a gis-based drastic model for assessing aquifer vulnerability

14
A GIS-based DRASTIC model for assessing aquifer vulnerability in Kakamigahara Heights, Gifu Prefecture, central Japan Insaf S. Babiker * , Mohamed A.A. Mohamed, Tetsuya Hiyama, Kikuo Kato Hydrospheric Atmospheric Research Center, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan Received 6 July 2004; accepted 5 November 2004 Available online 21 January 2005 Abstract Vulnerability assessment to delineate areas that are more susceptible to contamination from anthropogenic sources has become an important element for sensible resource management and land use planning. This contribution aims at estimating aquifer vulnerability by applying the DRASTIC model as well as utilizing sensitivity analyses to evaluate the relative importance of the model parameters for aquifer vulnerability in Kakamigahara Heights, Gifu Prefecture central Japan. An additional objective is to demonstrate the combined use of the DRASTIC and geographical information system (GIS) as an effective method for groundwater pollution risk assessment. The DRASTIC model uses seven environmental parameters (Depth to water, net Recharge, Aquifer media, Soil media, Topography, Impact of vadose zone, and hydraulic Conductivity) to characterize the hydrogeological setting and evaluate aquifer vulnerability. The western part of the Kakamigahara aquifer was dominated by bHighQ vulnerability classes while the eastern part was characterized by bModerateQ vulnerability classes. The elevated north-eastern part of the study area displayed bLowQ aquifer vulnerability. The integrated vulnerability map shows the high risk imposed on the eastern part of the Kakamigahara aquifer due to the high pollution potential of intensive vegetable cultivation. The more vulnerable western part of the aquifer is, however, under a lower contamination risk. In Kakamigahara Heights, land use seems to be a better predictor of groundwater contamination by nitrate. Net recharge parameter inflicted the largest impact on the intrinsic vulnerability of the aquifer followed by soil media, topography, vadose zone media, and hydraulic conductivity. Sensitivity analyses indicated that the removal of net recharge, soil media and topography causes large variation in vulnerability index. Moreover, net recharge and hydraulic conductivity were found to be more effective in assessing aquifer vulnerability than assumed by the DRASTIC model. The GIS technique has provided efficient environment for analyses and high capabilities of handling large spatial data. D 2004 Elsevier B.V. All rights reserved. Keywords: Aquifer vulnerability; DRASTIC; GIS; Model sensitivity; Kakamigahara; Japan 0048-9697/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2004.11.005 * Corresponding author. Tel.: +81 52 789 3473; fax: +81 52 789 3436. E-mail address: [email protected] (I.S. Babiker). Science of the Total Environment 345 (2005) 127– 140 www.elsevier.com/locate/scitotenv

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Page 1: A GIS-Based DRASTIC Model for Assessing Aquifer Vulnerability

www.elsevier.com/locate/scitotenv

Science of the Total Environm

A GIS-based DRASTIC model for assessing aquifer vulnerability

in Kakamigahara Heights, Gifu Prefecture, central Japan

Insaf S. Babiker*, Mohamed A.A. Mohamed, Tetsuya Hiyama, Kikuo Kato

Hydrospheric Atmospheric Research Center, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan

Received 6 July 2004; accepted 5 November 2004

Available online 21 January 2005

Abstract

Vulnerability assessment to delineate areas that are more susceptible to contamination from anthropogenic sources has

become an important element for sensible resource management and land use planning. This contribution aims at estimating

aquifer vulnerability by applying the DRASTIC model as well as utilizing sensitivity analyses to evaluate the relative

importance of the model parameters for aquifer vulnerability in Kakamigahara Heights, Gifu Prefecture central Japan. An

additional objective is to demonstrate the combined use of the DRASTIC and geographical information system (GIS) as an

effective method for groundwater pollution risk assessment. The DRASTIC model uses seven environmental parameters (Depth

to water, net Recharge, Aquifer media, Soil media, Topography, Impact of vadose zone, and hydraulic Conductivity) to

characterize the hydrogeological setting and evaluate aquifer vulnerability. The western part of the Kakamigahara aquifer was

dominated by bHighQ vulnerability classes while the eastern part was characterized by bModerateQ vulnerability classes. The

elevated north-eastern part of the study area displayed bLowQ aquifer vulnerability. The integrated vulnerability map shows the

high risk imposed on the eastern part of the Kakamigahara aquifer due to the high pollution potential of intensive vegetable

cultivation. The more vulnerable western part of the aquifer is, however, under a lower contamination risk. In Kakamigahara

Heights, land use seems to be a better predictor of groundwater contamination by nitrate. Net recharge parameter inflicted the

largest impact on the intrinsic vulnerability of the aquifer followed by soil media, topography, vadose zone media, and hydraulic

conductivity. Sensitivity analyses indicated that the removal of net recharge, soil media and topography causes large variation in

vulnerability index. Moreover, net recharge and hydraulic conductivity were found to be more effective in assessing aquifer

vulnerability than assumed by the DRASTIC model. The GIS technique has provided efficient environment for analyses and

high capabilities of handling large spatial data.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Aquifer vulnerability; DRASTIC; GIS; Model sensitivity; Kakamigahara; Japan

0048-9697/$ - s

doi:10.1016/j.sc

* Correspondi

E-mail addr

ent 345 (2005) 127–140

ee front matter D 2004 Elsevier B.V. All rights reserved.

itotenv.2004.11.005

ng author. Tel.: +81 52 789 3473; fax: +81 52 789 3436.

ess: [email protected] (I.S. Babiker).

Page 2: A GIS-Based DRASTIC Model for Assessing Aquifer Vulnerability

I.S. Babiker et al. / Science of the Total Environment 345 (2005) 127–140128

1. Introduction

Groundwater has been considered as an important

source of water supply due to its relatively low

susceptibility to pollution in comparison to surface

water, and its large storage capacity (US EPA, 1985).

However, there are significant sources of diffuse and

point pollution of groundwater from land use

activities, particularly agricultural practices. The

intrusion of these pollutants to groundwater alters

the water quality and reduces its value to the

consumer (Melloul and Collin, 1994). Prevention

of contamination is therefore critical for effective

groundwater management. Spatial variability and

data constraints preclude monitoring all groundwater

and make remediation activities expensive and often

impractical. Vulnerability assessment has been rec-

ognized for its ability to delineate areas that are more

likely than others to become contaminated as a result

of anthropogenic activities at/or near the earth’s

surface. Once identified, these areas can be targeted

by careful land-use planning, intensive monitoring,

and by contamination prevention of the underlying

groundwater.

The Kakamigahara aquifer, which is located in the

southern part of Gifu Prefecture, central Japan,

provides a source of water for domestic, industrial

and agricultural use. High nitrate levels exceeding

the 13 mg/l concentration (the human affected value)

have been encountered in groundwater of the

Kakamigahara aquifer (Terao et al., 1985, 1993;

Mohamed et al., 2003) while some concentrations

have already exceeded the maximum acceptance

level (44 mg/l NO3�) according to Japan regulations

(Babiker et al., 2004). The dominant source of nitrate

contamination has been identified as agricultural land

use, particularly vegetable cultivation with some

possibility of urban sources (residential, commercial,

and industrial; Babiker et al., 2004). To ensure that

this aquifer can remain as a source of water for the

Kakamigahara area, it is necessary to estimate

whether certain locations in this groundwater basin

are more susceptible to receive and transmit pollu-

tion. Therefore, the first objective of this study is to

evaluate the Kakamigahara aquifer vulnerability

using Depth to water, net Recharge, Aquifer media,

Soil media, Topography, Impact of vadose zone, and

hydraulic Conductivity (DRASTIC), the empirical

model of the U.S. Environmental Protection Agency

(EPA; US EPA, 1985). The second objective is to

evaluate the relative importance of the DRASTIC

model parameters for assessing aquifer vulnerability

in Kakamigahara Heights through sensitivity analy-

sis. An additional objective is to demonstrate the

combined use of DRASTIC and geographical infor-

mation system (GIS) as an effective method for

groundwater pollution risk assessment and water

resource management.

2. Background

The concept of groundwater vulnerability was

first introduced in France by the end of the 1960s to

create awareness of groundwater contamination

(Vrba and Zoporozec, 1994). It can be defined as

the possibility of percolation and diffusion of

contaminants from the ground surface into the

groundwater system. Vulnerability is usually consid-

ered as an bintrinsicQ property of a groundwater

system that depends on its sensitivity to human and/

or natural impacts. bSpecificQ or bintegratedQ vulner-ability, on the other hand, combines intrinsic vulner-

ability with the risk of the groundwater being

exposed to the loading of pollutants from certain

sources (Vrba and Zoporozec, 1994). Groundwater

vulnerability deals only with the hydrogeological

setting and does not include pollutant attenuation.

The natural hydrogeologic factors affect the different

pollutants in different ways depending on their

interactions and chemical properties.

Many approaches have been developed to eval-

uate aquifer vulnerability. They include process-

based methods, statistical methods, and overlay and

index methods (Tesoriero et al., 1998). The process-

based methods use simulation models to estimate the

contaminant migration but they are constrained by

data shortage and computational difficulties (Barbash

and Resek, 1996). Statistical methods use statistics to

determine associations between spatial variables and

actual occurrence of pollutants in the groundwater.

Their limitations include insufficient water quality

observations, data accuracy and careful selection of

spatial variables. Overlay and index methods com-

bine factors controlling the movement of pollutants

from the ground surface into the saturated zone

Page 3: A GIS-Based DRASTIC Model for Assessing Aquifer Vulnerability

I.S. Babiker et al. / Science of the Total Environment 345 (2005) 127–140 129

resulting in vulnerability indices at different loca-

tions. Their main advantage is that some of the

factors such as rainfall and depth to groundwater can

be available over large areas, which makes them

suitable for regional scale assessments (Thapinta and

Hudak, 2003). However, their major drawback is the

subjectivity in assigning numerical values to the

descriptive entities and relative weights for the

different attributes.

DRASTIC is an index model designed to produce

vulnerability scores for different locations by com-

bining several thematic layers. It was originally

developed for manual overlay of semiquantitative

data layers however the simple definition of its

vulnerability index as a linear combination of factors

shows the feasibility of the computation using GIS

(Fabbri and Napolitano, 1995). GIS are designed to

collect diverse spatial data to represent spatially

variable phenomena by applying a series of overlay

analysis of data layers that are in spatial register

(Bonham-Carter, 1996).

3. Study area

Kakamigahara Heights is composed of low hills 20

to 60 m above sea level and located on the northern

border of the Nohbi plains in central Japan (Fig. 1).

Fig. 1. Location of Kakamigahara Heights.

The Kiso River intersects the southern edge of the

area, running from east to west towards the plains.

Kakamigahara City is the third largest city in Gifu

Prefecture having a population over 135,000 inhab-

itants. The dominant industries are aircraft, transport

equipment and industrial metal manufacturing. Inten-

sive vegetable cultivation, mainly carrots, is practiced

in the eastern side of the Heights, sustained by

nitrogen fertilizer applications. The area is character-

ized by a warm and mild climate with an average

annual temperature of 15.5 8C and a rainfall of 1915

mm, (mean of 30-year records from Gifu City rainfall

station, Japan Meteorological Agency).

Kuroboku soils, Kisogawa mudflow deposits,

and the Kakamigahara formation constitute the

Kakamigahara Terrace, which is underlain by the

Nohbi Second Gravel Formation, the Tokai Group

and the basement (Yokoyama and Makinouchi,

1991). The Kakamigahara Formation consists of

sands with pumice ranging in age from ca. 0.09 to

0.06 Ma. The Nohbi Second Gravel Formation is

composed of boulder gravels with high permeability

while the Tokai Group consists of weathered clayey

gravels with low permeability. The Kakamigahara

groundwater basin consists of the highly permeable

Nohbi Second Gravel formation and the overlying

coarse sediments. The aquifer forms an east–west

elongated valley-like topography deepening west-

ward. The aquifer thickness ranges from 15 meters

in the east to more than 90 m in the west. Due to

the lack of confining clay layers, the aquifer is

considered typically unconfined. The groundwater

flow is from east to west and the water velocity is

high on average (2�10�3 m/s; Babiker et al.,

2004). The aquifer is characterized by a high

recharge rate from direct infiltration (Mohamed,

2003).

4. Methods

A DRASTIC model applied in a GIS environment

was used to evaluate the vulnerability of the

Kakamigahara aquifer. The DRASTIC model was

developed by the U.S. Environmental Protection

Agency (EPA) to evaluate groundwater pollution

potential for the entire United States (Aller et al.,

1987). It was based on the concept of the hydro-

Page 4: A GIS-Based DRASTIC Model for Assessing Aquifer Vulnerability

I.S. Babiker et al. / Science of the Total Environment 345 (2005) 127–140130

geological setting that is defined as ba composite

description of all the major geologic and hydrologic

factors that affect and control the groundwater

movement into, through and out of an areaQ (Aller

et al., 1987). The acronym DRASTIC stands for the

seven parameters used in the model which are:

Depth to water, net Recharge, Aquifer media, Soil

media, Topography, Impact of vadose zone and

hydraulic Conductivity (Table 1). The model yields a

numerical index that is derived from ratings and

weights assigned to the seven model parameters. The

significant media types or classes of each parameter

represent the ranges, which are rated from 1 to 10

based on their relative effect on the aquifer vulner-

ability. The seven parameters are then assigned

weights ranging from 1 to 5 reflecting their relative

importance. The DRASTIC Index is then computed

Table 1

The DRASTIC model parameters

Factor Description Relative

weight

Depth to water Represents the depth from the ground

surface to the water table, deeper

water table levels imply lesser chance

for contamination to occur.

5

Net Recharge Represents the amount of water

which penetrates the ground surface

and reaches the water table, recharge

water represents the vehicle for

transporting pollutants.

4

Aquifer media Refers to the saturated zone material

properties, which controls the

pollutant attenuation processes.

3

Soil media Represents the uppermost weathered

portion of the unsaturated zone and

controls the amount of recharge that

can infiltrate downward.

2

Topography Refers to the slope of the land

surface, it dictates whether the runoff

will remain on the surface to allow

contaminant percolation to the

saturated zone.

1

Impact of

vadose zone

Is defined as the unsaturated zone

material, it controls the passage and

attenuation of the contaminated

material to the saturated zone.

5

Hydraulic

Conductivity

Indicates the ability of the aquifer to

transmit water, hence determines the

rate of flow of contaminant material

within the groundwater system.

3

applying a linear combination of all factors accord-

ing to the following equation:

DRASTIC Index ¼ DrDw þ RrRw þ ArAw þ SrSw

þ TrTw þ IrIw þ CrCw ð1Þ

where D, R, A, S, T, I, and C are the seven

parameters and the subscripts r and w are the

corresponding rating and weights, respectively.

This model was selected based on the following

considerations. DRASTIC uses a relatively large

number of parameters (seven parameters) to compute

the vulnerability index, which ensures the best

representation of the hydrogeological setting. The

numerical ratings and weights, which were established

using the Delphi technique (Aller et al., 1987), are

well defined and are used worldwide. This makes the

model suitable for producing comparable vulnerabil-

ity maps on a regional scale. The necessary informa-

tion needed to build up the several model parameters

was available in the study area or could easily be

inferred. Data analyses and model implementation

were performed using the GIS software of the

International Institute for Geo-Information Science

and Earth Observation (ITC), Netherlands, bIntegratedLand and Water Information SystemQ (ILWIS 3.1).

4.1. Preparation of the parameter maps

Several types of data were used to construct

thematic layers of the seven model parameters. A

summary of the data types, sources and usages is

available in Table 2. The location of the 134 water

wells was digitized from the accompanying topo-

graphic map and was linked to an attribute table

containing the depth to groundwater table. Essentially,

the elevation of the well and the mean water level

table were provided for the last 6 years (1997–2002).

The depth to water table was obtained by subtracting

the water table level from the elevation of the well and

averaging over a six-year period. In a relatively small

area and a fairly isotropic aquifer, the static water

table level exhibits a smooth and gradual change of

heads. Therefore, an exact interpolation scheme is

appropriate for generating a smooth surface represen-

tation for the high degree of spatial continuity of the

groundwater surface in an aquifer. The inverse

distance moving average interpolation technique was

Page 5: A GIS-Based DRASTIC Model for Assessing Aquifer Vulnerability

Table 2

Data used for constructing the seven parameter layers

Data type Source Format Scale Date Used to produce

Borehole data

(water table level)

Kakamigahara City Hall Table 1998~2002 D

Environmental Office, Division

of Human Health and Environment

Location map 1:15,000 1997

Annual rainfall

(mean)

Japan Meteorological Agency website:

(http://www.data.kishou.go.jp)

Table 1971~2000 R

Geology map Ministry of Land Infrastructure

and Transport website:

(http://tochi.milt.go.jp)

Digital 1:50,000 1998 A

I

Geological profiles Yokoyama and Makinouchi 1991 A and I

Soil map Ministry of Land Infrastructure

and Transport website:

(http://tochi.milt.go.jp)

Digital 1:50,000 1998 S

Land use and

topographic map

Japan Geographical Survey Institute Hard copy 1:50,000 1998 T and R

Pollution sources

Hydraulic conductivity Kakamigahara City Hall Table C

Environmental Office, Division of

Human Health and Environment

Map ~1:70,000

I.S. Babiker et al. / Science of the Total Environment 345 (2005) 127–140 131

therefore performed on the point data using a limiting

search distance of 3000 m to ensure including a high

number of input points. In the moving average

algorithm, every pixel in the output map is assigned

a value based on the weighted average of all points at

a distance smaller than the limiting distance. The

weight of an input point is proportional to its distance

from the output pixel. The inverse distance is the

weight function which ensures that relatively larger

weights are assigned to points close to an output pixel.

This better satisfies the interpolation scheme and helps

generate the smooth surface representation suitable for

such type of data. The depth to water table map was

then classified into ranges defined by the DRASTIC

model and assigned rates ranging from 1 (minimum

impact on vulnerability) to 10 (maximum impact on

vulnerability). The deeper the groundwater, the

smaller the rating value.

Because the Kakamigahara aquifer is recharged

mainly by direct infiltration from precipitation

(Mohamed, 2003, Master thesis), the recharge map

was constructed from the rainfall data according to the

following formula:

Net recharge ¼ ðrainfall� evapotranspirationÞ� recharge rate ð2Þ

The rainfall map was obtained by interpolating a

thirty years mean of annual precipitation (mm/year)

from seven representative rainfall stations in the south

of Gifu Prefecture (Gifu, Mino Kamo, Ogaki, Mino,

and Inigawa) and the north of Aichi Prefecture (Inabu

and Nagoya). The inverse distance moving average

interpolation technique described above was used to

construct the rainfall map. Evapotranspiration data

from the Kakamigahara was not available, so instead

the evapotranspiration value of 900 mm/year from a

representative area in central Japan (Kondoh and

Nishiyama, 2000) was used. The recharge rate was

assumed to be 20% for the urbanized area and 85%

for the rest of the study area (Babiker, 1998). The

recharge map was then classified into ranges and

assigned ratings from 1 to 10. High recharge rates

were assigned high numerical rates.

The Aquifer media and the Impact of vadose zone

were obtained using a subsurface geology map,

geological sections, and drilling profiles of the

Kakamigahara aquifer (Figs. 4 and 7 in Yokoyama

and Makinouchi, 1991). The subsurface geology map

was imported in digital format from the website of the

Japan Ministry of Land, Infrastructure and Transport

(http://tochi.mlit.go.jp), geo-referenced and on-screen

digitized to create a representation of the different

geological units. The geological sections were then

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I.S. Babiker et al. / Science of the Total Environment 345 (2005) 127–140132

used to encode the geological units according to the

DRASTIC model rating system. The coarse (saturated

or unsaturated) media was assigned a high rating

value compared to the fine media types.

The soil map was also imported in digital format

into the ILWIS program, referenced to area coor-

dinates and on-screen digitized to delineate the

different soil units. The texture of the various soil

types in Kakamigahara was obtained from a repre-

sentative description of World soil types provided by

the United States Department of Agriculture (USDA,

1999). The soil media types were then assigned

ratings from 1 to 10 according to their permeability.

Coarse soil media have high rates in comparison to

fine soil media.

The topography layer was constructed from the

topography and land use map by interpolation. The

topography and land use map was first scanned and

registered. The elevation contour lines were digitized

together with the elevation points to be used in the

interpolation. The contour interpolation process

involves two steps (ITC-ILWIS, 2001): rasterization

and interpolation. Both the segment contour map and

the point elevation map were rasterized and combined.

The inclusion of the elevation points in the contour

interpolation assist to avoid the creation of flat areas

on hilltops or bottom of depressions as a result of

being enclosed by the same contour line. The

interpolation process was then performed on the

combined map using the Borgefors distance method

(ITC-ILWIS, 2001) and yielded a digital elevation

map (DEM). The height value for each undefined

pixel between two contours was found by calculating

the shortest distance towards the two isolines. The

vertical and horizontal filters bDyQ and bDxQ were

applied on the DEM in order to calculate the

horizontal and vertical gradients. The two filter maps

were used to obtain the slope percentage according to

the following formula:

Slope %ð Þ ¼ ðHYPðDx; DyÞ=pixel sizeÞ � 100 ð3Þ

where HYP is a map calculation function of ILWIS to

calculate the positive root of the sum of square Dx

plus square Dy (Pythagoras rule) where Dx and Dy

are the horizontal and vertical gradients, respectively.

The slope map was then sliced into ranges and

assigned ratings ranging from 1 to 10. Flat areas

obtain high rates because they slow down the runoff

allowing more time for the contaminant to percolate

down to reach the groundwater, while steep areas

increase the runoff washing out the contaminant hence

are assigned low rates.

The hydraulic conductivity map was also scanned

and spatially registered. The different hydraulic

conductivity zones in the area were defined and

assigned ratings according to DRASTIC.

4.2. Aquifer vulnerability assessment

The DRASTIC vulnerability index was computed

according to Eq. (1). In order to understand the

vulnerability index, it is necessary to choose a

representation method which can expose the aquifer

vulnerability in an appropriate fashion and simulta-

neously allows comparability between different areas.

Basically, Aller et al. (1987) introduced a national

colour coding of the vulnerability maps. The DRAS-

TIC indices are first classified into ranges by

imposing arbitrary thresholds. Those ranges are then

assigned the colours of the pseudo colour look-up

table (ranging from violet to red). Here, the vulner-

ability scores are presented based on the classification

scheme introduced by Chung and Fabbri (2001). In

this method, the vulnerability indices are classified

based on a fixed interval of area percentage in the

study area. The vulnerability index values were first

sorted in a descending form and then the vulnerability

indices corresponding to each 5% of the total number

of pixels in the study area were taken as thresholds for

the classification. Colours were then assigned to the

ranges of the subsequent percentages of pixels. The

cool colours (shades of blue) indicate bLowQ vulner-ability, the shades of green indicates bModerateQvulnerability while the warm colours (shades of red)

indicate bHighQ vulnerability. Because this represen-

tation demonstrates the results without imposing

arbitrary thresholds, it is considered free of subjectiv-

ity and useful in comparing results from different

areas or vulnerability models.

According to Civita (1994), the integrated vulner-

ability can be obtained by overlying a representation

of the actual pollution sources, which are subdivided

on the basis of their pollution potential (e.g., urban

areas, cultivated areas, waste dumps, industrial com-

plexes, and the like), on the intrinsic vulnerability

map. The integrated vulnerability map was obtained

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I.S. Babiker et al. / Science of the Total Environment 345 (2005) 127–140 133

by combining the vulnerability map and the potential

pollution sources extracted from the land use map. In

Kakamigahara Heights, nitrate contamination of

groundwater was found to closely associate mainly

with vegetable cultivation and to a lesser extent with

urban land (Babiker et al., 2004). Therefore, the two

land use types were considered as potential pollution

sources of groundwater of different degrees. To obtain

the integrated or specific vulnerability, we assumed

that there is no risk of contamination from land use/

cover types rather than vegetable fields and urban

land. Vegetable fields are expected to impose a high

risk of contaminant loading onto the ground surface

(e.g., nitrogen fertilizers) compared to urban land. It

has been suggested that the vulnerability classes

should be critically considered only where a ground-

water contamination risk exists. The vulnerability

classes are not considered where there is no risk of

contamination.

4.3. Sensitivity analysis

One of the major advantages of the DRASTIC

model is the implementation of assessment using a

high number of input data layers (Evans and Myers,

1990) which is believed to limit the impacts of errors

or uncertainties of the individual parameters on the

final output (Rosen, 1994). Some authors (e.g.,

Barber et al., 1993; Merchant, 1994) however, have

argued that a DRASTIC-equivalent result can be

obtained using a lower number of input parameters

and can achieve a better accuracy at less cost. Some

studies employed the DRASTIC model for aquifer

vulnerability using a lower number of parameters

(McLay et al., 2001) or assuming the constancy of

the missing parameter (Secunda et al., 1998). More-

over, the unavoidable subjectivity associated with the

selection of the seven parameters, the ratings, and

the weights used to compute the vulnerability index

has also been criticized (Napolitano and Fabbri,

1996). Here, we attempt to evaluate whether it was

really necessary to use all of the seven DRASTIC

parameters to assess the Kakamigahara aquifer

vulnerability by performing model sensitivity analy-

sis. The rated DRASTIC parameters were first

evaluated for interdependence and variability.

According to Rosen (1994), the independency of

DRASTIC parameters decreases the probability of

misjudgment. In fact, most of the DRASTIC

parameters are naturally closely related.

Two sensitivity tests were performed; the map

removal sensitivity analyses introduced by Lodwick

et al. (1990) and the single-parameter sensitivity

analysis introduced by Napolitano and Fabbri (1996).

The map removal sensitivity measure identifies the

sensitivity of the suitability map (vulnerability map)

towards removing one or more maps from the

suitability analysis and is computed in the following

way:

S ¼ ðjV=N � V V=nj=V Þ � 100 ð4Þ

where S is the sensitivity measure expressed in terms

of variation index, V and VV are the unperturbed and

the perturbed vulnerability indices respectively, and N

and n are the number of data layers used to compute V

and V V. The actual vulnerability index obtained using

all seven parameters was considered as an unper-

turbed vulnerability while the vulnerability computed

using a lower number of data layers was considered as

a perturbed one.

The single-parameter sensitivity measure was

developed to evaluate the impact of each of the

DRASTIC parameters on the vulnerability index. It

has been made to compare the beffectiveQ or brealQweight of each input parameter in each polygon with

the btheoreticalQ weight assigned by the analytical

model. The beffectiveQ weight of each polygon is

obtained using the following formula:

W ¼ ðPrPw=V Þ � 100 ð5Þ

where W refers to the beffectiveQ weight of each

parameter, Pr and Pw are the rating value and weight

of each parameter, and V is the overall vulnerability

index.

The implementation of the sensitivity analysis

requires a well-structured database and a GIS capable

of manipulating large tables. To avoid analysing the

large number of individual pixels in the study area

(639,794), the idea of bunique condition subareasQintroduced by Napolitano and Fabbri (1996) was used.

It is defined as one or more polygons (area) consisting

of pixels with a unique combination of Di, Ri, Ai, Si,

Ti, Ii, and Ci where Di, Ri, Ai, Si, Ti, Ii, and Ci are the

rating values of the seven parameters used to compute

the vulnerability index and 1ViV10. The cross-

operation of ILWIS 3.1 was used to obtain the

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I.S. Babiker et al. / Science of the Total Environment 345 (2005) 127–140134

subareas. The cross-operation performs an overlay of

two raster maps by combining pixels at the same

locations in both maps and tracking all the combina-

tions that occur between the different values or classes

in both maps (ITC-ILWIS, 2001). All the unique

combinations between the different parameters were

traced through multiple crossing. A total of 818

unique condition subareas have been found in the

study area; however, only 663 subareas that are larger

than 10 pixels in size (1000 m2) were considered in

the statistical analysis of the results.

5. Results and discussion

5.1. DRASTIC parameters and aquifer vulnerability

Fig. 2 shows examples of the rated parameter maps

used to obtain the DRASTIC aquifer vulnerability

index. The depth to water table in Kakamigahara

Heights is shallow (average is 26 m), decreasing

gradually from east to west. This makes the western

Fig. 2. Examples of the rated maps used to compute the DRASTIC vul

vulnerability while the score 10 indicates the maximum impact.

part of the Heights more susceptible to contamination

according to DRASTIC assumptions. The rating

scores ranges between 1 and 7 where the highest

scores were assigned to the far eastern part of the area

(depth to water is b9.1 m).

Although the study area is characterized by a high

annual rainfall (N1900 mm/year), the net recharge to

the groundwater aquifer is mainly controlled by the

type of land use/cover on the surface. The lowest

recharge rate (mean, 203 mm/year) was associated

with the urban land use because roofs and pavements

prevent the penetration of rainwater downward. The

rest of the study area had relatively higher recharge

rates (mean, 860 mm/year). The Kakamigahara

aquifer has generally high net recharge (N200 mm/

year) which was assigned high rating scores (8 and 9).

The gravel-rich alluvium of the Second Gravel

Formation mostly constitutes the Kakamigahara aqui-

fer and was assigned a high rating score (8). The

Mesozoic consolidated sediments and the tuff and

volcanic ash deposits were assigned low rating scores

ranging from 2 to 4.

nerability index. The rating score 1 implies a minimum impact on

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I.S. Babiker et al. / Science of the Total Environment 345 (2005) 127–140 135

The soil media is generally variable. The major soil

types are the Andosols, the Gleysols and the

Fluvisols. The different soil types were assigned rates

according to their permeability (depending on the

texture). A high score (10) was assigned to the

immature soil and the lowest score (1) was assigned

to the soil in the built-up areas. The loamy Andosols

and Gleysols were assigned moderate rating scores

(3~6).

The topography layer displayed a gentle slope

(0~6%) over most of the study area which has been

assigned the DRASTIC rating scores of 9 and 10. The

slope percentage increases northeast and northwest of

the study area associated with the mountain range.

These areas with steep slopes (N18%) were typically

assigned a low rating score (1) indicating their

minimal effect on the aquifer vulnerability.

In the impact of vadose zone layer, the gravel/sand-

rich alluvial deposits were assigned a high rating

value (8), the clayey–gravel deposits and the Meso-

zoic sandstone were assigned the moderate score 6,

while the low rating values 2, 3, and 4 were assigned

to the Mesozoic chert, mud-rich alluvium, and the tuff

and volcanic ash deposits, respectively.

Generally, the Kakamigahara aquifer is character-

ized by high hydraulic conductivity (N9.4�10�4 m/

s), therefore, was assigned the maximum rating score

of 10. The northern part of the aquifer has a

relatively lower conductivity (4.7�10�5~4.7�10�4

m/s) hence was assigned low rating values ranging

from 1 to 6.

Fig. 3. Aquifer vulnerability map

The DRASTIC aquifer vulnerability map clearly

shows the dominance of bHighQ vulnerability classes

(shades of red) in the western part of Kakamigahara

Heights while the eastern part is characterized by

bModerateQ vulnerability (shades of green; Fig. 3).

This pattern is mainly dictated by the variation in

depth to water from east to west. The elevated

northeastern part of the study area displays bLowQaquifer vulnerability. This is due to the combination of

deep water table, less-porous vadose and aquifer

media and steep topography. Ten vulnerability classes

are identified in the study area at a 5% interval. The

classification scale reflects the same level of detail for

the nine highest classes corresponding to 45% of the

total number of pixels in the study area. The least

vulnerable 55% of the study area is considered less

important and is assigned a one-class interval (N45%).

The integrated vulnerability map shows that the

eastern part of the Kakamigahara aquifer is under a

higher risk of contamination despite its moderate

intrinsic vulnerability (Fig. 4). The more vulnerable

western part of the aquifer is, however, under a lower

contamination risk. This is mainly due to the high

pollution risk associated with vegetable cultivation.

Recently, the analysis of nitrate concentration in 57

water samples from private, farm, monitoring, and

public water supply wells in Kakamigahara Heights

indicated that 90% of the water samples have shown

concentrations above the human affected value (13

mg/l NO3�) while 30% exceeded the permissible

concentration (44 mg/l NO3�) according to Japan

of Kakamigahara Heights.

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Fig. 4. Integrated vulnerability map combining the aquifer vulnerability with the potential pollution sources in Kakamigahara Heights.

Table 4

Summary of rank-order correlation analysis’ result between seven

I.S. Babiker et al. / Science of the Total Environment 345 (2005) 127–140136

regulation (Babiker et al., 2004). More than 70% of

those wells were associated with intensive vegetable

cultivation fields mainly on the eastern side of the

Heights.

5.2. Sensitivity of the DRASTIC model

The statistical summary of the seven rated param-

eter maps used to compute the DRASTIC index is

provided in Table 3. The highest risk of contamination

(mean value is 9) of groundwater in Kakamigahara

Heights originates from the net recharge parameter.

The soil media, topography, impact of vadose zone,

and hydraulic conductivity imply moderate risks of

contamination (5 and 6) while depth to water and

aquifer media impose a low risk of aquifer contam-

ination (4). Aquifer media, topography, soil media,

and hydraulic conductivity are highly variable (CV%

are 75, 66.7, 60 and 60, respectively) while depth to

water table and impact of vadose zone are moderately

variable (CV% are 50 and 40, respectively). Net

recharge is the least variable parameter (CV% is 11.1).

Table 3

A statistical summary of the DRASTIC parameter maps

D R A S T I C

Minimum 1 8 2 1 1 2 1

Maximum 7 9 8 10 10 8 10

Mean 4 9 4 5 6 5 5

SD 2 1 3 3 4 2 3

CV (%) 50 11.1 75 60 66.7 40 60

S.D. stands for standard deviation and CV for coefficient of

variation.

The low variability of the parameter implies a smaller

contribution to the variation of the vulnerability index

across the study area. The rank-order correlation

analysis (a summary of the result is provided in Table

4) between the seven DRASTIC parameters indicated

that a relatively strong relationship exists between

aquifer media and impact of vadose zone (r=0.81),

aquifer media and topography (r=0.73), and impact of

vadose zone and topography (r=0.56). The former

relationship can be attributed to the procedure

followed in order to construct the two layers from

the subsurface geology map, and the uniform and

unconfined nature of the Kakamigahara aquifer. The

latter two relationships can be explained by the fact

that the Kakamigahara aquifer occupies the alluvial

flood plain which has a typically low slope in

comparison to the mountain range in the north and

northeastern part of the area. Moderate correlation

was found between net recharge and soil media

DRASTIC parameters

Correlated parameters Correlation

coefficient, r

Significance

level, p

Aquifer media and impact of vadose 0.81 b0.0001

Aquifer media and topography 0.73 b0.0001

Impact of vadose and topography 0.56 b0.0001

Net recharge and soil media 0.46 b0.0001

Aquifer media and hydraulic

conductivity

0.30 b0.001

Depth to water and topography 0.29 b0.001

Only statistically significant (confidence level at/or more than 95%)

intercorrelations are tabulated.

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Table 6

Statistics of the map removal sensitivity analysis

Parameters used Variation index (%)

Mean Minimum Maximum S.D

D, R, S, T, I and C 4.5 0 11.3 3.3

D, R, S, T, and I 7 0 19.1 4.2

R, S, T and I 10.1 0 33.9 8

R, S and T 21 0 50.2 11

R and S 18.7 0 93.4 18.5

R 52.4 12.5 69.1 11.3

One or more parameters are removed at a time. Total of 633

subareas (z10 pixels in size) were considered. S.D. refers to the

standard deviation.

I.S. Babiker et al. / Science of the Total Environment 345 (2005) 127–140 137

(r=0.46) mainly due to the different recharge rates

assigned to the soil of the urban land cover and the

rest of the area (see Section 4.1). This agrees well with

the basic assumptions of DRASTIC model (see Table

1). As expected, depth to water table was slightly

correlated to the hydraulic conductivity (r=0.3).

Because of the relatively few significant correlations

at 95% confidence level (Table 4), the DRASTIC

parameters in Kakamigahara Heights were generally

considered independent.

5.3. Map removal sensitivity analysis

The results of the map removal sensitivity analysis

computed by removing one or more data layers at a

time are presented in Tables 5 and 6. The statistical

analysis of the variation index (sensitivity measure)

was applied only to those subareas which are 10

pixels or more in size (633 subareas). Table 4 displays

the variation of the vulnerability index as a result of

removing only one layer at a time. It is clear that high

variation of the vulnerability index is expected upon

the removal of the net recharge parameter from the

computation (mean variation index=15.1%). This

could mainly be attributed to the relatively high

theoretical weight assigned to this layer (Table 1) and

the high recharge rate characteristic of the Kakami-

gahara aquifer (rating score 9). The vulnerability

index seems to be sensitive to the removal of soil

media and topography layers although the two

parameters are considered theoretically less important

(weights 2 and 1, respectively). This also because of

their relatively high contamination risk (mean rating

scores 5 and 6, respectively). In Table 6, the variation

Table 5

Statistics of the map removal sensitivity analysis

Parameter

removed

Variation index (%)

Mean Minimum Maximum S.D.

D 7.9 0 16.3 4

R 15.1 4 41.7 7.5

A 4.5 0 11.3 3.3

S 11.6 3 15.4 2.8

T 11.2 2.3 16 3.3

I 10.7 0.5 24.3 5.2

C 7.2 0 24 4.9

One parameter is removed at a time. Total of 633 subareas (z10

pixels in size) were considered. S.D. refers to the standard deviation.

.

of the vulnerability index due to the removal of one or

more layers at a time from the model computation is

presented. The removal of the map(s) was based on

the previous map removal sensitivity measure (Table

5). The layers, which compel less variation of the final

vulnerability index, were preferentially removed. The

least average variation index resulted after removing

only the aquifer media layer (4.5%). As more data

layers are excluded from the computation of the

vulnerability index, the average variation index

increases. Although the layers assumed to be the

most effective were considered each time, the

interpretation of the growing average is not clear. It

might partly be attributed to one or more of the

following reasons: the internal variation of the

individual parameter, the weights assigned to them,

and their weak representation of the real world.

Conclusively, considerable variation in the vulner-

ability assessment is expected if a lower number of

data layers have been used.

5.4. Single-parameter sensitivity analysis

While the map removal sensitivity analysis pre-

sented in the previous section has confirmed the

significance of the seven parameters in the assessment

of the DRASTIC vulnerability index in the study area,

the single parameter sensitivity analysis compares

their beffectiveQ weights with their btheoreticalQweights. The beffectiveQ weight is a function of the

value of the single parameter with regard to the other

six parameters as well as the weight assigned to it by

the DRASTIC model. The beffectiveQ weights of the

DRASTIC parameters exhibited some deviation from

their btheoreticalQ weights (Table 7). The net recharge

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Table 7

Statistics of the single parameter sensitivity analysis

Parameter Theoretical

weight

Theoretical

weight (%)

Effective weight (%)

Mean Minimum Maximum S.D.

D 5 21.7 10.1 3.2 28.2 6.4

R 4 17.4 27.2 17.7 50 6.5

A 3 13.0 12.9 4.6 21.1 4.6

S 2 8.7 4.3 1.1 11.7 2.4

T 1 4.3 4.7 0.6 12.4 2.8

I 5 21.7 20.9 7.6 35.1 7.8

C 3 13.0 19.7 2.4 34.9 5.1

S.D. refers to the standard deviation. Total of 633 subareas (z10 pixels in size) were considered.

I.S. Babiker et al. / Science of the Total Environment 345 (2005) 127–140138

tends to be the most effective parameter in the

vulnerability assessment (mean effective wt.% is

27.2%) in agreement with the result from the map

removal sensitivity analysis. Its beffectiveQ weight

exceeds the theoretical weight assigned to it by

DRASTIC (17.4%). The hydraulic conductivity also

shows high beffectiveQ weight (19.7) that exceeds thebtheoreticalQ weight assigned by DRASTIC (13%).

The rest of the parameters, excluding topography,

exhibit lower beffectiveQ weights compared to the

btheoreticalQ weights. The significance of net rechargeand hydraulic conductivity layers highlights the

importance of obtaining accurate, detailed, and

representative information about these factors.

6. Conclusion

In this paper, we have attempted to assess the

aquifer vulnerability of the Kakamigahara ground-

water basin employing the empirical index DRASTIC

model of the U.S. Environmental Protection Agency

(EPA). Seven environmental parameters were used to

represent the natural hydrogeological setting of the

Kakamigahara aquifer; Depth to water, net Recharge,

Aquifer media, Soil media, Topography, Impact of

vadose zone, and hydraulic Conductivity. The DRAS-

TIC aquifer vulnerability map indicated that the

western part of the Kakamigahara aquifer is domi-

nated by bHighQ vulnerability classes while the easternpart was characterized by bModerateQ vulnerability

classes. The elevated northeastern part of the study

area displayed bLowQ aquifer vulnerability. The

integrated vulnerability map shows the high risk

imposed on the eastern part of the Kakamigahara

aquifer due to the high pollution potential of intensive

vegetable cultivation. The more vulnerable western

part of the aquifer is, however, under a lower

contamination risk. No contamination risk is expected

from land use/cover types other than vegetable fields

and urban lands in the Kakamigahara. In the particular

case of Kakamigahara Heights, land-use seems to be a

better predictor of groundwater contamination by

nitrate, although assessing natural aquifer vulnerabil-

ity is still necessary to avoid augmentation of the

problem and introduction of other pollutants.

The net recharge parameter inflicts the largest

impact (9) on the intrinsic vulnerability of the

Kakamigahara aquifer. Soil media, topography,

impact of vadose zone, and hydraulic conductivity

have moderate impacts (5 and 6) on aquifer vulner-

ability while depth to water table and aquifer media

have low impacts (4). The rank-order correlation

analysis between the seven DRASTIC parameters

indicated that a relatively strong relationship exists

between aquifer media and impact of vadose zone

(r=0.81), aquifer media and topography (r=0.73), and

impact of vadose zone and topography (r=0.56).

However, the relatively few significant correlations

at 95% confidence level between the DRASTIC

parameters in Kakamigahara Heights reveal their

independence. The map removal sensitivity analysis

indicated that the vulnerability index is highly

sensitive to the removal of net recharge, soil media,

and topography layers but is least sensitive to the

removal of the aquifer media layer. As more data

layers are excluded from the vulnerability index

computation, the average variation index increases.

Therefore, considerable variation in the vulnerability

assessment is expected if a lower number of data

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I.S. Babiker et al. / Science of the Total Environment 345 (2005) 127–140 139

layers have been used. The single-parameter sensitiv-

ity analysis has shown that net recharge and hydraulic

conductivity are the most significant environmental

factors which dictate the high vulnerability of the

Kakamigahara aquifer. This highlights the importance

of obtaining accurate, detailed, and representative

information about these factors.

The GIS technique has provided an efficient

environment for analyses and high capabilities in

handling a large quantity of spatial data. The seven

model parameters were constructed, classified and

encoded employing various map and attribute GIS

functions. The DRASTIC vulnerability index, which

is defined as a linear combination of factors, was

easily computed. Nevertheless, GIS greatly facilitated

the implementation of the sensitivity analysis applied

on the DRASTIC vulnerability index which otherwise

could have been impractical.

Acknowledgment

We thankfully acknowledge the assistance of the

Kakamigahara City Hall officials who have provided

the necessary materials to carry on this study. Thanks

are also due to Japan Meteorological Agency and

Ministry of Land Infrastructure and Transport, Japan

for making data available online to support different

kinds of research.

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