Application of DRASTIC Model Using GIS for Evaluation Of Ground Water Vulnerability: Study of Ahmedabad District

Abstract

Ahmedabad is one of the main district (zone) in state of Gujarat, India. Groundwater is considered to be one of the most important sources of water for domestic use, agriculture and industrial use in study area. The aim of this study is to determine the ground water vulnerability index using the DRASTIC model. The objective is to determine the susceptible ground water zone by integrating various layers using ArcGIS. Attributes such as depth to ground water, net recharge, aquifer media, soil media, topography, impact of vadose zone and hydraulic conductivity are considered for ground water vulnerability mapping for the study area. The ground water vulnerability index was classified into four classes viz. 100-120, 120-140, 140-160 and 160-180 corresponding to low, medium, high and very high vulnerability respectively. The vulnerability map depicts that 20.50%, 16.92%, 41.81% and 20.78% area of the Ahmedabad district falls under low, medium, high and very high vulnerability zones respectively. The present model can be used as a supplement for planning urban and industrial development in the study area.

Full text

Application of DRASTIC Model Using GIS for Evaluation Of Ground Water
Vulnerability: Study of Ahmedabad District
TUSHAR BOSEa *, YASH MAJEETHIAb and KHUSHBOO PATELa
a Faculty of Technology, CEPT University
b Infrastructure Engineering and Management, Faculty of Technology, CEPT University
KEY WORDS: DRASTIC model, Ahmedabad district, ArcGIS, aquifer, vulnerability
ABSTRACT
Ahmedabad is one of the main district (zone) in state of Gujarat, India.. Groundwater is considered to
be one of the most important sources of water for domestic use, agriculture and industrial use in
study area.. The aim of this study is to determine the ground water vulnerability index using the
DRASTIC model. The objective is to determine the susceptible ground water zone by integrating
various layers using ArcGIS.. Attributes such as depth to ground water, net recharge, aquifer media,
soil media, topography, impact of vadose zone and hydraulic conductivity are considered for ground
water vulnerability mapping for the study area.. The ground water vulnerability index was classified
into four classes viz. 100-120, 120-140, 140-160 and 160-180 corresponding to low, medium, high
and very high vulnerability respectively. The vulnerability map depicts that 20.50%, 16.92%, 41.81%
and 20.78% area of the Ahmedabad district falls under low, medium, high and very high vulnerability
zones respectively. The present model can be used as a supplement for planning urban and
industrial development in the study area.
1 Introduction to study area: Ahmedabad district
Ahmedabad district is located in the central
part of Gujarat state, India. The district is
surrounded by Gandhinagar- the state capital
and Mehsana district in north, Kheda as well
as Gulf of Khambhat in the south,
Sabarkantha district in north-east and
Bhavnagar and Surendranagar district in the
west. It lies between 22°55’ & 23°08’ north
latitude and 71°37’ & 72° 50’ east longitude.
Ahmedabad city is the largest city of Gujarat
state and the seventh largest agglomeration in
India. It is district headquarters and was also
state’s capital from May 1960 to May 1970,
before Capital was shifted to Gandhinagar 32
kilometres from Ahmedabad city. There are 11
talukas and 556 villages in Ahmedabad district. Total area of district is 7937.27 square kilometres
having population of 72,14,225 persons with 15,10,134 households.
2 Geohydrological profile
Sabarmati is the main river passing through the Ahmedabad district, originating from Aravalli ranges
of Rajasthan and meeting to sea in the Bay of Cambay. Geologically area is formed of Quaternary
alluvium deposits. The south west part of the district i.e., part of Barvada, Dhandhuka and Ranpur
talukas consist of Basalt rock, while the rest of the district consist of Alluvial formations which
consists of alternating beds of clay, gravels, cankers, sand and silt. The water is available in the form
of confined and unconfined aquifer under alluvial formations. Ahmedabad district heavily relies on

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deep tube-wells for water supply schemes, which in turn has lead to heavy drilling activity in the
region resulting in over extraction of ground water since last two decades. This has resulted into
sharp decline in water levels and in the quality of water.
Average rainfall of the district for the district is 758 mm and net groundwater recharge for year 2011
was 58309.42 million cubic metres. Out of 11 talukas- Ahmedabad city and Daskroi are categorized
as over exploited, Dholka lies in Critical category, Detroj-Rampura and Viramgam are categorized as
Semi critical, while Barwala, Bavla, Mandal, Ranpur and Sanand fall under Safe category. Overall the
district is categorized as Semi critical.
Sr.
No. Taluka
Net Annual
Ground Water
Availability
(mcm)
Annual
Ground Water
Demand
(mcm)
Stage of
Ground Water
Development
(%)
Category
1 Barwala
1521.54
942.90
61.97%
Safe
2 Bavla 4344.61
1596.40
36.74%
Safe
3 City-Daskroi 19258.28
19977.50
103.73% Over Exploited
4 Detroj-Rampura 5267.48
3998.60
75.91%
Semi critical
5 Dhandhuka Saline
6 Dholka 4393.43
4048.37
92.15%
Critical
7 Mandal
3419.24
1654.00
48.37%
Safe
8 Ranpur
4153.71
2833.90
68.23%
Safe
9 Sanand
11084.83
6682.00
60.28%
Safe
10 Viramgam
4866.29
3960.00
81.38%
Semi critical
Total 58309.42
45693.67
78.36%
Semi critical
3 Methodology
In 1987, an Environmental Protection Agency ( EPA) funded effort to research and develop a method
for evaluating pollution potential anywhere in United States which successfully produced the
DRASTIC model. It is one of the most widely used models to assess Groundwater Vulnerability.
Amongst many models prevalent for assessment, advantage of DRASTIC is that the inputs required
for its application are generally available or easily obtained from public agencies. DRASTIC is the
acronym for seven parameters of the hydrogeology factors which include Depth to water(D), Net
Recharge (R), Aquifer Media(A), Soil Media(S), Topography(T), Impact of the Vadose Zone(I),
Hydraulic Conductivity(C). On the basis of how significantly they influence pollution potential, a
combination of ratings and weights are assigned. The values of rating scale range from 1 to 10 and
weights assigned from 1 to 5. Weights are assigned according to generic model as analysis is being
carried out for non-pesticide model. Higher the ratings and weights indicate higher the risk of
vulnerability. In the study DRASTIC Model is developed by use of GIS wherein it creates data layers
using seven DRASTIC components and spatial analysis tool; based on mentioned criteria, layer for
each attribute was created in ArcGIS and in order to arrive at vulnerability index, all the layers were
added using Raster calculator in ArcGIS. The final vulnerability map is obtained from DRASTIC Index
(DI) equation as follows:

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Applied Geoinformatics for Society and Environment 2013 367
DRDW + RRRW + ARAW + SRSW + TRTW + IRIW + CRCW = Drastic Index (DI) or Vulnerability Index
Where,
R = Rating
W = Weight
DW = Relative weight of the depth to the water table
DR = Rating of the depth to the water table
RW = Relative weight of the net aquifer recharge
RR = Rating of the net aquifer recharge
AW = Relative weight of the Aquifer Media
AR = Rating of the Aquifer Media
SW = Relative weight of the Soil Media
SR = Rating of the Soil Media
TW = Relative weight of the Topography slope
TR = Rating of the Topography slope
IW = Relative weight of the Impact of Vadose Zone
IR = Rating of the Impact of Vadose Zone
CW = Relative weight of the Hydraulic Conductivity
CR = Rating of the Hydraulic Conductivity
3.1 Depth of Water (D)
Depth of water affects the present contaminants to undergo
various chemical and biological reactions. As the depth
increases, the travel time increases which leads to greater
chance for attenuation of contaminants. Data for Depth of
ground water level was obtained from CGWB, Ahmedabad for
95 stations of Ahmedabad district. The average depth of
ground water varies from 0.89 feet to 336.73 feet during post
monsoon and 3.78 feet to 342.21 feet during pre-monsoon
period.
Depth of Groundwater (feet)
Range Rating Index Area Area (km2)
0-5 10 50 3.97% 314.74
Mai 15 9 45 29.68% 2355.47
15-30 7 35 28.50% 2262.20
30-50 5 25 13.98% 1109.79
50-75 3 15 3.61% 286.79
75-100 2 10 2.77% 220.07
100+ 1 5 17.49% 1388.20

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3.2 Net Recharge (R)
Recharge is the amount of water which enters the aquifer and
it is the largest pathway for contaminant transport. It will also
dilute the contaminants which enters the aquifer. Thus, more
the percolation of water in the aquifer, greater is the potential
for contaminants to increase the pollution into the aquifer.
Recharge can be calculated using climate data as:
Net Recharge = Precipitation – Evaporation – Runoff
For calculations, rainfall is considered from the data obtained
from hydrology project report, Gujarat state. Estimation of
natural ground water recharge is normally subject to errors and
no single method for recharge estimation has been yet
identified. Here, the recharge has been estimated with various
methods and average value has been considered.
Zone Chaturvedi
Method
Revised
Chaturvedi
Method
NLR technique Krishna
Rao
Method
Direct
Method
Average
R= 2.0 (P -
15)0.4
R=1.35(P -
14)0.5
R=0.63(P-
15.28)0.76
R = K (P-
X)
R=f*P
Ahmedabad
City
6.32 5.85 5.54 4.25 3.28 5.05
Barwala 6.26 5.78 5.44 4.14 3.23 4.97
Bavla 5.45 4.92 4.16 2.88 2.73 4.03
Daskroi 5.85 5.34 4.77 3.47 2.96 4.48
Detroj-
Rampura
5.88 5.37 4.82 3.52 2.98 4.52
Dhandhuka 6.26 5.78 5.43 4.14 3.23 4.97
Dholka 5.00 4.45 3.51 2.25 2.49 3.55
Mandal 5.88 5.37 4.82 3.52 2.98 4.52
Ranpur 6.76 6.33 6.30 5.06 3.60 5.61
Sanand 6.08 5.59 5.15 3.85 3.11 4.76
Viramgam 5.21 4.67 3.81 2.55 2.60 3.77
where,
R = net recharge due to precipitation during the year (inch)
P = annual precipitation (inch)
K = coefficient (0.25 for areas with P values between 23.62 to 39.37 inch)
f = infiltration factor

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Net Recharge (inches)
Range Rating Index Area Area
(km2)
2-4 3 12 24.74% 1963.76
4-7 6 24 75.26% 5973.51
Weight:4
3.3 Soil Media (S)
Soil Media refers to the topmost layer of the earth to the uppermost layer of the aquifer. Soil Media
being the topmost layer allows for the transport of water and contaminants to the aquifer. It is a
habitat for micro organisms which in turn help in biodegradation of the contaminants. Type of soil
and the size of grains affect the rate of infiltration. Study area consists of clayey and loamy type of
soil having materials like sand, loam, silty loam, clay loam, sandy loam and gravels.
Soil Media
Range Rating Index Area Area (km2)
Shallow Clayey 3 6 1.62% 128.23
Medium Clayey 3 6 35.04% 2780.98
Deep loamy 5 10 35.58% 2824.10
Medium loamy 6 12 6.12% 485.96
Deep clayey 6 12 21.01% 1667.79
Rock outcrops 10 20 0.63% 50.21
Weight:2
3.4 Topography (T)
Topography defines variation in gradient or slope of the ground surface which in turn determine
whether ground water will run off or infiltrate the aquifer. Steeper the slope, higher is the run off and
it increases the attenuation for the infiltration of the contaminants. Digital Elevation Data (DEM) is
used to determine the slope and also GIS maps according to which the slope of study area varies
from 0 to 5.8 per cent. Major portion of the district consist of
slope ranging upto 2 per cent.
Topography (slope percent)
Range Rating Index Area Area (km2)
0-2 10 10 65.44% 5193.99
02. Jun 9 9 27.42% 2176.71
06. Dez 5 5 7.06% 559.97
Dez 18 3 3 0.08% Jun 44
18+ 1 1 0.002% 0.16
Weight:1

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3.5 Impact of Vadose Zone (I)
Vadose zone is the unsaturated layer above the water table consisting of layers of bed rock as well
as surface soil without groundwater holding capacity. The
attenuation characteristic of pollution is based on permeability
of the material within the vadose zone, porosity and thickness.
The material forming the vadose zone in study area consists of
clay, sand and silt. Major portion of the district consist of Low
level alluvium followed by Coastal alluvium and High level
alluvium type of aquifer media.
Impact of Vadoze zone
Range Rating Index Area Area (km2)
High Level Alluvium 6 30 2.77% 219.54
Low Level Alluvium 7 35 56.57% 4490.40
Coastal Alluvium 8 40 40.66% 3227.33
Weight:5
3.6 Hydraulic Conductivity (C)
Hydraulic Conductivity is a rate at which material transmits
within the aquifer. It determines the rate at which contaminants
transmit from the point of contact. Hydraulic Conductivity is
affected by the gradient of the aquifer, its material and porosity.
More than one half part of the district falls in the range of 700-
1000 gpd/ft2.
Hydraulic Conductivity (gpd/ft2)
Range Rating Index Area Area (km2)
300-700 4 12 2.77% 219.54
700-1000 6 18 56.57% 4490.40
1000-2000 8 24 40.66% 3227.33
Weight:3
3.7 Ground Water Vulnerability Index
The vulnerability index computed as the sum of the products of
weights and ratings assigned to each of the input considered
above. The index ranges from 101-179. Considering Low,
Medium, High and Very high vulnerability zones entire index
range is divided into four classes 100-120, 120-140, 140-160 and
160-180, respectively. Using the mentioned classes the map was
generated which indicates following distribution pattern:

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Vulnerability zone Area (km2) Area (%)
Low 1626.87 20.50
Medium 1342.61 16.92
High 3318.60 41.81
Very high 1649.20 20.78
4 Discussion
Ground water is a limited resource and already over exploited. The problem is further aggravated by
immense urban and industrial development pressure because it increases the extraction and
distribution of ground water pattern. Hence, the new development shall consider the vulnerability
index tool to determine the extent and type of development in the district.
Acknowledgement
The assistance received from Faculty of Technology, CEPT University, Ahmedabad is gratefully
acknowledged. The authors are thankful to the Central Ground Water Board, Ahmedabad for all the
data and assistance provided, which made this study possible.
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