use of modflow to simulate groundwater levels in an unconfined aquifer

1. SEMINAR PRESENTATION
on
“Modelling groundwater flow and advective
contaminant transport in the Bou-Areg unconfined
aquifer (NE Morocco)”
TF. El Yaouti , A. El Mandour , D. Khattach , O. Kaufmann (2008)
SHYAM MOHAN CHAUDHARY
17AG62R13
Land and Water Resources Engineering
Agricultural and Food Engineering Department
IIT KHARAGPUR

2. CONTENTS
• INTRODUCTION
• REVIEW OF LITERATURE
• PROBLEM STATEMENT
• OBJECTIVES
• METHODOLOGY
• RESULTS AND DISCUSSION
• CONCLUSIONS

3. INTRODUCTION
• Numerical groundwater modeling is an important
predictive tool for managing water resources in aquifers.
• Groundwater models can be used
– to test or refine different conceptual models,
– estimate hydraulic parameters.
– predict how the aquifer might respond to changes in
pumping and climate change.

4. continued….
The study was carried out on the groundwater of Bou-Areg
plain and that area was selected as the study area for
several reasons:
• Increased population growth and agriculture activities are
projected for this area in the future.
• Reliable information about the location, geometry, and
hydraulic properties of the aquifer system are available
• Extensive climate data, as well as data of measured water
levels at several monitoring wells are available.

5. REVIEW OF LITERATURE
AUTHORS YEAR STUDY
Pollock et. al. 1994
MODFLOW is a finite-difference
groundwater flow model that simulates
three-dimensional steady and transient
state flows in heterogeneous layered
aquifer systems, and predicts flow
paths using particle tracking
Gurwin et.al. 2004
stated that, by setting up a conceptual
model within the numerical model
environment and by applying a quasi-
3D solution, complex multi-aquifer
systems can be well and efficiently
modeled.

6. PROBLEM STATEMENT
• Since the groundwater there has been severely
contaminated by salinization, most water for agricultural
purposes is drawn from the river.
• Effective strategies for both management and protection
of groundwater resources are required to avoid future
irreversible environmental impacts, such as depletion
and deterioration of groundwater quality.

7. OBJECTIVES
• To employ MODFLOW within the framework of the
GMS to study the groundwater processes of the
hydrogeological system of Bou-Areg unconfined
aquifer
• To perform particle tracking in the aquifer using
MODPATH inside GMS framework .

8. METHODOLOGY
STUDY AREA
• The plain of Bou-Areg is located on the Mediterranean coast
of northeastern Morocco (35.1686° N, 2.9276° W)
• The coastal plain of Bou-Areg covers 160 km2.
• It is bordered to the northwest and the west by the massifs
of Gourougou and Ibn-Bou-Ifrour, to the east by the chains of
Kebdana, and to the north by the lagoon of Bou-Areg.

10. continued…..
• The waters of the lagoon are very salty, with average
salinity levels between 37 g/l and 42 g/l during the past
thirty years.
• The surface water consists of Selouane River, which
crosses the central plain of Bou-Areg and flows into in
the Bou-Areg lagoon.
• The climate of the study area is semi-arid to humid.
Average annual precipitation is around 346 mm.

11. GEOLOGICAL FRAMEWORK OF THE AQUIFER
The aquifer profile was divided into three classes using 30
boreholes logs
Formation I constitutes the upper layer, and is formed by
thin materials, such as silts, clayey silts, limestones and
calcareous tufa, which may contain gravels.
Formation II constitutes the bottom of the aquifer reservoir
and is formed by silts to gravels, coarser elements, pebbles,
gravels of volcanic or sedimentary origin and sands.
Formation III (reached only by some boreholes) is formed
by the marls. This formation constitutes the substratum of
aquifer.

12. HYDRODYNAMIC CHARACTERIZATION
• Estimation of the hydraulic conductivity and specific storage
of the unconfined aquifer system of Bou-Areg plain were
obtained from the analysis of pumping tests.
• The Neuman and Theis solutions are applied to obtain the
values of hydraulic conductivity, storage coefficient, and the
ratio of horizontal and vertical hydraulic conductivity .
• The measured hydraulic conductivity values range from
1.1 × 10−6 to 5 × 10−4 m/s.
• The vertical anisotropy varies from 3 to 15.
• The storativity values lie between 1.30 × 10−4 and 3 × 10−2.

13. ESTIMATION OF RECHARGE
The study area is an unconfined aquifer, and is therefore
capable of receiving direct recharge flux from precipitation.
The water balance technique is used to estimate the
recharge:
R=P−Q−RET−ΔW
where R is recharge (mm/year), P is precipitation (mm/year), Q is net runoff
(mm/year), RET is real evapotranspiration (mm/year), and ΔW is change in
soil moisture storage that is considered negligible in semi-arid regions.
Total recharge from precipitation and irrigation water
infiltrated to the aquifer is about 9.2 × 106 m3/year, on
average.

14. MODEL DEVELOPMENT
Numerical Methods
The saturated flow modeling is based transient on the three-
dimensional Darcy equation
where Kxx, Kyy, Kzz are the hydraulic conductivity along the x, y
and z axes ,
h is the hydraulic head,
Q represents the source/sink terms, and
S is the specific storage

15. continued…
• Groundwater flow in the Bou-Areg was simulated using
the modular groundwater flow model MODFLOW-2000.
• The partial-differential flow equation can be approximated by
replacing the derivatives with finite differences.
• MODFLOW-2000 solves the finite-difference equations
simultaneously. It accounts for groundwater flow between cells
and external sources or sinks of water, such as stream–aquifer
hydraulic interaction, aquifer recharge, or groundwater
withdrawal by wells.

17. SPATIAL AND TEMPORAL DISCRETIZATION
• To simulate groundwater flow to Bou-Areg aquifer,
a regularly spaced, finite-difference model grid was
constructed and rotated.
• Each cell is 500 m x 500 m in the horizontal plane. The grid
consists of 28 rows and 53 columns.
• The thin silt constitutes layer one with a thickness between
5 m and 11 m.
• The formation of silts, sand and gravels was included in
layers two and three, which are each 25 m thick.

18. BOUNDARY CONDITIONS
Grid, model boundary conditions and recharge zones of Bou-Areg aquifer system

19. RESULTS AND DISCUSSION
Model Calibration
The Bou-Areg groundwater model was calibrated in two
steps:
• Steady state calibration during 1990 and
• Calibration to transient conditions from 1990 to 2002.
The traditional trial-and-error method was used for the
steady state and transient calibration, whereas the
inverse modeling method was only used for steady state
calibration.

20. STEADY STATE FLOW
• Horizontal hydraulic conductivity values obtained from
inverse method by pilot points are interpolated by the
inverse distance weighted (IDW) method.
• Most values of the hydraulic conductivity range between
2 × 10−6 and 0.1 × 10−4 m/s for layer one and between
5.6 × 10−6 and 0.3 × 10−3 m/s for the layers two and three.
• These obtained values of hydraulic conductivity are in
tight agreement with the lithological boreholes data.

21. Estimated hydraulic conductivity (in 10−4 m/s) of layer 1 of Bou-Areg aquifer (obtained
from PEST optimization method)

22. Estimated hydraulic conductivity (in 10−4 m/s) of layer 2 of Bou-Areg aquifer (obtained
from PEST optimization method).

23. Scatter plot of observed weighted versus computed weighted values of
head for steady state calibration flow model (1990).

24. WATER BALANCE
The water budget of the entire aquifer obtained from the
groundwater flow model

25. TRANSIENT FLOW
• The transient calibration could be achieved using all of the
parameter values from the steady state calibration and
adjusting only storage coefficients and specific yields.

26. SENSITIVITY ANALYSIS

27. MODEL VERIFICATION

28. PARTICLE TRACKING SIMULATION
• The particle-tracking
simulation was done for
1990 steady state
conditions using
MODPATH incorporated
in GMS.
• In the period of 150
years, pollutants released
from the contact area
between the Gareb and
Bou-Areg plains can be
tracked 9216.5 m.

29. CONCLUSIONS
• The results of the model calibration show reasonable
agreement between observed and calculated water
levels for the observation wells.
• The horizontal hydraulic conductivity, obtained from the
model calibration, ranges between 3 × 10−4 and
2 × 10−6 m/s.
• The model aquifer is more sensitive to recharge than to
hydraulic conductivities and storativity.
• The MODPATH results show that migration of pollutants
from the upstream of the aquifer towards the lagoon is
very slow and could attain the Bou-Areg lagoon in 1000
years.

30. THANK YOU