groundwater flow and contaminant transport modeling of allen forrest zoo, kanpur
TRANSCRIPT
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Proceedings of International Conference of Benchmarks in Engineering Science and Technology (IC-BEST) 7-8 September 2012.
Groundwater Flow and Transport Modeling of Allen Forrest, Kanpur
Abhinav Srivastava, Dr. Deepesh Singh
B.Tech. Student, Assistant ProfessorDept. of Civil Engineering, H.B. Technological Institute, Kanpur (India)
[email protected], [email protected]
Abstract-This paper aims at studying the problem of
groundwater contamination of Allen Forest area,
Kanpur (U. P.) and suggests suitable observation well
management strategies for the area. In this respect,
groundwater flow and contaminant transport process
was simulated over the study area using a computer
based model, MOC v3.1. The study area was suitably
discretized into a block centered finite difference grid
which was limited to 1089m 961m. The simulationmodel utilizes the hydrogeological input data and
provides head and contaminant concentration values
for future time periods. It was observed that in 5
years simulation period around 42 percent of the total
study area is covered by the plume which crosses the
threshold limit of 300 mg/l. This work also utilizes the
breakthrough curves to explore suitable management
strategies for installation of observation wells in
different time periods depending upon the economic
constraints.
1. Introduction
Groundwater is a term used for the subsurface
water that occurs beneath the water table in soils andgeological formations that are fully saturated.
Groundwater is the most abundant source of fresh
water for mankind, with only 2.5% of water on earthbeing fresh, the total groundwater reserves account for
30% of this share [4]. As a result of our consumptive
way of life, the groundwater environment is being
assaulted with an ever increasing number of soluble
chemicals. From water quality viewpoint, degradation
of groundwater often requires long periods of timebefore the true extent of the problem is readily
detectable. It has thus become recognized as animportant environmental problem. With the increasingsense of awareness about the environment and the
recognition of the need for its protection, the study of
solute transport related to groundwater contaminationhas become the focus of numerous researchers.
Groundwater modelling is an effective way to
predict the flow of groundwater within an aquifer.
Groundwater modelling aims at studying the temporaland spatial distribution of such contaminants in the
aquifer and helps to formulate sustainable groundwater
management strategies. During the last three decades,
research activities in this area have accelerated to a
revolutionary level. Different investigators havestudied the solute transport from different perspectives.
Groundwater models can be divided into groundwater
flow models and solute transport models. Groundwaterflow models solve for the distribution of heads,
whereas solute transport models solve for
concentration of solute as affected by advection,dispersion and chemical reactions. Groundwater
models can be both analytical and numerical. While the
analytical models are wholly based on subjective
human judgments, numerical models simulate
groundwater flow indirectly by means of a governing
equation thought to represent the physical processesthat occur in the system, together with equations that
describe heads or flows along the boundaries of the
model [1].
After the contaminants and their behaviour havebeen detected, the well locations are monitored based
on which sustainable groundwater managementstrategies are devised. Along with these, efforts may
also be made for remediation of the problem by
implementing the three Es viz.engineering, education
and enforcement.
1.1. Objectives
This paper aims to address the spatial and temporal
distribution of water table and contaminant
concentrations in a confined aquifer with the following
objectives:i. Identification of various groundwater extraction,
recharge and contaminant sources in the area of
Allen Forest Zoo.ii. Implementation of a coding based numerical model
for groundwater flow and contaminant transport for
the area.
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Proceedings of International Conference of Benchmarks in Engineering Science and Technology (IC-BEST) 7-8 September 2012.
iii. Prediction of the head distribution, fate of the
contaminants in different time periods.
iv. Developing sustainable strategies for groundwater
observation well installation in the study area.
2. Theoretical background
2.1. Groundwater flow equation
The equation describing the transient two-
dimensional areal flow of a homogeneous
compressible fluid through a non-homogeneous
anisotropic aquifer in Cartesian tensor notation can be
written as [8]:
, 1,2 1
Where:
Tij =transmissivity tensor, [L2/T];
=
Kij
b;
Kij = hydraulic conductivity tensor, [LT-1
];b = saturated thickness of aquifer, [L];
h = hydraulic head, [L] ;
S = storage coefficient, (dimensionless);
t = time, [T];
W = volume flux per unit area (positive sign foroutflow and negative for inflow), [L/T];
and
xiandxjare the Cartesian coordinates, [L].
2.2. Contaminant transport equation
The equation used to describe the two dimensional
areal transport and dispersion of a given non-reactivedissolved chemical species in flowing ground water is
as follows [2] and [3]:
2
Where:
C = concentration of the dissolved chemical
species, [M/L3];
Dij = coefficient of hydrodynamic dispersion (a
second-order tensor), [L2/T];
b = saturated thickness of the aquifer, [L]; and
C = concentration of the dissolved chemical in a
source or, sink fluid, [M/L3].
2.3. Method of characteristics
The method of characteristics is used in this model
to solve the contaminant transport equation. This
method was developed to solve hyperbolic differentialequations. The approach taken by the method of
characteristics is not to solve equation 2 directly, but
rather to solve an equivalent system of ordinary
differential equations. Considering saturated thickness
as a variable and by expanding the convective transport
term, equation 2 can be written as [7]:
1
3
2.4. Assumptions considered in model
Following assumptions are considered in the model:
i. Darcys law is valid and hydraulic-head gradients
are the only significant driving mechanism for fluidflow.
ii. The porosity and hydraulic conductivity of the
aquifer are constant with time, and porosity is
uniform in space.iii.
Gradients of fluid density, viscosity and
temperature do not affect the velocity distribution.
iv.No chemical reactions occur that affect theconcentration of the solute, the fluid properties, or
the aquifer properties.
v. Ionic and molecular diffusion are negligible
contributors to the total dispersive flux.vi.Vertical variations in head and concentration are
negligible.
vii.The aquifer is homogeneous and isotropic with
respect to the coefficients of longitudinal and
transverse dispersivity.viii.The gradients of fluid density, viscosity and
temperature do not affect the velocity distribution.
2.5. Methodology
The study area is discretized into a block-centred
finite difference grid having rows and columns. A
pumping well is represented as withdrawal (discharge)
well and is specified as one pumping well per node.The model assumes that stresses developed in the
aquifer are constant with time during each pumping
period. But the total number of wells, as well as theirlocations, flux rates, and source concentrations, may be
changed for successive pumping periods. The model
specifies observation wells on potential locations.Other parameters like contaminant source, constant
head boundaries, no-flow boundaries, transmissivitycan be given as input in model as node identification
array [9].
An output file was obtained detailing the inputvalues, head values and concentration values. This
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Proceed
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Proceedings of International Conference of Benchmarks in Engineering Science and Technology (IC-BEST) 7-8 September 2012.
Table 3. Hydro-geological inputs for the model
Parameter Value
Aquifer Thickness 20 feet
Potentiometric heads inthe water table
130 feet in the north and
north-east boundary and100 feet on the south and
south-west boundary
Transmissivity 0.12 ft2/sec
LongitudinalDispersivity
100 feet
Transverse Dispersivity 30 feet
Effective Porosity 0.30
Storage Co-efficient0, due to steady state
conditions
Number of Observation
Wells10
Number of Pumping
Wells2
Discharge of each
pumping well20.96 ft3/sec
4. Results and analysis
4.1. Spatial distribution of contaminant
concentration
The concentration values at all the nodes wereinterpolated in the entire study area by method of
Kriging utilized by Surfer10. Kriging is a statisticalinterpolation method that chooses the best linear
unbiased estimate and unlike other interpolationmethods, it preserves the field value at measurement
points [1]. The contours join all points of same
concentration. The successive contour maps help to get
an idea about the areal extent of contamination in
ground water. The various contour maps obtained werethen carefully superimposed over the study area.
The values on contour lines represent concentrationvalues in mg/l. The threshold limit of groundwater
contamination is assumed to be 300 mg/l. It was
observed that a total of 167 cells, around 42 percent of
the total study area cross the threshold limit at the endof simulation period of 5 years.
The number of cells crossing the threshold limit
after every time interval of 2 months up to 2.5 years
and 6 months from then onwards has been shown in
Table 4.
Table 4. Number of cells above the threshold limitwith time
Time (Months)Number of cells above
threshold limit
2 1074 136
6 1508 156
10 157
12 160
14 157
16 161
18 16220 161
22 160
24 16126 162
28 163
30 164
36 16242 165
48 164
54 165
60 167
The results indicate that as the total time of
simulation increases, the concentration of the
contaminant gradually spreads throughout thefinite difference grid. The study area consists of
400 finite difference cells out of which 167 cells
were observed to be above the threshold limit of300 mg/l. Fig. 4 shows a curve depicting the
number of cells crossing the threshold limit of300 mg/l with time.
Figure 4. Curve showing the number of cellscrossing the threshold limit with time
The spatial distribution of threshold limit ofcontaminant concentration at the end of simulation
period of 5 years has been shown in Fig. 5.
0
25
50
75
100
125
150
175
200
225
250
2 6 10 14 18 22 26 30 42 54Numberofcellsabovethreshold
limit
Time (Months)
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Figure 5.the
4.2. Hea
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September 2012.
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Proceedings of International Conference of Benchmarks in Engineering Science and Technology (IC-BEST) 7-8 September 2012.
4.3. Temporal distribution of contaminant
concentration
For observing the temporal distribution ofconcentration, breakthrough curves were plotted at all
the ten identified potential locations for observation
well installations. A breakthrough curve can be definedas a graph between concentration and time at a
particular point at sampling location.
4.4. Observation well management strategies
An observation well is used to obtain representative
groundwater quality samples and hydrogeologicinformation. Observation wells are simple and
inexpensive monitoring tools that help in monitoring
groundwater trends. A properly designed, installed, and
developed groundwater observation well, provides thefollowing:
1. Representative samples of groundwater that can be
analyzed to determine physical properties andwater-quality parameters of the sample
2. Conducting aquifer tests used for the purpose of
determining the hydraulic properties of the
geologic materials.
4.4.1. Installation strategy of observation wells
For an entire study of the area it is needed to installobservation wells at different locations. As the
installation of observation wells involves a huge capital
investment it is many times not economical tocompletely utilise the observation wells. In this section
a methodology has been adopted to install the wellsstep by step with time. The methodology utilizes the
breakthrough curves. When the concentration value
crosses the threshold limit at a particular time, anobservation well is needed to be installed before that
time. It is assumed that a well can be installed one
month before that particular time.
With the help of breakthrough curves obtained, thewells which cross the threshold limit of 300 mg/l can
be predicted along with the time they would take tocross that limit. Due to economic constraints all the
wells cannot be installed at the same time. So, at a
time, observation wells need to be installed at those
locations only which cross the limit.With the help of breakthrough curves it can be seen
that observation well locations 1, 2 and 3 remain abovethe threshold limit throughout the simulation period so
an observation well should be installed at those
locations from the beginning of the simulation period.Well locations 4 and 5 cross the threshold limit
after 3 months of the simulation period so observation
wells should be installed at those locations during the
2ndmonth.
Well location 6 crosses the threshold limit after 4
months of the simulation period so observation wells
should be installed at this location during the 3rdmonth.
Well location 7 crosses the threshold limit after 45
months of the simulation period so observation wellsshould be installed at this location during the 44th
month.
Well locations 8, 9 and 10 never cross the threshold
limit during the simulation period so observation wellsneed not be installed at these locations.
4.5. Remedial measures
The other method for ensuring the wholesomenessof groundwater is the treatment of the influent water
which carries the contaminant being drained into the
lake. When situations arise like all the observation well
locations become unsafe, this method can be resortedto, though at a higher cost. A small treatment facilitycan be established anywhere in the course of the drain
or preferably, near its entrance in the lake. Although,
this method is expensive but it would benefit in thelong run because treating the contaminant at its very
source will reduce the concentration of contaminant
falling in the lake which, in turn, would gradually
render the groundwater safe.Finally, after the engineering aspect has been
covered, the next step should be to educate the
residents of the area about the problem so that they
remain cautious and use secondary methods to purify
the water before consuming it. As a long term planninga combined system with effluent treatment, regional
ordinances and observation wells may be adopted to
mitigate the groundwater contamination problem.
5. Conclusions
Two dimensional modelling was done forgroundwater flow and contaminant transport in the
study area, Allen Forest Zoo. The study area,
encompassing 48 acres was defined and a finitedifference grid was designed on it. The model predicts
the spread of plume after every specified time interval
starting from 2 months to 60 months (5 years) and alsothe head distribution at the end of the time step. The
simulation results indicate that as the total time fromthe beginning to the end of simulation increases, the
concentration of the contaminant gradually spreads
with the groundwater movement. At the end of thesimulation period, it was observed that around 42
percent of the total study area had crossed the threshold
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Proceedings of International Conference of Benchmarks in Engineering Science and Technology (IC-BEST) 7-8 September 2012.
limit of 300 mg/l. Ten potential observation well
locations were identified over the study area. At these
locations the model predicts the values of
concentration achieved throughout the simulation
period. This data was used for preparing breakthroughcurves which in turn helped to analyse the suitability of
installing an observation well at a particular location.
Considering the economic constraints, thepredictions of the simulation helped a lot to decide the
best management strategy for observation well
installation. It was suggested to install an observation
well, at a time, only at those locations where theconcentration of contaminant crosses the threshold
limit. It was also suggested that after all the well
locations cross the threshold limit, a small treatment
facility should be established at contaminant source.This would gradually improve the groundwater quality.
The aspects of education and enforcement were also
discussed very briefly for groundwater qualityimprovement which would spread awareness and
gradually reduce the outflow of contaminants in thewaste water.
REFERENCES
[1] Anderson M. P. and Woessner W. W. (1992) Applied
groundwater modeling: simulation of flow and advective transport.
Academic Press,San Diego, California.
[2] Bear, Jacob (1972). Dynamics of fluids in porous media,American Elsevier Publishing Co., New York, 764.
[3] Bredehoeft, J. D. and Pinder, G. F. (1973) Mass transport in
flowing groundwater Water Resources Research,9(1), 194-210.
[4] Chow, Ven Te; Maidment, David R. and Mays, Larry W. (1988).Applied Hydrology.New Delhi, Tata McGraw Hill, 4.
[5] Freeze,R.A.and Cherry, J. A. (1979). Groundwater. Englewood
Cliff, N. J. Prentice-Hall.[6] Golden Software Inc. (2011), SURFER version 10.0.
[7] Konikow, L. F. and Grove, D. B. (1977) Derivation of equations
describing solute transport in ground water U.S. Geological SurveyWater-Resources Investigatons 77-19, 30.
[8] Pinder,G.F. and Bredehoeft,J.D. (1968) Application of the
digital computer for aquifer evaluation Water Resources Research,
4(6), 1069-1093.[9] Singh, D. and Datta B. (2012). Linked Optimization Model for
Groundwater Monitoring Network Design, in proceedings of
International conference "ENSURE 2012: Environmentally
Sustainable Urban Ecosystems" IIT Guwahati, Assam, India
February 24-26, 2012 (in CD)