numerical analysis of groundwater-flow and solutetransport under skimming well

7/31/2019 Numerical Analysis of Groundwater-Flow and SoluteTransport under Skimming Well

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Science, Technology & Development Vol. 30, No. 1 (January-March) 2011

Science, Technology & DevelopmentVol. 30, No. 1 (January-March) 2011Section B: Biological & Agricultural Research

Numerical Analysis of Groundwater-Flow and Solute-Transport under Skimming Well

Zakir Hussain Dahri1, Ata-ur-Rehman Tariq2,

Bashir Ahmad1, Ghulam Ali1 and Shakil Ahmad3

The study employed three-dimensional finite-difference groundwater flow (MODFLOW) and

transport model (MT3D) to model the behavior of groundwater flow and solute transport

beneath a skimming well. Data regarding aquifer characterization, watertable elevations and

pumped-water quality for a double-strainer well located at Haroon farm, Khairpur, was used

to calibrate and verify the flow and transport models. A number of pumping-scenarios wereformulated to evaluate the performance of skimming well.

The results indicated that, for a shallow layer of fresh water, the pumped-water salinity

increases linearly with pumping time. The rise in salinity is more pronounced for a higher

well-discharge and greater well operational factor. Although the drawdown in the well

achieved steady-state condition within 0.75 day, but the quality of pumped-water continued to

deteriorate. The rise of saline-water mound also kept rising with pumping time. The study

revealed that, in the aquifers where salinity-difference of fresh and saline water is small,

upconing not only occurs at a rapid rate but also to a greater height. Furthermore, it was

observed that, in case when the saline-water cone had already intruded the fresh aquifer, any

reduction in well-discharge could neither ensure salt-free water-supply nor any fall in the

already raised saline-water mound could be observed; instead they kept rising on but at a

sluggish rate.Unlike the common belief and field observations, intermittent pumping could neither

control the upward movement of interface, nor was any improvement in quality of pumped

water observed; instead they remained more or less stagnant, nevertheless the technique is

helpful to avoid aquifer deterioration. Such contrast in the results of this study is mainly

attributed to the fact that MODFLOW/MT3D models do not take into account the density of

fresh and saline water, which is an important parameter in suppressing the upconing.

Therefore, it was believed that the models tend to over-estimate the solute movement and

quality of pumped water. Modification is suggested to be incorporated in MODFLOW/MT3D

programs.

INTRODU

ROUNDWATER is a hidden, but impor-

tant component of the hydrological cycle.

In many regions, the groundwater

resource is so huge that its occurrence and

hydrological significance cannot be overlooked in

the planning and management of water-resources.

1

CTION

Pakistan Agricultural Research Council, Islamabad, Pakistan.2 Centre of Excellence in Water Resources Engineering, University of Engineering and Technology, Lahore,

Pakistan.

12

3 National University of Science and Technology, Islamabad, Pakistan.

G


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13

Groundwater represents an exploitable resource,

provided its quality and depth permit its

economical development and optimal use. It can

serve as a source of dependable, flexible and

demand-based irrigation-water for boosting

agricultural production.

Groundwater has played a major role in

increasing the overall cropping-intensity inPakistan from about 63% in 1947 to 120% in

2010. Over 40% of the total irrigation-water at

farm-gate and over 70% of the total drinking-

water are currently met from groundwater. The

spatial distribution of groundwater-abstraction in

the Country is also highly variable. In some

regions, annual pumpage has surpassed the safe

annual yields and so watertables are declining.

There are however many areas where great

potential for groundwater-development still

exists. This requires controlled and optimal

groundwater-pumpage. Recent estimatesregarding availability and use of fresh

groundwater indicate that the resource has been

heavily exploited and is at the brink of

exhaustion. This large-scale indiscriminate,

uncontrolled and unregulated abstraction ofgroundwater has changed the policy-approach

from the development of groundwater to its

management. The major policy-issues now relate

to environmental sustainability and long-term

availability. The standard approaches to managegroundwater often require monitoring of

groundwater-aquifers, establishment of formalwater-rights and regulation mechanisms, so as to

bring the groundwater abstraction within

sustainable levels.

Sustainable groundwater development in the

Lower Indus Basin (LIB) of Pakistan is seriously

hampered by quality-problems, due to intrusion

of brackish groundwater from adjacent areas,

upconing of underlying saline water, backward

intrusion of seawater in coastal areas and disposal

of drainage effluents. The native groundwater

was originally brackish, because of the marine

origin of the underlying geologic formation. Thisnative brackish groundwater is now overlain by a

fresh-water layer of varying thicknesses, due to

seepage from conveyance system and percolation

from irrigation and precipitation. The depth of

that fresh-water layer is greater near the

recharging sources and decreases away from the

line source. Subsurface investigations in the LIB

show that, for about 4.66 m. ha (75.77 per cent)

of the irrigated area the underlain groundwater is

classified as hazardous and saline (TDS > 3,000

ppm), while for another 8.94 per cent of the area,

the groundwater is marginal (TDS from 1,500 to

3,000 ppm) and for only 15.29 per cent of the

area is the groundwater fresh (TDS < 1,500 ppm)(Zuberi and Sufi, 1992). The areas with fresh

groundwater thickness less than 45 m can be

termed as critical areas, where sustainable

groundwater development requires careful

thinking in the selection, design and operation of

irrigation-wells. Moreover, the spatial distribution

of its abstraction is highly variable. In some

regions, annual pumpage has surpassed the safe

annual yields and the watertables are declining.

There are however many areas where great

potential for groundwater-development still

exists. This requires controlled and optimalpumpage of groundwater.

Under natural conditions, the fresh and saline

groundwater layers are usually in a state of

dynamic equilibrium. Extraction from fresh-water

layer, however, disturbs that equilibrium and,

consequently, the adjacent or underlying saline

groundwater starts moving upward in the shape of

a cone. This phenomenon is termed as saline-

water upconing. The extent and intensity of

upconing is greatly dependent on pumping-rates,

degree of penetration, thickness of fresh

groundwater layer, quality of water-withdrawalzone, location of the fresh and saline water

interface, vertical hydraulic conductivity, and

extent of aquifer-recharge. Moreover, hydro-

dynamic-dispersion phenomenon significantly

affects the solute transport towards the well

(Kemper, et al., 1976; Mirbahar, et al., 1997 and

Sufi, et al., 1998). Therefore, many engineers and

researchers are of the view that the overlying

fresh groundwater must be abstracted in such a

way that the underlying saline groundwater is not

disturbed (Chandio and Chandio, 1992, Awan

1990). Properly designed and operated skimming-wells may offer an economical and sound

alternative over conventional deep wells, where

native saline groundwater exists at about 30 m

below ground-surface (Chandio and Chandio,

1992). However, very few researchers in Pakistan

have studied the upconing-phenomena beneath a


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skimming well, and only limited information is

available in this regard (Sufi, 1999). The present

research study is aimed at analyzing the

groundwater-flow and solute-transport

phenomena under a skimming well, and

evaluating the impact of operational strategies on

quality of pumped water and upconing of

underlying brackish water. The research outcomewill be helpful to formulate guidelines for

sustainable development of fresh groundwater in

the areas where a shallow layer of fresh water

overlies the native brackish groundwater.

THEORETICAL CONSIDERATIONS

The theoretical considerations regarding

fresh-water skimming can be divided into two

categories i.e. Immiscible and Miscible Theory.

The first formulation is less rigorous and less

realistic, assuming the application of DF

assumptions, and existence of sharp interfacehaving abrupt transition from a relatively fresh to

highly saline water. The second formulation is

more realistic but, at same time, more

complicated assuming change in salinity from salt

water to fresh water under the hydrodynamic

dispersion, and existence of the transition-zone of

relatively large thickness between the fresh and

salt water. The transition zone is characterized by

a gradual change in the concentration of water

from that of fresh water to lower saline water.

Both these formulations are, however, important

and are discussed in some detail in the followingsections.

Physics of groundwater flow and solute-

transport mechanisms

Groundwater flow is described by Darcys

law, which states that flow-rate is proportional to

the hydraulic gradient, where the constant of

proportionality is hydraulic conductivity

describing the hydraulic properties of the

medium. The hydraulic conductivity varies in

space in a manner that, to large degree, is tied up

with the spatial variation of the geologicalproperties. Darcys law is a well-proven

relationship, which has frequently been used for

quantitative assessments of groundwater flow.

Mathematically, it can be formulated in a tensor

notation as:

t

hSq

x

hK

xs

j

j

i

=+

; i,j = 1,2,3(1)

where q is specific discharge (LT-1

), Kij is

hydraulic conductivity (LT-1

), h is hydraulic head

(L), xi is space coordinate (L).

Combination of Darcys flow-equation withcontinuity-equation, which represents the

conservation of fluid mass, yields the following

partial differential equation describing the three-

dimensional movement of groundwater through

porous media:

szyx qz

hK

zy

hK

yx

hK

x+

+

+

t

hS s

=

....(2)

where Kx , Ky , Kz are the values of hydraulic

conductivity along x, y, z coordinates, which are

assumed to be parallel to the major axes of

hydraulic conductivity (LT-1); qs is the fluid

sink/source term or volumetric rate, at which

water is added or removed from the system per

unit volume of aquifer; Ss is the specific storage

or volume of water released from storage in a unit

volume of aquifer per unit decline in head.

While flow of groundwater is governed by

Darcys law, the transport of solute in a

groundwater-system is controlled by manyfactors. The most important mechanisms affecting

the transport of solute in a porous medium are:

advection or convection, hydrodynamic

dispersion, and various chemical reactions and

decay phenomena, which may be regarded as

source-sink phenomena for the solute. All these

phenomena cause changes in the concentration of

solute in the flowing fluid. In general, variations

in solute-concentration cause changes in liquids

density and viscosity. These, in turn, affect the

flow regime (i.e. velocity-distribution) that

depends on these properties. The partialdifferential equation describing the three-

dimensional transport of contaminants in

groundwater (Javendal et al., 1984) is written as

follows:

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( ) =

++

= N

k

ks

s

i

ii

ij

i

RCq

Cvxx

CD

xt

C

1

(3)

where,

C = concentration of contaminants dissolved in groundwater, M/L3

t = time, Txi = distance along respective cartesian-coordinate axis, L

Dij = hydrodynamic dispersion coefficient, L2/T

vi = seepage or linear pore-velocity, L/T

qs = volumetric water-flux per unit aquifer volume sources (+) & sinks (-), T-1

Cs = concentration of the sources or sinks, M/L3

= porosity of the porous medium, dimensionlessRk = chemical reaction sink/source term, representing the rate of change in solute mass of a

particular species, due to N chemical reactions.

METHODOLOGY

Data collection: The skimming well selected forthis study is located at Haroon Farm near Gambat

railway station, district Khairpur, and is part of

Khairpur SCARP area. During the past few years,

the area has been plagued by waterlogging and

salinity problems as the watertable generally

fluctuates between 0.5 and 1.0 m. The occurrence

of highly saline groundwater at depths ranging

from 25 to 35 m has restricted the installation of

large-capacity tubewells. The skimming-well

consists of two strainers, each bored at 12.2 m

apart and discharging 0.4265 cfs. Specification

and borehole lithology, along with profilicdistribution of salinity, is described Tables 1 and

2, respectively. The penetration of both the bores

was 19.82 m, with 10.67 m strainer surrounded

by coconut fiber. The internal diameter of the

blind pipe and strainers are 10.2 and 20.4 cm,

respectively. Both the bores are connected withcentrifugal pump, driven by 15 HP diesel engine.

The data-requirements of PMWIN are

categorized into time-constant and time-variant

data. Time-constant data include aquifer

geometry (areal and vertical distribution of

subsurface strata, aquifer-thickness, etc) and

hydraulic parameters (hydraulic conductivities,

effective porosity, specific yield, specific storage,

etc). Time-variant data are recharge, pumping,

evapotranspiration, groundwater levels, ground-

water quality, etc. Time-constant data was taken

from previous studies, such as Lower IndusReports, ACE 1997, ACE 2001, while time-

variant data for this skimming well was collected

by Drainage and Reclamation Institute of

Pakistan, Tando Jam.

Table-1: Specification of piezometers and boreholes dimensions.

Piezometer

No.Distance From the

Centre of Well (m)Diameter

(cm)Blind Pipe

(m)Strainer

(m)Total Length

(m)

PZ1 9 3.18 3.04 1.52 4.56

PZ2 26 3.18 3.04 1.52 4.56PZ3 77 3.18 3.04 1.52 4.56

PZ4 168 3.18 3.04 1.52 4.56

Bore-1 0 10.2 9.15 10.67 19.82

Bore-2 0 10.2 9.15 10.67 19.82

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16

Table-2: Soil lithology and salinity distribution in both the boreholes of the skimming well at

Haroon farm, Khairpur.

Bore-1 Bore-2Depth (m)

Soil Texture Salinity (ppm) Soil Texture Salinity (ppm)

0 3 silty loam 896 silty clay loam 768

3 6 very fine sand 960 fine sand 838

6 9 medium sand 1088 very fine sand 960

9 12 coarse sand 1152 very fine sand 1280

12 15 coarse sand 1024 Medium sand 1152

15 18 coarse sand 1152 coarse sand 1152

18 21 coarse sand 1024 coarse sand 1216

21 24 - - - 1152

24 27 - - - 1568

27 30 - - - 3410

60 - - - 8064

Model formulation: Numerical modeling is

commonly used for simulating the complex

groundwater systems, possessing non-linear

features, that may not be solved through

analytical approaches. It permits prediction of the

response of an aquifer to applied stresses and

evaluates alternative suggestions for its use. Of

the number of groundwater-models available, the

PMWIN (Chiang and Kinzelbach, 1996), which isa complete simulation system for modeling

groundwater-flow, with MODFLOW (Harbaugh

and McDonald, 1988, 1996), and solute-transport

processes with MT3D (Zheng, 1996), was

selected for this study. Using these models,

different scenarios can be developed and run to

study fluctuations of water-table and upconing

phenomenon in fresh and saline groundwater

systems.

The important steps for model-formulation

are: spatial and vertical discretization of the grid-

system; setting up initial and boundary

conditions; temporal discretization; specifying

aquifer-parameters; and assigning external stress

packages, like wells, rivers, recharges, evapo-

transpiration, advection, dispersion, etc.

The mesh was generated in a non-uniform

manner, the mesh spacing being smaller near the

well and larger away from the well at a constant

ratio of 1.4. Moreover, the grid spacing was kept

small enough to avoid any artificial oscillation

and numerical dispersion. The grid size varied

from 3.05 m to 11.72 m. The total grid domain

(417.55 m 417.55 m) was divided into 59columns and 59 rows. The generated model grid-

system is shown in Figure 1, while Table-3

describes spatial discretization of the model grid.

The aquifer was discretized into 8 layers

(Table-4) on the basis of soil-texture, salinity-

distribution, length of blind pipe and well-screen,

expected drawdown and ease in simulation, to

achieve the planned objectives.

The average water-table elevation within the

model domain, prior to pumping was 119.136 m.

This head was specified as the initial hydraulic

heads, throughout the domain. The spatial and

vertical distribution of initial concentration was

estimated from the bore log of the well and

piezometers installed around it. The heads and

concentrations specified as the starting values, at

the boundaries, were set to be constant, i.e. did

not change with time, throughout the simulation

period. A no-flow boundary was used at the base

of the system, representing the assumed zero

movement of water into or out of the relatively


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Constant head and constant concentration boundary

17

Figure 1. Spatial discretization of the grid system

Table-3: Spatial discretization of the grid system

S. No. No. of Columns No. of Rows Grid Size (m x m)

1 7 7 3.05 x 3.05

2 10 10 4.27 x 4.27

3 16 16 5.98 x 5.98

4 14 14 8.37 x 8.37

5 12 12 11.72 x 11.72

Total 59 59 417.55 x 417.55

Table-4: Vertical discretization of model domain

LayerNo.

Layer

Thickness

(m)

Cumulative

Thickness (m)Top

Elevation

Layer (m)

Bottom

Elevation

Layer (m)

Concentration

(ppm)

1 9.15 9.15 120 110.85 1000

2 5.335 14.485 110.85 105.515 1100

3 5.335 19.82 105.515 100.18 12304 8.18 28 100.18 92 1400

5 5 33 92 87 2000

6 7 40 87 80 3500

7 20 60 80 60 7000

8 60 120 60 0 10000


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18

impermeable bedrock. In case of transport-model,

a constant concentration-boundary was assigned

at the lowest layer. The skimming-well was

hydraulically simulated as a well pumping from

an infinite aquifer, where head and solute

concentration in the cells beyond the radius of

influence will remain constant and equal to the

initially determined values.The temporal discretization include the time-

unit and the number of stress-periods, time steps

and transport steps in each time step or stress

period. In MODFLOW, the simulation-time can

be divided into stress periods i.e., the time-

intervals during which all external excitations or

stresses are constant which, in turn, may be

divided into time-steps. In MT3D model, each

time step is further divided into smaller time-

increments, called transport-steps, because the

length of a time step used for a head solution may

be too large for a transport-solution. The study

used different scenarios and simulation-time,

number of stress-periods; and the number of time-

steps in each stress-period varied for each

scenario. The length of transport-step in each

simulation, however, was set to be calculated

automatically by the model itself.Site-specific data, regarding aquifer

parameters, is not available. However, as per

elaborated pumping tests performed around the

study area during the regional groundwater

investigations (during the Lower Indus Project),

some estimates are available. These were refined

during model calibration and verification. The

final values used for this study are summarized in

Table-5.

Table-5: Parameter values used for simulation

S. No. Parameter Value

1 Initial Watertable Elevation 119.136 m.

2 Initial Concentration As per bore log (Table 4)

3 Horizontal Hydraulic Conductivity(Kh ) 20.16 m/d

4 Vertical Hydraulic Conductivity (Kv ) 6.72 m/d

5 Anisotropy Ratio (Kh / Kv ) 3

6 Effective Porosity () 13 %7 Specific Yield (Sy) 7 %

8 Specific Storage (Ss) 110-5

9 Recharge 361 mm/year

10 Concentration of Recharge Flux 550 ppm

11 Advective parameters for particle placement/movement

As per users manual

12 Longitudinal Dispersivity 8 m

13 Ratio of horizontal transverse dispersivity tolongitudinal dispersivity

0.33

14 Ratio of vertical transverse dispersivity to

longitudinal dispersivity

0.1

15 Effective molecular diffusion coefficient 0.00139


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19

There are two well-points acting as double-

bore skimming-well for this particular study. The

bores are located 12.2 m apart. The length of

blind pipe for both the bores is 9.15 m, which is

represented by top model layer (layer-1). The

length of screen for both the well-points is 10.67

m, which is divided into two model layers

(layers-2 and 3). The designed discharge for eachbore-point is 43.5 m3/hr (0.4265 cfs).

MODFLOW assumes uniform distribution of

water entering into the well-screen throughout its

length, which is not realistic as, in actual field

conditions, more water enters into the well-screen

from the top and gradually decreases towards the

bottom. Li (1954) and Soliman (1965) analyzed

the flow to a well and showed that the entrance-

velocity in the upper 10 % of the well-screen was

70 times that in the lower 10% in an ideal aquifer.

If the open area of the screen is constant

throughout, we can approximate the distributionof flow rate into the screen. Based on Li and

Solimans statement, it was found that about 77%

of the water enters from the upper half of the

well-screen (layer-2) and the remaining 23% from

the lower half of the screen (layer-3). Thisdistribution of water-entry to the screen was used

to determine the quality of pumped water.

Model calibration and verification: Calibration

is an inverse modeling process, in which certain

model input-parameters are adjusted until model-

output parameters/dependent variables match the

field-observed values to an acceptable degree.The data regarding watertable elevations,

pumped-water salinity, and profilic distribution of

groundwater-quality were used to calibrate both

the flow (MODFLOW) and transport (MT3D)

models. Simple manual trial-and-error method

was used to sequentially calibrate both the

models, against the data observed after 15.67 hrs.

of continuous well-operation, because at this

stage the well-drawdown tends to achieve steady-state condition.

Model verification is a process in which the

calibrated model is shown to be capable of

reproducing a set of field-observations

independent of that used in model calibration.

The calibrated models were verified against the

observed data at different pumping-periods

(10.92, 13.67 and 17.67 hrs.) through an

interactive procedure of sequential calibration and

recalibration, until the models were capable of

reproducing all the data-sets used for model

verification, including the one used for modelcalibration. Besides graphical comparison of

observed and simulated values, some statistical

measures of goodness-of-fit were also used for

qualititative comparison and assessment of the

model calibration. Figures 2 (c-f) and 3 presentcomparison of observed and simulated hydraulic

heads and pumped-water salinities at different

pumping periods, while Table 6 shows statistical

analysis of model calibration and verification.

There is good agreement between the observedand simulated values, and so the models are

capable of reproducing the field observations atany specified time.

Table-6: Statistical analysis of model calibration and verification.

Simulation

Time(hrs)

Liner Correlation

Coefficient(r)

Root Mean Squared

Residual Errors (RMS)

Maximum

Error

(ME)

Modeling

Efficiency

(EF)

MODFLOW for Hydraulic Heads

10.92 0.522064534 0.042077524 7.92E-08 0.98307

13.67 0.5322365 0.033377298 9.04E-11 0.98956115.67 0.522615471 0.044022472 1.63E-07 0.981527

17.67 0.513007667 0.055299168 2.09E-06 0.970306

MT3D for Pumped Water Salinity

1.0 - 17.67 0.99313918 6.091711582 4.96 0.962462


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Calibration (15.67 hrs)

118

118.2

118.4

118.6

118.8

119

119.2

119.4

0 50 100 150 200

Distance from centre of wells (m)

Piezome

trichead(m)

Initial

Obseved

Simulated

Verification (10.92 hrs)

118

118.2

118.4

118.6

118.8

119

119.2

119.4

0 50 100 150 200


Piezom

etrichead(m)

Observed

Simulated

c. Hydraulic heads after 15.67 hrs of operation d. Hydraulic heads after 10.92 hrs of operationVerification (13.67 hrs)

118

118.2

118.4

118.6

118.8

119

119.2

119.4

0 50 100 150 200

Distance from Well (m)

PiezometricHead(m

.

Observed

Simulated

Verification (17.67 hrs)

118

118.2

118.4

118.6

118.8

119

119.2

119.4

0 50 100 150 200


Piezometriche

ad(m)

Observed

Simulated

e. Hydraulic heads after 10.92 hrs of operation f. Hydraulic heads after 17.67 hrs of operationFigure 2 Observed and simulated hydraulic heads at various pumping times.

Calibration and verification

1100

1130

1160

1190

1220

1250

1280

0 2 4 6 8 10 12 14 16 18 20

Pumping time (hrs.)

Pumpedwatersalinity(ppm)

Observed

Simulated

Figure 3. Observed and simulated pumped water salinities at various pumping times

20


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RESULTS AND DISCUSSION

It is imperative to maintain the original

dynamic equilibrium between the layers of fresh

and brackish water, if groundwater is to be

developed on sustainable basis. Thus, there is an

important need to investigate the mechanics of

solute-transport in response of fresh groundwater

withdrawal. For this purpose, an effort has beenmade to replicate the actual field-conditions of

pumpage by the numerical model, so that one

may have a close look at the flow and solute-

transport phenomena under a skimming well.

Model simulations were performed for three

different scenarios.

The 1st scenario represents an ideal situation

where the well is operated intermittently and

exact time of its operation and closure is

specified. The total simulation-time is divided

into 60 stress-periods, each representing the timein which the well was either on or off. The main

problem associated with this type of scenario is

small simulation period, as PMWIN can simulate

a maximum of 80 stress-periods while in actual

field-conditions the majority of wells are operated

on daily basis (operated for some time and closed

for rest of the day). In this way, the maximum

simulation-time can be only 40 days. Anyway, to

get an idea of the situation, simulation time of

scenario-1 was kept 30 days, having 60 stress-

periods.

The simulation time of scenario-2 was alsokept 30 days, but the entire time was considered

single-stress period, in which the well was

operated continuously at a reduced discharge i.e.

designed well-discharge was multiplied with the

well-operational factor. The total volume of

pumped water for both the scenarios, however, is

the same. Although, this scenario is not realistic,

but it does provide an opportunity to simulate the

models for a desirable time-period. This scenario

was designed just to compare its outcome with

the previous one. Figure 4 depicts the pumping

schedule and well discharge for both thescenarios, while pumped-water quality is

presented in Figure 5.

0

400

800

1200

1600

2000

0 5 10 15 20 25 30

Pumping Duration (days)

WellDischarge(m3/day)

Scenario-1 Scenario-2

Figure 4. Graphical representation of well discharge for scenarios - 1 & 2

21


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1100

1200

1300

1400

1500

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30


PumpedWate

rQuality

(ppm)

Scenario-1 Scenario-2

Figure 5. Graphical representation of pumped-water quality for scenarios - 1 & 2

The pumped water quality for both the

scenarios clearly indicates insignificant difference

between their outcomes. This insignificant

difference encouraged us to design and adopt 3rd

scenario, by combining the 1st and 2nd scenarios.

This scenario can employ longer simulation

period and very closely matches the actual field-

conditions, where temporal variation of pumping

is quite high, which is mainly dependent on

availability of canal water, crop water-

requirements and climatic conditions. The total

simulation time of 565 days is divided into 10

stress-periods. An increased well-operation factor

is assigned for periods during which canal water

supplies are very low and the demand is high. The

well is kept closed for periods during which either

there was no crop in the field or, because of

heavy rainfall in monsoon season, crops do not

require supplemental irrigation. The pumping

schedule, well-discharge and quality of pumped

water for 3rd scenario are presented in the

Figure 6, whereas temporal variability in the

aquifer salinity at different observation-depths is

presented in Figure 7.

0

100

200

300

400

500

600

0 100 200 300 400 500 600


W

ellDischarge(m3/day)

0

500

1000

1500

2000

2500

3000

Pum

pedWaterQuality(ppm)

Well Discharge Pumped Water Quality

Figure 6. Temporal variation of well discharge and pumped-water quality for scenario-3

22


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500

1500

2500

3500

4500

5500

6500

0 100 200 300 400 500 600


Salinityrise(ppm)

4.575 m

11.82 m17.15 m

23.91 m

30.5 m

Obsevation

depth from

ground

surface

Figure 7. Temporal variation of aquifer salinity at different observation depths

Figures 5, 6, and 7 further indicate that, for

shallow fresh-water layer, the salinity rise is alinear function of operational time. The rise in

salinity is more pronounced for higher well-

discharge or greater well-operational factor. The

results also indicated that the design of the

selected skimming-well is faulty for the given

aquifer properties and profilic distribution of the

salinity within the aquifer. Zuberi and McWhorter

(1973) suggested that, for aquifer properties in

Pakistan, the discharge of individual skimming

well should be in the range of 0.1 to 0.3 cfs,

whereas the discharge of the selected skimming

wells was 0.853 cfs, which seems too high.Moreover, there is very little available fresh-

water cushion below the well-screen. The

optimum well-penetration ratio of the site under

study was found to be 23 %, against the present

penetration of 33%.

It can also be observed from Figure 7 that,despite the fact that penetration of well-screen

started from the depth of 9.15 m to 19.82 m from

the ground surface, the salinity of the upper fresh

layer above 9.15 m depth was increased, whereas,

it is quite clear that flow in that portion is radially

downward. Moreover, surface recharge alsooccurs in that layer, so the question is what

prompted the salinity to increase there. Salinity

rise in the top layer above well-screen may be

attributed to the process of molecular diffusion

and transverse dispersivity, which generally

propagate in the direction orthogonal to the flow.

Effect of drawdown: The drawdown for

different discharges is shown in Figure 8, whichindicated that, for each well discharge, there

existed a limit on drawdown at which the

discharge and recharge balanced each other and

ultimately steady-state condition was achieved.

Figure 9 shows the spatial extent of upconing at

various observation depths after 335 days of

simulation. It can be observed from this figure

that the radius of influence of the saline-water

mound at all observation depths is about 150 m,

which is roughly half of the radius of influence of

the cone of depression. Therefore, it can be

approximated that if the well is to be replaced byanother well, due to salinity rise under the well,

then the new well must be 300 to 500 m away

from the previous one.

But, unlike the steady-state condition of

drawdown, the quality of pumped water

continued to deteriorate and also the rise of salt-water mound, which kept rising on with pumping

time. It is commonly believed that smaller

upcoming occurs for smaller drawdowns and that,

to limit the upconing, it is essential to limit the

drawdown. Nevertheless, it is not directly

proportional to the drawdown. In fact, drawdownitself is a dependent variable and is the function

of well-discharge and certain aquifer properties,

and can be limited by decreasing the well-

discharge; whereas upconing is related with many

factors, including those which affect drawdown.

Therefore, relating it with drawdown or any other

factor alone is not correct.

23


13/17


118.90

118.95

119.00

119.05

119.10

119.15

119.20

-300 -200 -100 0 100 200 300

Distance from Well (m)

Watertab

leElevation

(m)

well Q = 522 m3/d

well Q =348.7 m3/d

well Q = 261 m3/dWell is closed

Figure 8. Cone of depression for different well-discharges after 335 days of simulation

After 335 days of simulation

0

2500

5000

7500

10000

12500

15000

-300 -200 -100 0 100 200 300

Distance from the centre of wells (m)

EC(ppm)

118.0

118.2

118.4

118.6

118.8

119.0

119.2

Hydraulichead(m)

4.575 m

11.82 m

17.15 m

23.91 m

30.5 m

Observation

depth from

ground

surfacecone of dipressionupconing

Figure 9. Rise of salt water mound at various observation-depths, after 335 days of pumping

Bower (1978) declared that, if the

groundwater withdrawal from the coastal aquifer

exceeds the safe yield and the water-levels

decline, the salt-water would rise 40 m for everymeter of drop in the watertable. However, during

this pumping-scenario, the maximum drawdown

only 0.22 m occurred against the discharge of 522m3/d, but the fresh and salt water interface was

raised by 19.18 m in 335 days of well operation,

which is 87 times greater than the drawdown.

The starting salinity-level of the fresh and saline

interface is considered to be 3,500 ppm. The large

difference between Bowers statement and the

simulated results of this scenario can be attributed

to the fact that his statement is primarily based onthe Ghyben-Herzberg relation, which does not

take into account the vertical component of

groundwater-flow; hence, it underestimates theextent of upconing. Furthermore, the small

difference between the salinities of fresh and

saline waters of the aquifer under study, viz-a-viz

seawater, also causes the saline water to move

24


14/17


25

more quickly. It reveals that in the aquifers where

salinity-difference of fresh and saline waters is

small, upconing not only occurs at a rapid rate,

but also to a greater height.

Effect of discharge reduction: Obviously, the

higher discharges induce greater upconing.

Therefore, if the well discharge is low, right from

the beginning of pumping, then the rise ofinterface will be slower. What of the situations

where rise of interface has already taken place

due to higher well-discharge: can any reduction

in well-discharge suppress the upcoming? This

question is answered in the following paragraphs.

At the initial stage of pumping, much of thewater contribution is from top of the screen. As

the pumping continues, the area and depth of the

propagation of streamlines are increased. At the

interface, the streamlines tend to flow tangentially

to the top of interface. All other streamlines,

which originate below these ones, will bring salt

water to the well, thereby deteriorating the quality

of pumped water and cause rising of salt-water

mound. Now if the salt water has already intruded

into the fresh-water zone, reduction in well

discharge will neither ensure salt-free water

supply nor can any fall in the raised mound of salt

water be observed. Figs. 7 and 8 confirm such a

statement.

These figures showed that, even when the

well-discharge was halved from 522 m3/d to 261

m

3

/d, no improvement in pumped water qualityand recession of salt-water cone could be

observed; instead they kept on rising. This is

because, even though the discharge was reduced,

the flow towards the well continued through the

same streamlines but at a sluggish rate. The

streamlines, which originated from salt-water

zone, will vanish eventually only when the well is

stopped and groundwater is fully recovered, but

the position of interface would be higher than the

initial one. Therefore, when the well is started

next time, it might have less available thickness

of fresh water below the well-screen and chances

of upconing will be greater.

Effect of intermittent pumping: Generally

speaking, when a partially penetrating well

continuously discharges water from an

unconfined aquifer, in which fresh water is

underlain by salt water and is separated by a well-

defined interface located within the reach of

streamlines (generated due to pumping of well

from the fresh water zone), then ultimately a time

must come when the salt-water of interface will

enter the well-screen, causing deterioration of

pumped water quality. It is commonly believed

and also has been witnessed during the field

studies (Saeed, et al., 2002 and Ashraf, et al.,

2001) that intermittent pumping may control theupward movement of fresh and salt-water

interface. The well discharge and operational

factor, however, must be low enough, not

allowing the streamlines to reach the interface.

During the periods of well-closure, the generated

streamlines will tend to stabilize but the time

required for their complete stabilization may be

quite long. At complete stabilization of

streamlines, the well may be turned on again for a

certain safe period, at which regenerated

streamlines are not allowed to reach the interface.

This safe period of pumping depends on well-discharge and operational factor, aquifer

characteristics, fluid properties and fresh-water

cushion below the well-screen. There is need to

determine this safe pumping period for a given set

of conditions, but this might be a very difficult

task, and some-times not practicable. McWhorter

(1980) recommended a minimum of 5 times the

length of pumping-period as the rest period for

the well between the two pumping periods.

The well under study was operated

intermittently at varying operation-factors and

was closed for sufficiently long time, during andafter the pumping, to observe the rates of rise and

recession of the salt-water cone. The quality of

pumped water deteriorated linearly as the

pumping continued. During the periods of well-

closure, the cone remained more or less stagnant.

Again from the Figures 4.8 and 4.9, one can

easily judge that the impact of well-closure on

well-water quality and fall of salt-water cone is

negligible, which is totally in contrast with the

actual field conditions. This is well supported by

the fact that, like many other wells in the Region,

this well too discharges an acceptable quality ofgroundwater for some time and is closed for a

sufficient time to allow the fresh groundwater to

regain its original position, through surface as

well as lateral recharge. Moreover, Hafeez et al.,

1986 conducted field studies at Mona project,

Bhalwal, to determine the rates of rise and fall of


15/17


26

the salt-water cone beneath a skimming well.

They witnessed the rise of cone by 30 feet in 15

days of continuous operation at 0.5 cfs discharge,

whereas, the recession of cone was much slower

than its rise: it took 164 days to fall only two-

third (20 ft) of rise. They also showed that about

half of the rise could be made to recede by

stopping the well for a period equal to twice theoperation time.

The negligible effect of well-closure on fall

of the salt-water mound, indicated by the model

results, is clearly because the MT3D model does

not consider the density of water, which creates a

downward potential during well-closure, thereby

forcing the cone to recede. The effect of density

will be large if the salinity difference between

fresh and saline water is greater. It is therefore

believed that the MT3D model might

overestimate rise of cone and can not properly

simulate the intermittent pumping of well.Nevertheless, the model is a valuable tool for

simulating the sources or sinks, which are usually

continuous in nature.

Suggested modification: The results of the

previous scenarios led to the conclusion that the

MT3D model could not accurately simulate the

intermittent pumping, whereas its role in

abstraction of fresh groundwater, while keeping

the rising of underlying salt-water well under

control, can not be overlooked. . Therefore, it was

strongly felt that there is need to introduce some

modification so that the model is able to simulatemore accurately the well that is not continuously

operating. The idea was primary based on the

theory described by Sehni (1973) that the two

fluids (fresh and saline water) are miscible and in

reality, at their contact, they tend to mix with

each other by molecular diffusion and

macroscopic dispersion. Thus, they are not

separated by an oil-water type interface; they do

not constitute distinct fluid phases, and there is no

pressure discontinuity where they are

encountered. Since it is assumed that no pressure

discontinuity exist across the interface, thepressure at any point within the aquifer can be

taken as constant:

P1 = P2 = P3 = = Pn

1gh1 = 2gh2 = 3gh3 = . = nghn

The model domain of this study was divided

into eight layers, each having different salinity.

The above equation can be reduced as,

1h1=2h2 = -------------------- = 8h8

If h1 is considered as the watertable elevation

in the top layer, which is known, then the

hydraulic heads in the lower layers can beapproximated by putting the value of density of

water in each layer in the above equation.

Rubin and Pinder (1977) assumed a linear

relationship between salinity and specific weight

as follows;

= f(1 + c)

where, c is salinity, and is coefficient relatingchanges in density to concentration.

Sufi (1999) determined the densities of saline

waters of varying concentrations. From these

known values of densities, the density of fresh

water and were calculated as 1.00166845gm/cm3 and 0.000000785 respectively. Putting

these values in the above equation, one can

calculate the density of any specified

concentration. The corrected head (h*) becomes

as h* = h/, where h is the head computed byMODFLOW.

The MODFLOW and MT3D programs are

operated in sequence. The MODFLOW generates

head-distribution for all cells for all time-steps.

The MT3D program subsequently uses thesehead-distributions to generate time-variant flow-

velocity field and then determines the solute

movement, due to advection and dispersion. It is

required that MODFLOW and MT3D should be

made to execute recursively over small time-

increments. At each time-increment (including

initial condition), the hydraulic head potential be

adjusted according to the salt-contents in each

cell, as per above equation. This will cause the

hydraulic gradient of cells in lower saline-water

layers to be smaller than for upper fresh-water

layers. The solution then should proceed withtime, solving head and salinity equations

recursively.

CONCLUSIONS & RECOMMENDATIONS

The following conclusions have been drawn

from the results of this modeling study, conducted

to analyze the solute transport phenomenon under


16/17


27

skimming wells, installed in the shallow fresh

groundwater areas:

1. In case, the saline water mound hasalready risen, any reduction in well-

discharge would neither decrease salinity

of pumped water nor suppress upconing,

but would reduce their rate of rise.

2. In the areas having moderate values ofaquifer parameters and fresh-water

thickness less than 30 m, the

development of fresh groundwater

through skimming wells is not

sustainable. It is only the matter of time

up to which the well will discharge good

quality groundwater.

3. The optimum well-penetration ratio ofthe site under study was found to be 23 %

against the present penetration of 33 %.

Thus, closing the lower 3 to 5 m length ofscreen and reducing well discharge by 40

to 50 % can ensure acceptable pumped

water-quality on sustainable basis.

4. MODFLOW/MT3D predicted gradualrise in pumped-water salinity and

upconing of underlying saline water, for

all pumping scenarios. It erroneously

provides no decline in fresh-saline water

interface after well closure.

5. The MT3D model does not properlysimulate skimming wells, mainly due to

the reason that it does not take intoaccount the density difference between

fresh and saline groundwater. For

evaluating skimming wells, MODFLOW

and MT3D programs need to be made to

run, recursively, over small time-steps

when potentials computed for various

cells are corrected to account for

calculated salinity levels.

In view of the above discussion, the

following are recommended:

1. A thorough understanding of thehydrological properties of the aquifer at the

specific location, where the skimming well

facility is contemplated, must be achieved

before the design and installation of

skimming well;

2. In shallow fresh groundwater areas, the well-penetration ratio should not be greater than

20 per cent, to avoid any upconing.

Moreover, the well should be operated at a

reasonably low discharge and operational

factor. The low discharges from these wells

can be effectively utilized for irrigation,

through pressurized irrigation systems.

REFERENCES

ACE & Halcrow, 2001. Exploitation and Regulation of

Fresh Groundwater. Draft Final Report. Sector

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Ashraf, M., M. Aslam, M. M. Saeed, M. S. Shafique,

2001. Effect of Intermitted Pumping on the Water

Quality of Multi Strainer Skimming Wells.

Proceedings 2nd National Seminar on Drainage inPakistan held in University of Agriculture,

Faisalabad from April 18-19, 2001. pp. 200-210.Awan, N.M., 1991. Salt Water Intrusion, Centre of

Excellence in Water Resources Engineering,Engineering University, Lahore

Bouwer, H., 1978. Groundwater Hydrology.

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numerical analysis of groundwater-flow and solutetransport under skimming well

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