REPORT
Comparison of the AVI, modified SINTACS and GALDIT
vulnerability methods under future climate-change scenarios
for a shallow low-lying coastal aquifer in southern Finland
Samrit Luoma
1
& Jarkko Okkonen
2
& Kirsti Korkka-Niemi
3
Received: 14 February 2016 /Accepted: 28 August 2016 /Published online: 26 September 2016
#
The Author(s) 2016. This article is published with open access at Springerlink.com
Abstract A shallow unconfined low-lying coastal aquifer in
southern Finland surrounded by the Baltic Sea is vulnerable to
changes in groundwater recharge, sea-level rise and human
activities. Assessment of the intrinsic vulnerability of ground-
water under climate scenarios was performed for the aquifer
area by utilising the results of a published study on the impacts
of climate change on groundwater recharge and sea-level rise
on groundwater–seawater interaction. Three intrinsic vulnera-
bility mapping methods, the aquifer vulnerability index (AVI),
a modified SINTACS and GALDIT, were applied and com-
pared. According to the results, the degree of groundwater
vulnerability is greatly impacted by seaso nal variations in
groundwater recharge during the year, and also varies depend-
ing on the climate-change variability in the long term. The
groundwater is potentially highly vulnerable to contamination
from sources on the ground surface during high groundwater
recharge rates after snowmelt, while a high vulnerability to
seawater intrusion could exist when there is a low groundwa-
ter recharge rate in dry season. The AVI results suggest that a
change in the sea level will have an insignificant impact on
groundwater vulnerability compared with the results from the
modified SINTACS and GALDIT. The modified SINTACS
method could be used as a guideline for the groundwater vul-
nerability assessment of glacial and deglacial deposits in in-
land aquifers, and in combination with GALDIT, it could pro-
vide a useful tool for assessing groundwater vulnerability to
both contamination from sources on the ground surface and to
seawater intrusion for shallow unconfined low-lying coastal
aquifers under future climate-change conditions.
Keywords Aquifer vulnerability
.
Vulnerability mapping
.
Climate change
.
Coastal aquifer
.
Finland
Introduction
Shallow permeable aquifers located in low-lying coastal areas
are vulnerable not only to contamination from sources that are
located on the ground surface, but also to seawater intrusion
and/or flooding of coastal areas either due to sea-level rise or
storm surges (e.g. Luoma and Okkonen
2014; Luoma et al.
2013;OudeEssink1999, 2001;Barlow2003; Pulido-Leboeuf
2004;OudeEssinketal.2010; Rasmussen et al. 2013;
Ferguson and Gleeson 2012; Ataie-Ashtiani et al. 2013).
These events will presumably be accelerated by the changing
climate, including changes in precipitation, temperature and
groundwater recharge, as well as sea-level rise and an increas-
ing frequency of storm surges (IPCC
2000, 2007; Nicholls
et al. 2007). Besides these, the increasing demand for water
by the population and industry, as well as changing land-use
practises as a result of human activities, could expose shallow
aquifers to contamination.
The intrinsic vulnerability of an aquifer is the relative de-
gree of natural protection of an aquifer from contamination by
anthropogenic sources at the land surface. It is defined as a
* Samrit Luoma
samrit.luoma@gtk.fi
Jarkko Okkonen
jarkko.okkonen@gtk.fi
Kirsti Korkka-Niemi
kirsti.kokka-niemi@helsinki.fi
1
Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland
2
Geological Survey of Finland, P.O. Box 97,
FI-67101 Kokkola, Finland
3
Department of Geosciences and Geography, University of Helsinki,
P.O. Box 64, FI-00014 Helsinki, Finland
Hydrogeol J (2017) 25:203–222
DOI 10.1007/s10040-016-1471-2
function of the hydrogeological characteristics of the aquifer,
without considering the type and intensity of human activities
at the surface (Vrba and Zaporozec
1994). Although the vul-
nerability of an aquifer to contamination is based not only on
hydrogeological factors but also on land-use factors (Vrba and
Zaporozec
1994), the hydrogeological factors would not
change appreciably over time, whereas land use would. For
the sustainable and effective management and protection of
groundwater resources, an intrinsic vulnerability assessment
should be performed for any aquifer area, as an indicator of
aquifer vulnerability and the need for detailed investigations.
Particularly in low-lying permeable coastal aquifers, where
the groundwater level is close to the ground surface, a small
increase in groundwater recharge and sea-level rise may in-
crease aquifer vulnerability.
A shallow aquifer in the Hanko area in southern Finland, the
case study area, is confronting these issues, and there is an
attempt to maintain water supply quality within the drinking-
water standards in the long term. Referring to Luoma and
Okkonen (
2014), a rise in the sea level due to global climate
change would cause some parts of the Hanko aquifer to be
below the sea level, compromising groundwater quality. This,
together with the predicted increase in precipitation, would
increase groundwater recharge and raise the water table, con-
sequently contributing to the potential deterioration of grou nd-
water quality or potential flooding in the low-lying aquifer area.
A number of methods have been used to assess the intrinsic
vulnerability of aquifers. Among these, DRASTIC (Aller et al.
1987), SINTACS (Civita 1994), GOD (Foster 1987) and the
AVI (Van Stempoort et al.
1993) are well-known and suitable
methods for aquifers in clastic sedimentary environments. The
DRASTIC method is usually used to determine the vulnera-
bility of groundwater to contamination from anthropogenic
sources from the ground surface. However, it does not take
into account factors associated with watercourses such as
lakes or rivers that are connected to the aquifer. SINTACS is
a modified DRASTIC method with more options for the
weight strings, including additional factors associated with
human activities and watercourses, while the rating system
of each parameter is still preserved as in the original
DRASTIC method. For coastal aquifers, however, both
DRASTIC and SINTACS have no parameters to determine
contamination from seawater intrusion, which is a different
and more complicated process compared with contamination
via sources from the ground surface (Werner et al.
2013).
The GALDIT index (Chachadi et al.
2003;Lobo-Ferreira
et al. 2007), a system of weights and ratings similar to
DRASTIC and SINTACS, is a well-known method for
assessing the vulnerability to seawater intrusion of coastal
aquifers. The GALDIT vulnerability index map indicates the
aquifer area along the coastline that is most likely to be affect-
ed by seawater intrusion and provides recommendations for
detailed site investigations of aquifer area s. Although
GALDIT does not take into account the rate of groundwater
withdrawal relative to the total amount of freshwater recharge
to the aquifer, or the freshwater–saltwater interface in the sea-
water intrusion process, the simplicity of this method makes it
attractive for assessing aquifer vulnerability to seawater intru-
sion (Ivkovic et al.
2013), and it has been used for many
coastal aquifer areas around the world (e.g. Chachadi et al.
2003; Chachadi and Lobo-Ferreira 2007; Lobo-Ferreira et al.
2007; Dörfliger et al. 2011; Najib et al. 2012;Kuraetal.2015;
Recinos et al. 2015). According to the National Research
Council (
1993), vulnerability assessment methods must be
evaluated in order to verify the assigned vulnerability rating
and increase the reliability of assessments. However, for all of
the afore-mentioned methods, only a few cases have reported
the validation of the method, and the most commonly used
method has been a comparison of the results with geochemical
data from groundwater samples (Allouche et al.
2015). The
distribution of indicators of seawater intrusion such as total
di
ssolved solids (TDS), Cl and the Na/Cl ratio, and the extent
of the freshwater–seawater interface from the Ghyben-
Herzberg model have also been compared with the GALDIT
vulnerability index maps (Trabelsi et al.
2016).
In Finland, the shallow groundwater resides in Quaternary
sediments deposited during the Weichselian glaciation and
deglaciation. The sediments consist of glacial gravel, sand, till
and clay, and in some areas with postglacial littoral gravel,
sand and clay. The aquifer areas are very often located next
to watercourses such as lakes or rivers (Okkonen and Kløve
2010) or human infrastructure (e.g. urban areas, industries,
highways). Lavapuro et al. (2008) modified the DRASTIC
method to assess the intrinsic vulnerability of an aquifer in
an esker deposit where the sediments mainly consist of gravel
and sand. This modified DRASTIC method nevertheless did
not represent the overall depositional patterns of the shallow
aquifers in Finland, where the deposits consist not only of
esker material (with the deposition of coarse-grained sedi-
ments, e.g. gravel and sand), but also of ice-marginal end
formations (with the deposition of gravel, sand, glacial till, silt
and clay layers) or postglacial littoral sediments (gravel, sand
and clay). A vulnerability assessment method that can be ap-
plied for the whole depositio nal environment o f shallow
groundwater areas such as in Finland, as mentioned in the
previous section, and a method that can provide the same
standard for all shallow aquifers, is still needed.
Although SINTACS appears better suited than DRASTIC
to the vulnerability assessment of shallow glaciogenic aqui-
fers, the rating classifications in SINTACS are still not repre-
sentative of the depositional environments of shallow ground-
water areas—for example, the aquifer media of such areas do
not include basalt or massive sandstone or limestone. In this
study, the rating classification of SINTACS was modified for
three parameters: the soil media, aquifer media and the atten-
uation capa city of the unsaturated zone based on the
204 Hydrogeol J (2017) 25:203–222
superficial deposit map of Finland. The aim was to make it
more suitable for aquifers formed in glacial deposits and de-
positional environments following deglaciation.
This study examined how the impacts of climate change on
groundwater recharge, sea-level rise and the water table affect
the vulnerability of the shallow low-lying coastal glaciogenic
aquifer in Hanko, southern Finland. The results of a previous
assessment of climate change impacts on the Hanko aquifer
by Lu oma and Okkonen (
2014) were used to provide the
inputs for three index models, the AVI, the modified version
of SINTACS and GALDIT, and the results from the models
were compared. The numerical approaches could be useful
tools to predict contaminant transport in both space and time
in the coastal aquifers. However, these approaches often have
data and computational demands that are not easy to meet (e.g.
Sanford and Pope
2010). A recent study by Beebe et al. (2016)
suggests that analytical approaches are not always reliable. If
framed properly, index models could be able to support these
types of vulnerability assessments. In addition, index models
are generally used instead of numerical groundwater flow and
transport models for the assessment of the groundwater vul-
nerability because they are easy to use and the index maps can
be simply overlaid and integrated with thematic maps, such as
land use and hydrogeological maps, in order to provide infor-
mation to support the decisions of land users and land-use
managers in groundwater risk assessment in the area.
In order to assess the potential impacts of climate change
on the vulnerability of an aquifer and its ability to sustain
groundwater development in the future, vulnerability index
maps under climate variability and change should be
prepared and examined, which can be done by first
assessing climate change impacts on the aquifer and then
using the outcomes in different vulnerability assessment
methods. This study applied the outcomes of Luoma and
Okkonen (
2014) to demonstrate the impacts of climate vari-
ability and climate change on aquifer vulnerability. In addi-
tion, hydrogeochemical data, as well as field investigation and
monitoring data, including the temperature, water level and
electrical conductivity (EC) of groundwater, were used to con-
firm the degree of seawater intrusion and validate the vulner-
able areas of the coastal Hanko aquifer. The vulnerability in-
dex map was used to assess the vulnerability of groundwater
to potential sources of contamination in the groundwater area
at present and also those predicted for the future under differ-
ent climate change scenarios.
Study area
Background
The study area is located on the Hanko peninsula on the south-
ern co ast of Finla nd at app roximately 59°53″N 23°10 ″ E
(Fig.
1). The shallow, unconfined, low-lying coastal aquifer
in Hanko consists of porous gravels and sands of an ice-
marginal end deposit, and is located in a low-lying coastal
area bounded by the Baltic Sea. It is an important source of
drinking water and the water supply for residents of the town
Hanko and the local industries. The economy of Hanko town
is based on services (61 %) and industry (38 %) and the pop-
ulation in 2016 was 9109 (Hanko 2016). Hanko is a popular
summer resort, and the population considerably increases dur-
ing the summer due to the arrival of holiday homeowners and
tourists. The Hanko area belongs to the temperate coniferous-
mixed forest climate zone with cold, wet winters. The mean
annual temperature is 6 °C, with mean minimum and maxi-
mum temperatures of −4.2 and 16.6 °C, respectively. The
average annual precipitation was 620 mm during the period
1971–2000. Forestry, mainly of Scots pine (Pinus sylvestris),
is the main land use in the aquifer area (approximately 80 % of
land use). Additionally, the existing potential anthropogenic
impacts from human activities in the area, namely gravel ex-
cavation pits, local industries and salt (NaCl) used for de-icing
on the highway that runs through the middle of the ground-
water area, could pose a contamination risk to groundwater
quality.
Geology and hydrogeology
The Quaternary deposits in the Hanko area are underlain by
the basement of the Precambrian crystalline igneous and meta-
morphic rocks (Fig.
2). The Precambrian bedrock mainly con-
sists of granite, quartz diorite and granodiorites, forming a
sharp unconformity with the Quaternary deposit with some
outcrops in the area (Kielosto et al.
1996). The aquifer in the
study area is in the First Salpausselkä ice-marginal formation,
deposited during the Weichselian and Holocene deglaciation
of the Scandinavian Ice Sheet (Fyfe
1991; Saarnisto and
Saarinen 2001). The formation consists of gravel, sand, glacial
till, silt and clay, and of postglacial littoral gravel, sand and
clay (Fig.
1) that was originally deposited as sub-glacial out-
wash fan deposits (Fyfe 1991).
The primary ice-marginal formation in Hanko was
formed in deep water as a low narrow ridge (Fyfe
1991).
When the ice sheet withdrew from the area, this deep-water
deposit was covered by fine-grained sediments, silt and
clay layers of the Ancylus Lake and Littorina Sea. The
sea level has been regressive since deglaciation because
of isostatic land uplift. The primary deposit of the First
Salpausselkä formation was exposed to sea waves and also
to wind (Kielosto et al.
1996). The well-sorted gravels in-
dicate r eworked materials from the high energy o f waves
and storm activities, and are found over a large area in
Santala, while the fine sand from aeolian deposits covers
a large area in the east (Fig.
1;Fyfe1991; Kielosto et al.
1996).Thelakeandwetlandsinthemiddleoftheaquifer
Hydrogeol J (2017) 25:203–222 205
are located in a depression that forms part of the First
Salpausselkä formation and the sand dune terrain. The lake
has a surface area of about 1.8 km
2
, with an average depth
of approximately 1–2m.
Fig. 1 Location and Quaternary geological deposit map of the study area in the eastern Baltic Sea region. Cross-section lines A–A′ to C–C′ are presented
in Fig.
2 and D–D′ in the last figure
Fig. 2 V isualisation of the
bedrock surface, groundwater
level and sediments at drilled
wells in the Santala area. Cross-
section lines A–A′, B–B′ and C–C′
represent the thicknesses of
saturated and unsaturated zones
of the Quaternary sediment. The
locations of cross-section lines
and observation wells (Obs)are
indicated in Fig.
1
206 Hydrogeol J (2017) 25:203–222
The topographic landform of the study area varies between
10 and 14 m above sea level (a.s.l.) along the northern ridge of
the First Salpausselkä formation, and its elevation decreases
to less than 2 m along the northern coastline, while in the
south and southeast the elevation gradually decreases to 5–
7 m a.s.l. The shallow aquifer in Hanko is unconfined, with
the thickness of the Quaternary deposits varying from less
than 1–75 m, and the average thickness being about 25 m.
The sediments are generally thick on the western side, in a
NE–SW direction conforming to the First Salpausselkä for-
mation, and their thickness decreases eastwards to less than
1 m in the eastern coastal area. The simulated groundwater
levels under mean and dry (summer) climate conditions at
present (1971–20 00) are shown in Fig.
3 (Luoma and
Okkonen 2014). The water table varies between 2 and 10 m
below the ground surface in the inland area, and is less than
2 m below the ground surface in the coastal area, wh ere
groundwater discharges into the Baltic Sea. In many parts of
the aquifer, the groundwater level is close to the ground sur-
face, and water intake areas are located along the coastline,
where the groundwater level may often fall below the sea
level. According to the results of well testing and soil sample
analysis, the hydraulic conductivity of the aquifer ranges from
0.3 to 4.8 m day
−1
in silty sand and fine sand, and is up to
100 m day
−1
in sand and gravel (Luoma and Pullinen 2011).
Groundwater recharge mainly occurs twice a year, during
spring (late March to early April) and late autumn
(November to early December) from the infiltration of snow-
melt and rainfall, respectively (Luoma and Okkonen
2014).
Groundwater mainly flows northward into the coastal area
and also towards the south–southeast into wetlands and
peatlands, as well as towards the Baltic Sea in the east. The
groundwater level rapidly responds to a rise in the sea level, as
well as to recharge from the spring snowmelt and rainfall
(Backman et al.
2007; Luoma et al. 2013;Luomaand
Okkonen 2014).
Fig. 3 Simulated groundwater
levels based on data for the
present (1971–2000) under a
mean and b dry conditions
Hydrogeol J (2017) 25:203–222 207