Projection
None (3D with free surface) Problem
Class Combined flow and mass transport Time
Class Unsteady flow Unsteady mass
transport Upwinding Full upwinding Element
Type 6-noded triangular prism Mesh
Elements 52656 Mesh Nodes 32053 Incorporate
fluid viscosity dependencies Extended Boussinesq
approximation applied to density coupling Time
Stepping Scheme Automatic time step
control via predictor-corrector schemes - Forward
Adams-Bashforth/backward trapezoid (AB/TR time
integration scheme) Initial time step
length 0.001 d Final time 3650
d Unconfined (phreatic) aquifer(s) Slice
No. Status 1 Free movable
2 Unspecified 3 Unspecified
4 Unspecified 5 Unspecified
6 Fixed 7 Fixed
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Three-dimensional Fringes for Hydraulic Head at
the End of Simulation Period
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Three-dimensional Plot for Mass Distribution
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Mass at Top Section
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Mass at Middle Section
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Mass at Bottom Section
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Simulated Extent of Seawater Intrusion
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Conclusions

• Presently, seawater intrusion in Bardez taluk of
  North Goa is confined only upto 290 m from the
  coast under normal rainfall conditions and
  present draft pattern. It may slightly extend
  farther for low rainfall years.
• Seawater intrusion may further advance inland if
  withdrawals of groundwater by builders, hotels
  and other tourist establishments continue to
  increase in the coming years.
• Groundwater salinity needs to be continuously
  monitored near the coastal area, especially
  within 2 km from the coast.
• Corrective measures with proper planning and
  management of groundwater resources in the area
  need to be initiated so that it may not turn to
  be a major water quality problem in the coming
  times.

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Thanks...